AbstractID: 7498 Title: An Implementation of Solid Filter
(compensator)-Based Intensity Modulated Radiation Therapy,
IMRT, for Clinical Use
During the past year and a half, an IMRT system that uses solid filters to modulate the beams has been implemented in our clinic. The
IMRT treatment delivery utilizes high resolution compensators mounted in the wedge slot (41.6 cm from the source) of the Siemens
MD accelerator. The planning system uses a convolution algorithm to calculate dose and the simultaneous-iterative-inversetreatment-planning method for IMRT optimization. The planner must choose beam angles and write a prescription for targets and
critical structures. The planning system optimizes beam weights and modulation patterns and generates files for milling compensators
out of brass, which are fabricated by a contracted company. Quality assurance (QA) measurements of absolute dose and dose profiles
along two orthogonal lines for each beam are made with a diode array, which has 46 diodes in a line. Comparisons are made to QA
data generated by the planning system. Compensator IMRT has been used for head and neck treatments. Experience has shown that
treatments with five fields can produce very conformal dose distributions. These compensator-based IMRT treatments can be
delivered in a 15-min time slot, the same time necessary for conventional 3-D treatments. The compensator thickness file has spatial
resolution of 2 mm, with points interpolated in between, resulting in 5 mm resolution isocenter. A 1 mm machining resolution in the
compensator thickness results in modulation resolution of approximately 4% for a 6 MV beam. IMRT plans carried out with
compensator modulation are as good or better than are those using MLC modulation.
Research partially supported by Computerized Medical Systems, Inc.