SU‐GG‐T‐176: Influence of the Multileaf Collimator Step Size in Step&Shoot IMRT Prostate Treatments

Abstract

Introduction Reducing the beamlet size during step&shoot IMRT planning may lead to improve dose conformity. The beamlet length can be adjusted in the treatment planning system (TPS) with the leaf step‐size along the MLC travel direction. The aim of this paper was to analyse the step‐size influence during prostate treatments. Materials and methods Ten patients treated for prostate cancer with 6MV‐IMRT fields were selected. Each treatment was calculated with the Konrad v2.2, Siemens, TPS using leaves step sizes of 3, 5 and 10mm, and delivered with a Primus, Siemens, linac associated to an Optifocus MLC (10mm leaf width). For each step‐size, the plans were calculated so as to equal the dose received by 95% of the PTV. Plans were compared in terms of PTV dose uniformity, organ‐at‐risk doses, treatment time and MU number. For each plan, the beams were imported in a water‐equivalent phantom and the doses calculated were compared with Farmer (0.6cc) and Pinpoint (0.015cc) ion chambers measurements. Results Reducing the step‐size from 10 to 5 or 3mm leads to a better PTV dose uniformity (31.2% and 35.8% dose standard deviation increase) and higher prostate minimum dose (+0.2% and +0.6%). Rectum V40 (V65) decreased of 3.2% (3.6%) and 6.9% (12%) for 5mm and 3mm. Bladder V65 and femoral heads V50 decreased of 9.5% and 26% for 5mm and 18% and 42% for 3mm. The treatment time and MU number increased of about 10% and 27% for 5mm and 20% and 48% for 3mm. The dose measurement verification showed that both chambers can be used for all step‐sizes with errors less than 1.3%. However, the PinPoint chamber use for 3mm beamlet sizes decreases the error from 1.3% to 0.3%. Conclusion Using 5mm step‐size seems to be a good compromise by improving the IMRT plan quality without treatment time increase.