QUALITY ASSURANCE OF SERIAL TOMOTHERAPY FOR HEAD AND NECK
.D., K. S. C
, M.D., S
, M.S., R
Mallinckrodt Institute of Radiology, Division of Radiation Oncology, 510 South Kingshighway Blvd., St. Louis, MO 63110
Purpose: A commercial serial tomotherapy intensity-modulated radiation therapy (IMRT) treatment planning
(Peacock, NOMOS Corp., Sewickley, PA) and delivery system is in clinical use. The dose distributions are highly
conformal, with large dose gradients often surrounding critical structures, and require accurate localization and
dose delivery. Accelerator and patient-speciﬁc quality assurance (QA) procedures have been developed that
address the localization, normalization, and delivery of the IMRT dose distributions.
Methods and Materials: The dose distribution delivered by serial tomotherapy is highly sensitive to the accuracy
of the longitudinal couch motion. There is also an unknown sensitivity of the dose distribution on the dynamic
mutlileaf collimator alignment. QA procedures were implemented that assess these geometric parameters.
Evaluations of patient positioning accuracy and stability were conducted by exposing portal ﬁlms before (single
exposure) and after (single or double exposure) treatments. The ﬁlms were acquired with sequential exposures
using the largest available ﬁxed multileaf portal (3.36 ؋ 20 cm
). Comparison was made against digitally
reconstructed radiographs generated using independent software and appropriate beam geometries. The deliv-
ered dose was veriﬁed using homogeneous cubic phantoms. Radiographic ﬁlm was used to determine the
localization accuracy of the delivered isodose distributions, and ionization chambers and thermoluminescent
dosimetry (TLD) chips were used to verify absolute dose at selected points. Ionization chamber measurements
were conﬁned to the target dose regions and TLD measurements were obtained throughout the irradiated
volumes. Because many more TLD measurements were made, a statistical evaluation of the measured-to–
calculated dose ratio was possible.
Results: The accelerator QA techniques provided adequate monitoring of the geometric patient movement and
dynamic multileaf collimator alignment and positional stability. The absolute delivered dose as measured with
the ionization chamber varied from 0.94 to 0.98. Based on these measurements, the delivered monitor units for
both subsequent QA measurements and patient treatments were adjusted by the ratio of measured to calculated
dose. TLD measurements showed agreement, on average, with the ionization chamber measurements. The
distribution of TLD measurements in the high-dose regions indicated that measured doses agreed within 4.2%
standard deviation of the calculated doses. In the low-dose regions, the measured doses were on average 5%
greater than the calculated doses, due to a lack of leakage dose in the dose calculation algorithm.
Conclusions The QA system provided adequate determination of the geometric and dosimetric quantities
involved in the use of IMRT for the head and neck. Ionization chamber and TLD measurements provided
accurate determination of the absolute delivered dose throughout target volumes and critical structures, and
radiographic ﬁlm yielded precise dose distribution localization veriﬁcation. Portal ﬁlm acquisition and subse-
quent portal ﬁlm analysis using 3.36 ؋ 20 cm
portals proved useful in the evaluation of patient immobilization
quality. Adequate bony landmarks were imaged when carefully selected portals were used. © 1998 Elsevier
Intensity-modulated radiation therapy, Photon dose-calculation algorithm veriﬁcation, Treatment veriﬁcation,
A commercial serial tomotherapy planning and delivery
was recently installed and commissioned in our
clinic (1). The system is used to deliver conformal dose
distributions using serial tomotherapy, and includes a treat-
ment-planning system and computer-driven tertiary dy-
namic multileaf collimator (DMLC)
. The DMLC consists
of 40 leaves, arranged in two parallel adjacent banks of 20
leaves each, and is oriented so that a line connecting the
projected leaf centers is parallel to the gantry rotation plane.
The leaves are pneumatically driven and move in a direction
normal to the gantry rotation plane, each projecting on our
Presented at the 1997 ASTRO Meeting.
Reprint requests to: Daniel A. Low, Ph.D., Division of Radia-
tion Oncology, Mallinckrodt Institute of Radiology, 510 South
Kingshighway Blvd., St. Louis, MO 63110.
Accepted for publication 9 July 1998.
Peacock, NOMOS Corp., Sewickley, PA.
MIMiC, NOMOS Corp., Sewickley, PA.
Int. J. Radiation Oncology Biol. Phys., Vol. 42, No. 3, pp. 681–692, 1998
Copyright © 1998 Elsevier Science Inc.
Printed in the USA. All rights reserved
0360-3016/98 $19.00 ϩ .00