Clinical applications of IMRT to adenocarcinoma of the prostate: Portal dose verification and intensity modulated neutron radiotherapy

Abstract

Intensity modulated radiotherapy (IMRT) provides an improvement in the conformality of radiotherapy dose distributions. Its application to photon radiotherapy for prostate adenocarcinoma is well established. A quality assurance tool for verifying photon IMRT treatment and the potential application of intensity modulation to neutron radiotherapy (IMNRT) to prostate cancer are investigated here. This study evaluates the use of an amorphous silicon flat panel imager for dose verification of photon IMRT fields. Various correction factors were developed to allow accurate estimation of the absorbed dose using this portal imager. The ratio of the dose measured with the portal imager to that measured using an ionization chamber was found to be for 23 measured IMRT fields. The study also yielded an accurate estimate of the relative beamlet intensity (fluence) at the plane of the detector. The raw difference between the relative beamlet intensity predicted by the EPID and that of the planning system for 23 IMRT fields was found to be . These results demonstrate the capabilities of this imager as a robust IMRT quality assurance tool. An in‐house optimization algorithm was used to optimize forward planned segments for the treatment of prostate cancer using IMNRT. The applicability of two different algorithms was investigated for IMNRT dose calculation, namely, the differential scatter air ratio (DSAR) and the finite size pencil beam (FSPB) algorithms. Measured profiles and absolute point doses were compared to results calculated by the treatment planning system. Dual ion‐chamber measurements were performed to determine the individual neutron and gamma doses and to estimate the whole body dose equivalent. IMNRT plans retrospectively calculated for five prostate cancer patients provided dose distributions superior to conventional fast neutron therapy. When normalized to provide equivalent target coverage, the volume of the rectum and bladder receiving 80% of the prescription dose in the IMNRT plans was reduced by an average of 16% and 13%, respectively. Measured and calculated profiles for IMNRT fields matched to within with the exception of the penumbral region. Absolute point dose measurements also agreed with calculated doses to within at various points within the modulated dose distribution. Whole body dose estimates for IMNRT plans generated in this study were only an average of 16% higher than conventional fast neutron therapy and no significant variations in RBE are expected based on current measurements. Based on the results presented here, clinical implementation of IMNRT is currently achievable.