Compensating filters for IMRT I: Material characterization and process verification

The clinical implementation of Intensity Modulated Radiation Therapy (IMRT) may be accomplished with existing, well-understood dose modulation techniques such as compensating filters. The initial evaluation of a commercial 3D treatment planning system for producing remotely manufactured aluminum compensating filters is reported here as a first step toward clinical use of attenuator-based IMRT. Time and labor concerns cited as detriments to compensator techniques are addressed. Internet-based communication between the treatment planning system and the remote manufacturing facility provides an efficient, convenient means of quickly implementing compensated beams for any given treatment plan. The intermediary support role of the treatment planning vendor and the reliability of modern shipping services contribute a measure of stability important for daily clinical operations. Characterization of the aluminum stock used in the process has been done for 6 and 18 MV photon beams, with average effective linear attenuation coefficients of 0.108 cm^-1 and 0.073 cm^-1 respectively. The field size, tissue deficit, and depth in phantom dependencies of the attenuation coefficients, tissue reduction ratios, and transmission curves were measured with an ion chamber in solid water. Compensation factors calculated for individual beams using a single, average effective linear attenuation coefficient agreed with measured values to within 1% in six of seven cases (2% for the seventh). Initial tests of the effectiveness of the dose compensation algorithm used in the current version of the software were carried out using radiographic film dosimetry, showing agreement within 5% for the Clarkson algorithm using a single attenuation coefficient.