METHOD FOR OCCUPATIONAL SKIN DOSE ESTIMATION IN UPPER EXTREMITY 131I-MIBG CONTAMINATION

METHOD FOR OCCUPATIONAL SKIN DOSE ESTIMATION IN UPPER EXTREMITY 131I-MIBG CONTAMINATION ABSTRACT This study examines upper extremity skin contamination of nuclear medicine and radiation safety staff during 131I-Metaiodobenzylguanidine (MIBG) therapy. Utilizing retrospective data, a methodology for performing a rapid assessment of the radiation dose to the skin of the upper extremities is presented. Using the skin contamination measurements and calculated skin dose for each contamination incident at our facility, a conversion factor (XE) was derived that estimates skin dose (DE) based on the initial contamination measurement. This methodology yields an estimate of the final skin dose accounting for radioactive decay, decontamination and other factors, such as skin sloughing. As a standard practice multiple time-point measurements from initial contamination to background should be used to calculate the total attributable skin dose. However, to provide an early projection of the expected skin dose, the dose can be reasonably estimated to be <0.10% mSv cpm−1 (10% mrem cpm−1) of the initial contamination measurement. INTRODUCTION 131I-Metaiodobenzylguanidine (131I-MIBG) is a therapeutic radiopharmaceutical used in the treatment of neuroendocrine tumors such as paraganglioma, pheochromocytoma and neuroblastoma.(1) Due to its principal gamma emission at 364 keV, 131I-MIBG may also be useful for diagnostic imaging, however, 123I-MIBG and other pharmaceuticals are typically used for detection, localization and staging.(2, 3) In pediatric patients with relapsed or refractory neuroblastoma, investigational dosages range from 111 to 703 MBq kg−1 (3–19 mCi kg−1) and may be administered as a single treatment or in conjunction with radiosensitizing chemotherapy agents.(4, 5) In addition to the extensive treatment planning performed by the patient’s medical team, involvement of professionals with expertise in radiation safety, i.e. health physicists, is necessary to ensure safety and compliance with regulatory requirements.(6) Frequent personnel contamination surveys should be performed during 131I-MIBG prep, therapy and room decontamination post therapy. If skin contamination is identified, the area is typically decontaminated with tepid water and mild soap. The skin is surveyed after each wash cycle, with the goal being to continue the decontamination process until the efficacy of the washing process is no longer reducing the contamination by 50%, or until it is determined that the remaining readings result from non-removable contamination(7). A wipe test to determine the amount of removable contamination can also be used to establish an endpoint for non-invasive decontamination techniques. Since our institution resides in the United States, skin dose calculations are consequently performed from the documented measurements to show compliance with Title 10 of the US Code of Federal Regulations (CFR) Part 20.1201.(8) This regulation mandates that an occupational radiation worker’s shallow dose equivalent (SDE) limit is 0.5 Sv per annum to any extremity skin. The SDE is defined as an external dose to the skin of the whole body or extremity at a tissue depth of 0.07 mm, which is similar to the personal dose equivalent, as defined by the International Commission on Radiation Units and Measurements, at the same tissue depth.(9) The SDE limit is set lower than the estimated threshold for deterministic effects to occur in the skin, which is 2–3 Sv.(7) The National Council on Radiation Protection and Measurements Report 130 states that the stochastic risk is negligible, but a deterministic effect such as acute epidermal necrosis could lead to infection.(10) The purpose of this study is to provide a practical guideline for an early skin dose estimate, so that proper decontamination techniques may be assessed to mitigate potential regulatory compliance issues and in extremely rare cases deleterious effects. To these ends, this retrospective study investigates upper extremity SDE, or skin dose, from 131I-MIBG contamination of nuclear medicine and radiation safety staff during the dose preparation, patient administration and facility decontamination following 131I-MIBG therapy. METHODS Contamination and detection Data was collected retrospectively from 131I-MIBG skin contamination incidents occurring at a pediatric facility to which the researchers provided radiation safety support from 2003 to 2014. This study includes only contaminations to the upper extremities such as the forearm, wrist and hand as defined by 10 CFR Part 20.1003 of both nuclear medicine and radiation safety staff members.(11) The historical data included count rate measurements taken at a distance of 1 cm from the contaminated skin surface using a calibrated survey meter with an internal or external thin-window pancake Geiger Muller (GM) probe. These types of survey meters are readily available in a nuclear medicine department. The corresponding skin location, date and time for each incident measurement were obtained from the records. Also obtained were the post decontamination skin surveys performed at multiple time-points during the following days until the skin region was at, or near, background radiation levels. The count rate measurements were converted to disintegration rates using an efficiency of 15% for beta detection. This efficiency was derived from the analysis of instrument calibration data of 18 pancake GM survey meters similar in model to the meters that were used by staff during the 10 years from which the contamination data was retrospectively gathered. The methodology for this assessment was based on NUREG-1507.(12) NUREG-1507 was developed as a guidance for US health physicists to quantify various external factors on the detection sensitivity of typical field radiation detection equipment. The guidance discusses multiple types of detection equipment commonly used in field measurements, and how to determine parameters for specific equipment such as detection efficiency and minimum detectable concentrations. The GM response data was collected by positioning the probe 1 cm above beta reference sources, in a geometry similar to that used during the measurement of skin contamination. The counting efficiencies for a range of peak beta energies from 156 to 1 711 keV were established using 14C, 99Tc, 36Cl and 32Si–32P sources. Figure 1 shows the average detection efficiency with one standard deviation across all 18 GM instruments. Detection efficiencies were interpolated by fitting this data with a three-parameter hyperbolic curve fit. Since the maximum energy of beta particles emitted by 131I-MIBG is 606 keV, the efficiency for beta detection is ~15% (Figure 1). Figure 1. View largeDownload slide GM detector efficiencies for maximum beta energy (Emax) calibrations using 14C, 99Tc, 36Cl and 32Si–32P sources. Figure 1. View largeDownload slide GM detector efficiencies for maximum beta energy (Emax) calibrations using 14C, 99Tc, 36Cl and 32Si–32P sources. Activity-to-skin dose conversion Radiation doses to the skin were calculated in accordance with NUREG/CR-6918 using the VARSKIN 4.0 computer code.(13, 14) Varskin is available through the Radiation Safety Information Computational Center at Oak Ridge National Laboratory, and is approved by the US Nuclear Regulatory Commission for use in performing radiation skin dose calculations.(14) Varskin 4.0 allows the user to define a variety of parameters including source geometry, isotope, and whether the source is in contact with the skin or if there was an intermediary (air gap, protective clothing, etc.). Tissue depth doses are calculated with photon and beta dosimetry models. Using the NUREG guidance the 131I-MIBG contaminate was simulated as an infinitely thin disk geometry for the radiation source in direct contact with the skin surface. The skin thickness was simulated at the SDE for the basal cell layer at a depth of 7 mg cm−2 (0.07 mm), and the area of contamination was assumed to be 1 cm2.(11) To comply with 10 CFR Part 20.1201, the dose from the highest contamination measurement must be averaged over a contiguous 10 cm2. Due to averaging over this area, variances in the geometric size of the radiation skin contamination below 10 cm2 yield negligible differences to the dose conversion factor, as shown in Table 1. Therefore, assuming the contaminated area of an upper extremity is 1 cm2 is adequate for these calculations. Table 1. VARSKIN calculated skin dose variations by geometric area for an infinitely thin disk modeling 131I skin contamination. Contaminate area (cm2) Disk diameter (cm) Skin dose (averaged over 10 cm2) Sv Bq−1 h−1 rem μCi−1 h−1 1 1.13 1.43 × 10−6 5.30 × 10−1 2 1.60 1.43 × 10−6 5.30 × 10−1 3 1.95 1.44 × 10−6 5.31 × 10−1 4 2.26 1.44 × 10−6 5.31 × 10−1 5 2.52 1.44 × 10−6 5.31 × 10−1 6 2.76 1.44 × 10−6 5.32 × 10−1 7 2.99 1.44 × 10−6 5.32 × 10−1 8 3.19 1.44 × 10−6 5.32 × 10−1 9 3.39 1.44 × 10−6 5.32 × 10−1 10 3.57 1.42 × 10−6 5.25 × 10−1 Contaminate area (cm2) Disk diameter (cm) Skin dose (averaged over 10 cm2) Sv Bq−1 h−1 rem μCi−1 h−1 1 1.13 1.43 × 10−6 5.30 × 10−1 2 1.60 1.43 × 10−6 5.30 × 10−1 3 1.95 1.44 × 10−6 5.31 × 10−1 4 2.26 1.44 × 10−6 5.31 × 10−1 5 2.52 1.44 × 10−6 5.31 × 10−1 6 2.76 1.44 × 10−6 5.32 × 10−1 7 2.99 1.44 × 10−6 5.32 × 10−1 8 3.19 1.44 × 10−6 5.32 × 10−1 9 3.39 1.44 × 10−6 5.32 × 10−1 10 3.57 1.42 × 10−6 5.25 × 10−1 View Large Table 1. VARSKIN calculated skin dose variations by geometric area for an infinitely thin disk modeling 131I skin contamination. Contaminate area (cm2) Disk diameter (cm) Skin dose (averaged over 10 cm2) Sv Bq−1 h−1 rem μCi−1 h−1 1 1.13 1.43 × 10−6 5.30 × 10−1 2 1.60 1.43 × 10−6 5.30 × 10−1 3 1.95 1.44 × 10−6 5.31 × 10−1 4 2.26 1.44 × 10−6 5.31 × 10−1 5 2.52 1.44 × 10−6 5.31 × 10−1 6 2.76 1.44 × 10−6 5.32 × 10−1 7 2.99 1.44 × 10−6 5.32 × 10−1 8 3.19 1.44 × 10−6 5.32 × 10−1 9 3.39 1.44 × 10−6 5.32 × 10−1 10 3.57 1.42 × 10−6 5.25 × 10−1 Contaminate area (cm2) Disk diameter (cm) Skin dose (averaged over 10 cm2) Sv Bq−1 h−1 rem μCi−1 h−1 1 1.13 1.43 × 10−6 5.30 × 10−1 2 1.60 1.43 × 10−6 5.30 × 10−1 3 1.95 1.44 × 10−6 5.31 × 10−1 4 2.26 1.44 × 10−6 5.31 × 10−1 5 2.52 1.44 × 10−6 5.31 × 10−1 6 2.76 1.44 × 10−6 5.32 × 10−1 7 2.99 1.44 × 10−6 5.32 × 10−1 8 3.19 1.44 × 10−6 5.32 × 10−1 9 3.39 1.44 × 10−6 5.32 × 10−1 10 3.57 1.42 × 10−6 5.25 × 10−1 View Large Skin dose estimation methods Two skin dose methods were used to evaluate a third proposed method, which allows the estimation of the resultant skin dose during the decontamination phase. The commonly used methods include the decay-based skin dose calculation and the multiple time-point skin dose method (gold standard). Of these two methods, only the decay-based method allows estimation of the resultant skin dose during the decontamination phase. The proposed pre-decontamination skin dose method would theoretically yield a skin dose more closely related to the gold standard than a method that only takes radioactive decay into account. All three methods were evaluated as follows: Decay-based skin dose (DDC) Simple skin dose calculations assume the major removal of contamination is through the physical decay process for 131I. Using this method, the radiation dose from the gamma and beta particles through the full decay process is calculated by integrating the initial skin dose (D0) over an infinite time period (t) utilizing the following equation: Df=∫i∞D0e−λt (1) where λ accounts only for the specific physical decay constant of 131I. The contamination activity (Ai) was determined by converting the counts per minute (cpm) from the initial contamination measurements to activity (kBq) using the detector efficiency described previously. The Ai along with the physical half-life of 131I (T) and an activity-to-dose conversion factor (CFVarskin) from Varskin can be applied to equation (1) to yield a decay-based skin dose (DDC): DDC=1.44×T×Ai×CFVarskin (2) Multiple time-point skin dose (DMTP) To account for biological and physical contamination removal processes, such as skin sloughing, multiple measurements of the skin contamination level are recorded from the time of initial contamination discovery until the area is indistinguishable from background radiation levels. The skin dose (Dx) may be calculated independently for each time interval (tX) between measurement with the following equation: Dx=tx×Ax×CFVarskin (3) The skin dose (DX) for each specific time period was calculated using the activity (Ax) determined from the skin measurement, the time period (tx) during which the activity is assumed to be present on the skin, and the activity-to-dose conversion factor (CFVarskin). The sum of these incremental doses (Dx) is the total attributable skin dose from the multiple time-point measurements (DMTP) from the contaminate: DMTP=∑ifDxi+…+Dxf (4) Pre-decontamination skin dose estimate (DE) Since the DMTP method for calculating skin dose requires data from multiple time-points post contamination, quick estimates of skin dose during the decontamination process are not possible using this method. Furthermore, the previously mentioned DDC method allows only for radioactive decay, which results in dose estimates that are unreasonably high for real-world situations. To determine an acceptable estimate of skin dose during the decontamination phase, the retrospectively gathered DMTP for each incident was normalized to its corresponding initial contamination measurement (M0) in cpm: XE=DMTPM0 (5) This new conversion factor (XE) can then be applied as a contamination measurement-to-dose conversion factor for estimating the skin dose during the preliminary stages of the decontamination by using the following equation: DE=XE×M0 (6) where DE is the skin dose estimate in milli-Sieverts (mSv), and M0 is the initial contamination reading prior to any decontamination procedures of the area. M0 is units of cpm, which is readily available on typical GM survey instruments found in a nuclear medicine department. RESULTS Contamination and detection A total of 520 131I-MIBG therapies were performed between 2003 and 2014, with administered activities ranging from 1.15 GBq (31 mCi) to 46.0 GBq (1244 mCi). In total, 28 contamination events occurred during this period. For the purposes of this study, each individual upper extremity contamination location identified during these events was treated as a separate incident. This resulted in 43 upper extremity staff contamination incidents yielding a contamination rate of ~8 incidents per 100 therapies (8.3%). A summary of the number of contamination incidents, total therapies, and range of administered activities per year can be found in Table 2. Table 2. Summary of 131I-MIBG therapies and the number of upper extremity staff skin contaminations by year. Year Total 131I-MIBG Therapies Range of administered activities (GBq) Number of upper extremity contaminations 2003 35 8.95–45.4 9 2004 38 6.73–44.4 4 2005 40 7.92–44.5 0 2006 39 3.42–27.5 1 2007 60 1.15–31.2 14 2008 41 2.75–45.8 2 2009 45 2.29–44.8 5 2010 35 2.18–46.0 1 2011 50 7.18–42.6 5 2012 54 7.18–41.4 1 2013 45 1.92–44.3 1 2014 38 2.10–24.7 0 Year Total 131I-MIBG Therapies Range of administered activities (GBq) Number of upper extremity contaminations 2003 35 8.95–45.4 9 2004 38 6.73–44.4 4 2005 40 7.92–44.5 0 2006 39 3.42–27.5 1 2007 60 1.15–31.2 14 2008 41 2.75–45.8 2 2009 45 2.29–44.8 5 2010 35 2.18–46.0 1 2011 50 7.18–42.6 5 2012 54 7.18–41.4 1 2013 45 1.92–44.3 1 2014 38 2.10–24.7 0 View Large Table 2. Summary of 131I-MIBG therapies and the number of upper extremity staff skin contaminations by year. Year Total 131I-MIBG Therapies Range of administered activities (GBq) Number of upper extremity contaminations 2003 35 8.95–45.4 9 2004 38 6.73–44.4 4 2005 40 7.92–44.5 0 2006 39 3.42–27.5 1 2007 60 1.15–31.2 14 2008 41 2.75–45.8 2 2009 45 2.29–44.8 5 2010 35 2.18–46.0 1 2011 50 7.18–42.6 5 2012 54 7.18–41.4 1 2013 45 1.92–44.3 1 2014 38 2.10–24.7 0 Year Total 131I-MIBG Therapies Range of administered activities (GBq) Number of upper extremity contaminations 2003 35 8.95–45.4 9 2004 38 6.73–44.4 4 2005 40 7.92–44.5 0 2006 39 3.42–27.5 1 2007 60 1.15–31.2 14 2008 41 2.75–45.8 2 2009 45 2.29–44.8 5 2010 35 2.18–46.0 1 2011 50 7.18–42.6 5 2012 54 7.18–41.4 1 2013 45 1.92–44.3 1 2014 38 2.10–24.7 0 View Large Of the 43 contamination incidents, four were discarded from the study based on insufficient data. The majority of the upper extremity contaminations were to a finger or thumb (74%) followed by the palm of the hand (20%) and wrist (6%). Of the 43 incidents, 44% were from events where multiple skin locations were contaminated simultaneously during a single event. Review of the contamination event reports showed that most of the 28 events occurred while staff were wearing proper personnel protective equipment (PPE). Reports for six of the events detailed either incorrect PPE or failure of the PPE. Based on these events, the prescribed PPE has changed over the years to include at least two pairs of disposable nitrile gloves with sleeve covers over laboratory coats to ensure coverage of the wrist when handling 131I-MIBG and potentially contaminated objects. The reported initial net count rate prior to decontamination ranged from 483 ± 22.0 to 299 950 ± 548 cpm. The majority of the reported data showed initial count rates <10 000 cpm (72%), and 8% of the incidents had count rates in excess of 100 000 cpm. The calculated skin doses for all incidents were well below the threshold for deterministic effects. Skin dose calculation methods Effective half life The effective half-life (Teff) for the purpose of this study is defined as a combination of the initial decontamination, biological removal, and radiological decay. Figure 2 shows the effective half-life (Teff) for each time-point measurement that was taken after the initial contamination measurement. The maximum Teff of 47.6 h occurred at 22 h post contamination and is only 25% of the time required for removal by TP. Figure 2. View largeDownload slide Calculated effective half-life (Teff) for each individual time-point measurement for all incidents versus time post contamination. Figure 2. View largeDownload slide Calculated effective half-life (Teff) for each individual time-point measurement for all incidents versus time post contamination. Figure 3 shows four different contamination incidents, each with at least eight consecutive measurements extending for an 8–15-day period post incident. In each case, the measured count rate for multiple time-points is shown as well as the predicted count rate calculated assuming only radioactive decay of 131I-MIBG contaminates over the same specified time interval based on the initial activity. The most substantial difference between the measured count rate and decay corrected count rate occurs at the final time-point with a 200% difference. Figure 3. View largeDownload slide Comparison of actual contamination count rates measured on the skin versus the predicted count rate based on an 131I physical decay calculation from the initial measurement with one standard deviation shown. Figure 3. View largeDownload slide Comparison of actual contamination count rates measured on the skin versus the predicted count rate based on an 131I physical decay calculation from the initial measurement with one standard deviation shown. DDC, DMTP, DE Figure 4 shows the DMTP per incident with the corresponding calculated DDC and proposed pre-decontamination skin dose estimate DE. The DDC skin doses assumed an infinite exposure time to account only for radioactive decay, and this resulted in a maximum dose of 1360 mSv for one of the incidents. The DMTP from equation (4) ranged from 9.56 to 19.9 mSv. Figure 4. View largeDownload slide Skin dose as a function of the initial contamination measurement using the multiple time-point methodology (DMTP), decay only correction method (DDC) and dose estimation method (DE). Figure 4. View largeDownload slide Skin dose as a function of the initial contamination measurement using the multiple time-point methodology (DMTP), decay only correction method (DDC) and dose estimation method (DE). The proposed method (DE) for predicting the pre-decontamination skin dose from the initial measurement is to use a normalized dose conversion factor (XE) as shown in equation (5). To derive the XE factor, each DMTP dose was normalized to the initial contamination measurement (M0) in cpm. This was done for ease of use, since the readily available GM meters in a nuclear medicine department can measure in cpm. Table 3 lists the distribution of XE factors as percentiles. The DE skin dose estimates are included in Figure 4 along with the DMTP and DDC shown for each incident for comparison. Table 3. Percentile distribution of the calculated values for the skin dose estimation conversion factor (XE). Percentile XE (mSv cpm−1) 25th 3.38 × 10−5 50th 1.90 × 10−4 75th 5.98 × 10−4 90th 1.02 × 10−3 Percentile XE (mSv cpm−1) 25th 3.38 × 10−5 50th 1.90 × 10−4 75th 5.98 × 10−4 90th 1.02 × 10−3 View Large Table 3. Percentile distribution of the calculated values for the skin dose estimation conversion factor (XE). Percentile XE (mSv cpm−1) 25th 3.38 × 10−5 50th 1.90 × 10−4 75th 5.98 × 10−4 90th 1.02 × 10−3 Percentile XE (mSv cpm−1) 25th 3.38 × 10−5 50th 1.90 × 10−4 75th 5.98 × 10−4 90th 1.02 × 10−3 View Large DISCUSSION According to the International Commission on Radiation Protection, the adult thyroid gland has an organ dose conversion factor of 72 mGy MBq−1 for an individual with 5% uptake of free iodine.(15) The organ dose conversion factor for MIBG bound 131I is substantially lower at 0.07 mGy MBq−1 since the thyroid is not the target organ of the intact, bounded radiopharmaceutical.(16) In most of the incidents studied, thyroid bioassays were conducted at 7–72 h post contamination. None of these resulted in positive uptakes (>1.48 kBq or 40 nCi) of absorbed disassociated 131I from the MIBG compound through the skin. Therefore, the concern for skin contamination from the intact radiopharmaceutical is greater for the degradation of the skin integrity rather than exposure of the healthy thyroid. However, at skin doses below 3 Sv, deterministic effects such as erythema, dry or moist desquamation, and necrosis are not likely, thus, making skin dose calculations important for demonstrating compliance with regulatory limits.(17) The idea of a biological removal process has been suggested to be both radiopharmaceutical dependent as well as dependent on the exposed skin surface.(18, 19) The Teff calculated in this study takes into account the loss of isotope through physical decay as well as biological removal, which suggests that calculating a skin dose based primarily on radioactive decay (DDC) as the most significant means of removal will potentially result in large overestimates. The variability in the removal efficiency of the 131I-MIBG contaminates occurring during the first 5–24-h post contamination in Figure 2 is most likely attributable to the initial decontamination process in addition to biological removal, which is all accounted for during this initial period. This is demonstrated again in the results in Figure 3. It is important to note that the decay predicted count rate was calculated based off the initially recorded contamination measurement. Post decontamination, the rate of removal of the skin contaminate is largely dependent on the biological factors affecting its removal efficiency, such as skin sloughing. These factors have a substantial effect and must be taken into account when making a reasonable estimate of the attributable skin dose. The maximum DDC calculated at 1360 mSv would have exceeded the US regulatory annual limit by almost a factor of 3, and provides a gross overestimation as compared to the corresponding DMTP skin dose of 7.09 mSv. Performing multiple measurements at various times until the contaminated skin reaches the background level allows the individual biological factors to be accounted for, as well as physical decay, providing a conservative yet more accurate skin dose. For the pre-decontamination skin dose estimate, DE, using the XE from the 75th percentile would have underestimated 18% of the corresponding DMTP doses. To be conservative, the 90th percentile XE yields a DE that does not underestimate any of the DMTP doses from this study. This particular XE factor can also conveniently be remembered as 0.10% in mSv (10% in mrem) of the initial contamination reading (M0) in cpm. That is, multiplying the initial contamination reading (cpm) by 0.10% (0.0010) mSv cpm−1 will result in a conservative estimate of the total skin dose in mSv. For example, in the incident discussed earlier the skin dose estimate based on an M0 of 300 000 cpm prior to decontamination using the traditional DDC method yielded a dose of 1360 mSv possibly suggestive of the potential need for a more invasive decontamination regimen. However, applying the 90th percentile XE factor from Table 3 to M0 using equation (6) DE=XE×M0=(1.02×10−3mSv/cpm)×300 000cpm would calculate a DE of 300 mSv. As a rule of thumb the DE (in mSv) is 0.10% of the initial contamination measurement M0. This DE still overestimates the final DMTP of 7.09 mSv by a factor of 42. However, it is a better skin dose predictor than the DDC immediately prior to beginning a decontamination regimen. CONCLUSION One of the challenges of a 131I-MIBG contamination is that the contamination activities have the potential to be substantial due to the high activities of the patient therapy doses and the long half-life of the nuclide. Having the ability to predict a reasonable skin dose estimate using the initial contamination reading is essential. Such an estimate would provide a guideline during the decontamination process to allow invasive measures, such as the removal of the upper layers of contaminated skin through abrasion or harsh detergents, to be avoided when the skin dose will not warrant such actions. An early indication of the expected skin dose (DE) from 131I-MIBG contamination can be estimated as 0.10% mSv cpm−1 (10% mrem cpm−1) of the initial contamination measurement, accounting for biological and physical decay processes. Table 4 provides a quick reference of the M0 count rates from the initial pre-decontamination measurement that would trigger skin doses (DE) at various percentages of the US regulatory annual limit. Since most accidental exposures of this nature will not exceed the threshold for deterministic effects, the relevant concern becomes establishing regulatory compliance. Table 4. Reference of initial contamination measurements (M0) that would yield estimated skin doses (DE) corresponding to specific percentages of the US regulatory annual limit. Percent of annual limita (%) Estimated skin dose, DE (Sv) M0 (cpm) 10 0.05 4.91 × 104 25 0.13 1.23 × 105 50 0.25 2.45 × 105 75 0.40 3.68 × 105 90 0.45 4.42 × 105 100 0.50 4.91 × 105 Percent of annual limita (%) Estimated skin dose, DE (Sv) M0 (cpm) 10 0.05 4.91 × 104 25 0.13 1.23 × 105 50 0.25 2.45 × 105 75 0.40 3.68 × 105 90 0.45 4.42 × 105 100 0.50 4.91 × 105 a0.5 Sv US regulatory shallow dose equivalent limit per year. View Large Table 4. Reference of initial contamination measurements (M0) that would yield estimated skin doses (DE) corresponding to specific percentages of the US regulatory annual limit. Percent of annual limita (%) Estimated skin dose, DE (Sv) M0 (cpm) 10 0.05 4.91 × 104 25 0.13 1.23 × 105 50 0.25 2.45 × 105 75 0.40 3.68 × 105 90 0.45 4.42 × 105 100 0.50 4.91 × 105 Percent of annual limita (%) Estimated skin dose, DE (Sv) M0 (cpm) 10 0.05 4.91 × 104 25 0.13 1.23 × 105 50 0.25 2.45 × 105 75 0.40 3.68 × 105 90 0.45 4.42 × 105 100 0.50 4.91 × 105 a0.5 Sv US regulatory shallow dose equivalent limit per year. View Large DE provides an initial guideline for determining the appropriateness of the level of decontamination, as well as, aiding in the decision for determining the necessary time intervals between measurements required to calculate a total attributable skin dose, DMTP. Shorter time intervals provide more data points for the DMTP calculation thus making it more accurate, which may be more or less necessary depending on the initial dose estimate in relation to the regulatory limit. As a standard practice, this early estimate DE should be followed by multiple time-point measurements from initial discovery to background radiation levels to calculate the total attributable skin dose (DMTP). ACKNOWLEDGEMENTS The authors would like to acknowledge, and thank Jessica Kendrick for her contributions during the initial stages of this work. REFERENCES 1 Carrasquillo , J. A. , Pandit-Taskar , N. and CHEN , C. C. Radionuclide therapy of adrenal tumors . J. Surg. Oncol. 106 , 632 – 642 ( 2012 ) 10.1002/jso.23196 . Google Scholar CrossRef Search ADS PubMed 2 Taieb , D. et al. . EANM 2012 guidelines for radionuclide imaging of phaeochromocytoma and paraganglioma . Eur. J. Nucl. Med. Mol. Imaging 39 , 1977 – 1995 ( 2012 ) 10.1007/s00259-012-2215-8 . 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Available on https://rsicc.ornl.gov (25 January 2018, date last accessed). 15 International Commission on Radiological Protection . Radiation dose to patients from radiopharmaceuticals. ICRP Publication 53 . Ann. ICRP 18 ( 1–4 ), 259 – 278 ( 1987 ). 16 International Commission on Radiological Protection . Radiation dose to patients from radiopharmaceuticals: addendum 3 to ICRP publication 53. ICRP Publication 106 . Ann. ICRP 38 ( 1–2 ), 153 – 157 ( 2008 ). 17 International Atomic Energy Agency . Health surveillance of persons occupationally exposed to ioninzing radation: guidance for occupational physicians. Safety Reports Series No. 5 ( 1998 ). 18 Covens , P. , Berus , D. , Caveliers , V. , Struelens , L. and Verellen , D. The contribution of skin contamination dose to the total extremity dose of nuclear medicine staff: first results of an intensive survey . Radiat. Meas. 46 , 1291 – 1294 ( 2011 ) 10.1016/j.radmeas.2011.07.007 . Google Scholar CrossRef Search ADS 19 Hession , H. , Byrne , M. , Cleary , S. , Andersson , K. G. and Roed , J. Measurement of contaminant removal from skin using a portable fluorescence scanning system . J. Environ. Radioact. 85 , 196 – 204 ( 2006 ) 10.1016/j.jenvrad.2004.07.008 . Google Scholar CrossRef Search ADS PubMed © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

METHOD FOR OCCUPATIONAL SKIN DOSE ESTIMATION IN UPPER EXTREMITY 131I-MIBG CONTAMINATION

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Abstract

ABSTRACT This study examines upper extremity skin contamination of nuclear medicine and radiation safety staff during 131I-Metaiodobenzylguanidine (MIBG) therapy. Utilizing retrospective data, a methodology for performing a rapid assessment of the radiation dose to the skin of the upper extremities is presented. Using the skin contamination measurements and calculated skin dose for each contamination incident at our facility, a conversion factor (XE) was derived that estimates skin dose (DE) based on the initial contamination measurement. This methodology yields an estimate of the final skin dose accounting for radioactive decay, decontamination and other factors, such as skin sloughing. As a standard practice multiple time-point measurements from initial contamination to background should be used to calculate the total attributable skin dose. However, to provide an early projection of the expected skin dose, the dose can be reasonably estimated to be <0.10% mSv cpm−1 (10% mrem cpm−1) of the initial contamination measurement. INTRODUCTION 131I-Metaiodobenzylguanidine (131I-MIBG) is a therapeutic radiopharmaceutical used in the treatment of neuroendocrine tumors such as paraganglioma, pheochromocytoma and neuroblastoma.(1) Due to its principal gamma emission at 364 keV, 131I-MIBG may also be useful for diagnostic imaging, however, 123I-MIBG and other pharmaceuticals are typically used for detection, localization and staging.(2, 3) In pediatric patients with relapsed or refractory neuroblastoma, investigational dosages range from 111 to 703 MBq kg−1 (3–19 mCi kg−1) and may be administered as a single treatment or in conjunction with radiosensitizing chemotherapy agents.(4, 5) In addition to the extensive treatment planning performed by the patient’s medical team, involvement of professionals with expertise in radiation safety, i.e. health physicists, is necessary to ensure safety and compliance with regulatory requirements.(6) Frequent personnel contamination surveys should be performed during 131I-MIBG prep, therapy and room decontamination post therapy. If skin contamination is identified, the area is typically decontaminated with tepid water and mild soap. The skin is surveyed after each wash cycle, with the goal being to continue the decontamination process until the efficacy of the washing process is no longer reducing the contamination by 50%, or until it is determined that the remaining readings result from non-removable contamination(7). A wipe test to determine the amount of removable contamination can also be used to establish an endpoint for non-invasive decontamination techniques. Since our institution resides in the United States, skin dose calculations are consequently performed from the documented measurements to show compliance with Title 10 of the US Code of Federal Regulations (CFR) Part 20.1201.(8) This regulation mandates that an occupational radiation worker’s shallow dose equivalent (SDE) limit is 0.5 Sv per annum to any extremity skin. The SDE is defined as an external dose to the skin of the whole body or extremity at a tissue depth of 0.07 mm, which is similar to the personal dose equivalent, as defined by the International Commission on Radiation Units and Measurements, at the same tissue depth.(9) The SDE limit is set lower than the estimated threshold for deterministic effects to occur in the skin, which is 2–3 Sv.(7) The National Council on Radiation Protection and Measurements Report 130 states that the stochastic risk is negligible, but a deterministic effect such as acute epidermal necrosis could lead to infection.(10) The purpose of this study is to provide a practical guideline for an early skin dose estimate, so that proper decontamination techniques may be assessed to mitigate potential regulatory compliance issues and in extremely rare cases deleterious effects. To these ends, this retrospective study investigates upper extremity SDE, or skin dose, from 131I-MIBG contamination of nuclear medicine and radiation safety staff during the dose preparation, patient administration and facility decontamination following 131I-MIBG therapy. METHODS Contamination and detection Data was collected retrospectively from 131I-MIBG skin contamination incidents occurring at a pediatric facility to which the researchers provided radiation safety support from 2003 to 2014. This study includes only contaminations to the upper extremities such as the forearm, wrist and hand as defined by 10 CFR Part 20.1003 of both nuclear medicine and radiation safety staff members.(11) The historical data included count rate measurements taken at a distance of 1 cm from the contaminated skin surface using a calibrated survey meter with an internal or external thin-window pancake Geiger Muller (GM) probe. These types of survey meters are readily available in a nuclear medicine department. The corresponding skin location, date and time for each incident measurement were obtained from the records. Also obtained were the post decontamination skin surveys performed at multiple time-points during the following days until the skin region was at, or near, background radiation levels. The count rate measurements were converted to disintegration rates using an efficiency of 15% for beta detection. This efficiency was derived from the analysis of instrument calibration data of 18 pancake GM survey meters similar in model to the meters that were used by staff during the 10 years from which the contamination data was retrospectively gathered. The methodology for this assessment was based on NUREG-1507.(12) NUREG-1507 was developed as a guidance for US health physicists to quantify various external factors on the detection sensitivity of typical field radiation detection equipment. The guidance discusses multiple types of detection equipment commonly used in field measurements, and how to determine parameters for specific equipment such as detection efficiency and minimum detectable concentrations. The GM response data was collected by positioning the probe 1 cm above beta reference sources, in a geometry similar to that used during the measurement of skin contamination. The counting efficiencies for a range of peak beta energies from 156 to 1 711 keV were established using 14C, 99Tc, 36Cl and 32Si–32P sources. Figure 1 shows the average detection efficiency with one standard deviation across all 18 GM instruments. Detection efficiencies were interpolated by fitting this data with a three-parameter hyperbolic curve fit. Since the maximum energy of beta particles emitted by 131I-MIBG is 606 keV, the efficiency for beta detection is ~15% (Figure 1). Figure 1. View largeDownload slide GM detector efficiencies for maximum beta energy (Emax) calibrations using 14C, 99Tc, 36Cl and 32Si–32P sources. Figure 1. View largeDownload slide GM detector efficiencies for maximum beta energy (Emax) calibrations using 14C, 99Tc, 36Cl and 32Si–32P sources. Activity-to-skin dose conversion Radiation doses to the skin were calculated in accordance with NUREG/CR-6918 using the VARSKIN 4.0 computer code.(13, 14) Varskin is available through the Radiation Safety Information Computational Center at Oak Ridge National Laboratory, and is approved by the US Nuclear Regulatory Commission for use in performing radiation skin dose calculations.(14) Varskin 4.0 allows the user to define a variety of parameters including source geometry, isotope, and whether the source is in contact with the skin or if there was an intermediary (air gap, protective clothing, etc.). Tissue depth doses are calculated with photon and beta dosimetry models. Using the NUREG guidance the 131I-MIBG contaminate was simulated as an infinitely thin disk geometry for the radiation source in direct contact with the skin surface. The skin thickness was simulated at the SDE for the basal cell layer at a depth of 7 mg cm−2 (0.07 mm), and the area of contamination was assumed to be 1 cm2.(11) To comply with 10 CFR Part 20.1201, the dose from the highest contamination measurement must be averaged over a contiguous 10 cm2. Due to averaging over this area, variances in the geometric size of the radiation skin contamination below 10 cm2 yield negligible differences to the dose conversion factor, as shown in Table 1. Therefore, assuming the contaminated area of an upper extremity is 1 cm2 is adequate for these calculations. Table 1. VARSKIN calculated skin dose variations by geometric area for an infinitely thin disk modeling 131I skin contamination. Contaminate area (cm2) Disk diameter (cm) Skin dose (averaged over 10 cm2) Sv Bq−1 h−1 rem μCi−1 h−1 1 1.13 1.43 × 10−6 5.30 × 10−1 2 1.60 1.43 × 10−6 5.30 × 10−1 3 1.95 1.44 × 10−6 5.31 × 10−1 4 2.26 1.44 × 10−6 5.31 × 10−1 5 2.52 1.44 × 10−6 5.31 × 10−1 6 2.76 1.44 × 10−6 5.32 × 10−1 7 2.99 1.44 × 10−6 5.32 × 10−1 8 3.19 1.44 × 10−6 5.32 × 10−1 9 3.39 1.44 × 10−6 5.32 × 10−1 10 3.57 1.42 × 10−6 5.25 × 10−1 Contaminate area (cm2) Disk diameter (cm) Skin dose (averaged over 10 cm2) Sv Bq−1 h−1 rem μCi−1 h−1 1 1.13 1.43 × 10−6 5.30 × 10−1 2 1.60 1.43 × 10−6 5.30 × 10−1 3 1.95 1.44 × 10−6 5.31 × 10−1 4 2.26 1.44 × 10−6 5.31 × 10−1 5 2.52 1.44 × 10−6 5.31 × 10−1 6 2.76 1.44 × 10−6 5.32 × 10−1 7 2.99 1.44 × 10−6 5.32 × 10−1 8 3.19 1.44 × 10−6 5.32 × 10−1 9 3.39 1.44 × 10−6 5.32 × 10−1 10 3.57 1.42 × 10−6 5.25 × 10−1 View Large Table 1. VARSKIN calculated skin dose variations by geometric area for an infinitely thin disk modeling 131I skin contamination. Contaminate area (cm2) Disk diameter (cm) Skin dose (averaged over 10 cm2) Sv Bq−1 h−1 rem μCi−1 h−1 1 1.13 1.43 × 10−6 5.30 × 10−1 2 1.60 1.43 × 10−6 5.30 × 10−1 3 1.95 1.44 × 10−6 5.31 × 10−1 4 2.26 1.44 × 10−6 5.31 × 10−1 5 2.52 1.44 × 10−6 5.31 × 10−1 6 2.76 1.44 × 10−6 5.32 × 10−1 7 2.99 1.44 × 10−6 5.32 × 10−1 8 3.19 1.44 × 10−6 5.32 × 10−1 9 3.39 1.44 × 10−6 5.32 × 10−1 10 3.57 1.42 × 10−6 5.25 × 10−1 Contaminate area (cm2) Disk diameter (cm) Skin dose (averaged over 10 cm2) Sv Bq−1 h−1 rem μCi−1 h−1 1 1.13 1.43 × 10−6 5.30 × 10−1 2 1.60 1.43 × 10−6 5.30 × 10−1 3 1.95 1.44 × 10−6 5.31 × 10−1 4 2.26 1.44 × 10−6 5.31 × 10−1 5 2.52 1.44 × 10−6 5.31 × 10−1 6 2.76 1.44 × 10−6 5.32 × 10−1 7 2.99 1.44 × 10−6 5.32 × 10−1 8 3.19 1.44 × 10−6 5.32 × 10−1 9 3.39 1.44 × 10−6 5.32 × 10−1 10 3.57 1.42 × 10−6 5.25 × 10−1 View Large Skin dose estimation methods Two skin dose methods were used to evaluate a third proposed method, which allows the estimation of the resultant skin dose during the decontamination phase. The commonly used methods include the decay-based skin dose calculation and the multiple time-point skin dose method (gold standard). Of these two methods, only the decay-based method allows estimation of the resultant skin dose during the decontamination phase. The proposed pre-decontamination skin dose method would theoretically yield a skin dose more closely related to the gold standard than a method that only takes radioactive decay into account. All three methods were evaluated as follows: Decay-based skin dose (DDC) Simple skin dose calculations assume the major removal of contamination is through the physical decay process for 131I. Using this method, the radiation dose from the gamma and beta particles through the full decay process is calculated by integrating the initial skin dose (D0) over an infinite time period (t) utilizing the following equation: Df=∫i∞D0e−λt (1) where λ accounts only for the specific physical decay constant of 131I. The contamination activity (Ai) was determined by converting the counts per minute (cpm) from the initial contamination measurements to activity (kBq) using the detector efficiency described previously. The Ai along with the physical half-life of 131I (T) and an activity-to-dose conversion factor (CFVarskin) from Varskin can be applied to equation (1) to yield a decay-based skin dose (DDC): DDC=1.44×T×Ai×CFVarskin (2) Multiple time-point skin dose (DMTP) To account for biological and physical contamination removal processes, such as skin sloughing, multiple measurements of the skin contamination level are recorded from the time of initial contamination discovery until the area is indistinguishable from background radiation levels. The skin dose (Dx) may be calculated independently for each time interval (tX) between measurement with the following equation: Dx=tx×Ax×CFVarskin (3) The skin dose (DX) for each specific time period was calculated using the activity (Ax) determined from the skin measurement, the time period (tx) during which the activity is assumed to be present on the skin, and the activity-to-dose conversion factor (CFVarskin). The sum of these incremental doses (Dx) is the total attributable skin dose from the multiple time-point measurements (DMTP) from the contaminate: DMTP=∑ifDxi+…+Dxf (4) Pre-decontamination skin dose estimate (DE) Since the DMTP method for calculating skin dose requires data from multiple time-points post contamination, quick estimates of skin dose during the decontamination process are not possible using this method. Furthermore, the previously mentioned DDC method allows only for radioactive decay, which results in dose estimates that are unreasonably high for real-world situations. To determine an acceptable estimate of skin dose during the decontamination phase, the retrospectively gathered DMTP for each incident was normalized to its corresponding initial contamination measurement (M0) in cpm: XE=DMTPM0 (5) This new conversion factor (XE) can then be applied as a contamination measurement-to-dose conversion factor for estimating the skin dose during the preliminary stages of the decontamination by using the following equation: DE=XE×M0 (6) where DE is the skin dose estimate in milli-Sieverts (mSv), and M0 is the initial contamination reading prior to any decontamination procedures of the area. M0 is units of cpm, which is readily available on typical GM survey instruments found in a nuclear medicine department. RESULTS Contamination and detection A total of 520 131I-MIBG therapies were performed between 2003 and 2014, with administered activities ranging from 1.15 GBq (31 mCi) to 46.0 GBq (1244 mCi). In total, 28 contamination events occurred during this period. For the purposes of this study, each individual upper extremity contamination location identified during these events was treated as a separate incident. This resulted in 43 upper extremity staff contamination incidents yielding a contamination rate of ~8 incidents per 100 therapies (8.3%). A summary of the number of contamination incidents, total therapies, and range of administered activities per year can be found in Table 2. Table 2. Summary of 131I-MIBG therapies and the number of upper extremity staff skin contaminations by year. Year Total 131I-MIBG Therapies Range of administered activities (GBq) Number of upper extremity contaminations 2003 35 8.95–45.4 9 2004 38 6.73–44.4 4 2005 40 7.92–44.5 0 2006 39 3.42–27.5 1 2007 60 1.15–31.2 14 2008 41 2.75–45.8 2 2009 45 2.29–44.8 5 2010 35 2.18–46.0 1 2011 50 7.18–42.6 5 2012 54 7.18–41.4 1 2013 45 1.92–44.3 1 2014 38 2.10–24.7 0 Year Total 131I-MIBG Therapies Range of administered activities (GBq) Number of upper extremity contaminations 2003 35 8.95–45.4 9 2004 38 6.73–44.4 4 2005 40 7.92–44.5 0 2006 39 3.42–27.5 1 2007 60 1.15–31.2 14 2008 41 2.75–45.8 2 2009 45 2.29–44.8 5 2010 35 2.18–46.0 1 2011 50 7.18–42.6 5 2012 54 7.18–41.4 1 2013 45 1.92–44.3 1 2014 38 2.10–24.7 0 View Large Table 2. Summary of 131I-MIBG therapies and the number of upper extremity staff skin contaminations by year. Year Total 131I-MIBG Therapies Range of administered activities (GBq) Number of upper extremity contaminations 2003 35 8.95–45.4 9 2004 38 6.73–44.4 4 2005 40 7.92–44.5 0 2006 39 3.42–27.5 1 2007 60 1.15–31.2 14 2008 41 2.75–45.8 2 2009 45 2.29–44.8 5 2010 35 2.18–46.0 1 2011 50 7.18–42.6 5 2012 54 7.18–41.4 1 2013 45 1.92–44.3 1 2014 38 2.10–24.7 0 Year Total 131I-MIBG Therapies Range of administered activities (GBq) Number of upper extremity contaminations 2003 35 8.95–45.4 9 2004 38 6.73–44.4 4 2005 40 7.92–44.5 0 2006 39 3.42–27.5 1 2007 60 1.15–31.2 14 2008 41 2.75–45.8 2 2009 45 2.29–44.8 5 2010 35 2.18–46.0 1 2011 50 7.18–42.6 5 2012 54 7.18–41.4 1 2013 45 1.92–44.3 1 2014 38 2.10–24.7 0 View Large Of the 43 contamination incidents, four were discarded from the study based on insufficient data. The majority of the upper extremity contaminations were to a finger or thumb (74%) followed by the palm of the hand (20%) and wrist (6%). Of the 43 incidents, 44% were from events where multiple skin locations were contaminated simultaneously during a single event. Review of the contamination event reports showed that most of the 28 events occurred while staff were wearing proper personnel protective equipment (PPE). Reports for six of the events detailed either incorrect PPE or failure of the PPE. Based on these events, the prescribed PPE has changed over the years to include at least two pairs of disposable nitrile gloves with sleeve covers over laboratory coats to ensure coverage of the wrist when handling 131I-MIBG and potentially contaminated objects. The reported initial net count rate prior to decontamination ranged from 483 ± 22.0 to 299 950 ± 548 cpm. The majority of the reported data showed initial count rates <10 000 cpm (72%), and 8% of the incidents had count rates in excess of 100 000 cpm. The calculated skin doses for all incidents were well below the threshold for deterministic effects. Skin dose calculation methods Effective half life The effective half-life (Teff) for the purpose of this study is defined as a combination of the initial decontamination, biological removal, and radiological decay. Figure 2 shows the effective half-life (Teff) for each time-point measurement that was taken after the initial contamination measurement. The maximum Teff of 47.6 h occurred at 22 h post contamination and is only 25% of the time required for removal by TP. Figure 2. View largeDownload slide Calculated effective half-life (Teff) for each individual time-point measurement for all incidents versus time post contamination. Figure 2. View largeDownload slide Calculated effective half-life (Teff) for each individual time-point measurement for all incidents versus time post contamination. Figure 3 shows four different contamination incidents, each with at least eight consecutive measurements extending for an 8–15-day period post incident. In each case, the measured count rate for multiple time-points is shown as well as the predicted count rate calculated assuming only radioactive decay of 131I-MIBG contaminates over the same specified time interval based on the initial activity. The most substantial difference between the measured count rate and decay corrected count rate occurs at the final time-point with a 200% difference. Figure 3. View largeDownload slide Comparison of actual contamination count rates measured on the skin versus the predicted count rate based on an 131I physical decay calculation from the initial measurement with one standard deviation shown. Figure 3. View largeDownload slide Comparison of actual contamination count rates measured on the skin versus the predicted count rate based on an 131I physical decay calculation from the initial measurement with one standard deviation shown. DDC, DMTP, DE Figure 4 shows the DMTP per incident with the corresponding calculated DDC and proposed pre-decontamination skin dose estimate DE. The DDC skin doses assumed an infinite exposure time to account only for radioactive decay, and this resulted in a maximum dose of 1360 mSv for one of the incidents. The DMTP from equation (4) ranged from 9.56 to 19.9 mSv. Figure 4. View largeDownload slide Skin dose as a function of the initial contamination measurement using the multiple time-point methodology (DMTP), decay only correction method (DDC) and dose estimation method (DE). Figure 4. View largeDownload slide Skin dose as a function of the initial contamination measurement using the multiple time-point methodology (DMTP), decay only correction method (DDC) and dose estimation method (DE). The proposed method (DE) for predicting the pre-decontamination skin dose from the initial measurement is to use a normalized dose conversion factor (XE) as shown in equation (5). To derive the XE factor, each DMTP dose was normalized to the initial contamination measurement (M0) in cpm. This was done for ease of use, since the readily available GM meters in a nuclear medicine department can measure in cpm. Table 3 lists the distribution of XE factors as percentiles. The DE skin dose estimates are included in Figure 4 along with the DMTP and DDC shown for each incident for comparison. Table 3. Percentile distribution of the calculated values for the skin dose estimation conversion factor (XE). Percentile XE (mSv cpm−1) 25th 3.38 × 10−5 50th 1.90 × 10−4 75th 5.98 × 10−4 90th 1.02 × 10−3 Percentile XE (mSv cpm−1) 25th 3.38 × 10−5 50th 1.90 × 10−4 75th 5.98 × 10−4 90th 1.02 × 10−3 View Large Table 3. Percentile distribution of the calculated values for the skin dose estimation conversion factor (XE). Percentile XE (mSv cpm−1) 25th 3.38 × 10−5 50th 1.90 × 10−4 75th 5.98 × 10−4 90th 1.02 × 10−3 Percentile XE (mSv cpm−1) 25th 3.38 × 10−5 50th 1.90 × 10−4 75th 5.98 × 10−4 90th 1.02 × 10−3 View Large DISCUSSION According to the International Commission on Radiation Protection, the adult thyroid gland has an organ dose conversion factor of 72 mGy MBq−1 for an individual with 5% uptake of free iodine.(15) The organ dose conversion factor for MIBG bound 131I is substantially lower at 0.07 mGy MBq−1 since the thyroid is not the target organ of the intact, bounded radiopharmaceutical.(16) In most of the incidents studied, thyroid bioassays were conducted at 7–72 h post contamination. None of these resulted in positive uptakes (>1.48 kBq or 40 nCi) of absorbed disassociated 131I from the MIBG compound through the skin. Therefore, the concern for skin contamination from the intact radiopharmaceutical is greater for the degradation of the skin integrity rather than exposure of the healthy thyroid. However, at skin doses below 3 Sv, deterministic effects such as erythema, dry or moist desquamation, and necrosis are not likely, thus, making skin dose calculations important for demonstrating compliance with regulatory limits.(17) The idea of a biological removal process has been suggested to be both radiopharmaceutical dependent as well as dependent on the exposed skin surface.(18, 19) The Teff calculated in this study takes into account the loss of isotope through physical decay as well as biological removal, which suggests that calculating a skin dose based primarily on radioactive decay (DDC) as the most significant means of removal will potentially result in large overestimates. The variability in the removal efficiency of the 131I-MIBG contaminates occurring during the first 5–24-h post contamination in Figure 2 is most likely attributable to the initial decontamination process in addition to biological removal, which is all accounted for during this initial period. This is demonstrated again in the results in Figure 3. It is important to note that the decay predicted count rate was calculated based off the initially recorded contamination measurement. Post decontamination, the rate of removal of the skin contaminate is largely dependent on the biological factors affecting its removal efficiency, such as skin sloughing. These factors have a substantial effect and must be taken into account when making a reasonable estimate of the attributable skin dose. The maximum DDC calculated at 1360 mSv would have exceeded the US regulatory annual limit by almost a factor of 3, and provides a gross overestimation as compared to the corresponding DMTP skin dose of 7.09 mSv. Performing multiple measurements at various times until the contaminated skin reaches the background level allows the individual biological factors to be accounted for, as well as physical decay, providing a conservative yet more accurate skin dose. For the pre-decontamination skin dose estimate, DE, using the XE from the 75th percentile would have underestimated 18% of the corresponding DMTP doses. To be conservative, the 90th percentile XE yields a DE that does not underestimate any of the DMTP doses from this study. This particular XE factor can also conveniently be remembered as 0.10% in mSv (10% in mrem) of the initial contamination reading (M0) in cpm. That is, multiplying the initial contamination reading (cpm) by 0.10% (0.0010) mSv cpm−1 will result in a conservative estimate of the total skin dose in mSv. For example, in the incident discussed earlier the skin dose estimate based on an M0 of 300 000 cpm prior to decontamination using the traditional DDC method yielded a dose of 1360 mSv possibly suggestive of the potential need for a more invasive decontamination regimen. However, applying the 90th percentile XE factor from Table 3 to M0 using equation (6) DE=XE×M0=(1.02×10−3mSv/cpm)×300 000cpm would calculate a DE of 300 mSv. As a rule of thumb the DE (in mSv) is 0.10% of the initial contamination measurement M0. This DE still overestimates the final DMTP of 7.09 mSv by a factor of 42. However, it is a better skin dose predictor than the DDC immediately prior to beginning a decontamination regimen. CONCLUSION One of the challenges of a 131I-MIBG contamination is that the contamination activities have the potential to be substantial due to the high activities of the patient therapy doses and the long half-life of the nuclide. Having the ability to predict a reasonable skin dose estimate using the initial contamination reading is essential. Such an estimate would provide a guideline during the decontamination process to allow invasive measures, such as the removal of the upper layers of contaminated skin through abrasion or harsh detergents, to be avoided when the skin dose will not warrant such actions. An early indication of the expected skin dose (DE) from 131I-MIBG contamination can be estimated as 0.10% mSv cpm−1 (10% mrem cpm−1) of the initial contamination measurement, accounting for biological and physical decay processes. Table 4 provides a quick reference of the M0 count rates from the initial pre-decontamination measurement that would trigger skin doses (DE) at various percentages of the US regulatory annual limit. Since most accidental exposures of this nature will not exceed the threshold for deterministic effects, the relevant concern becomes establishing regulatory compliance. Table 4. Reference of initial contamination measurements (M0) that would yield estimated skin doses (DE) corresponding to specific percentages of the US regulatory annual limit. Percent of annual limita (%) Estimated skin dose, DE (Sv) M0 (cpm) 10 0.05 4.91 × 104 25 0.13 1.23 × 105 50 0.25 2.45 × 105 75 0.40 3.68 × 105 90 0.45 4.42 × 105 100 0.50 4.91 × 105 Percent of annual limita (%) Estimated skin dose, DE (Sv) M0 (cpm) 10 0.05 4.91 × 104 25 0.13 1.23 × 105 50 0.25 2.45 × 105 75 0.40 3.68 × 105 90 0.45 4.42 × 105 100 0.50 4.91 × 105 a0.5 Sv US regulatory shallow dose equivalent limit per year. View Large Table 4. Reference of initial contamination measurements (M0) that would yield estimated skin doses (DE) corresponding to specific percentages of the US regulatory annual limit. Percent of annual limita (%) Estimated skin dose, DE (Sv) M0 (cpm) 10 0.05 4.91 × 104 25 0.13 1.23 × 105 50 0.25 2.45 × 105 75 0.40 3.68 × 105 90 0.45 4.42 × 105 100 0.50 4.91 × 105 Percent of annual limita (%) Estimated skin dose, DE (Sv) M0 (cpm) 10 0.05 4.91 × 104 25 0.13 1.23 × 105 50 0.25 2.45 × 105 75 0.40 3.68 × 105 90 0.45 4.42 × 105 100 0.50 4.91 × 105 a0.5 Sv US regulatory shallow dose equivalent limit per year. View Large DE provides an initial guideline for determining the appropriateness of the level of decontamination, as well as, aiding in the decision for determining the necessary time intervals between measurements required to calculate a total attributable skin dose, DMTP. 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