SU-E-T-616: Shifting MLC to Follow the Prostate Movements while Concurrently Treating Pelvic Lymph Nodes

Abstract

Purpose: To evaluate an MLC tracking method and compare it to conventional iso‐center shift methods using daily kilo‐voltage cone beam CT (KV‐CBCT) for concurrent prostate and lymph nodes treatment. Methods: A total of 124 kV‐CBCT images from six patients were studied, and three verification plans were created for each KV‐CBCT. Contours of prostate, bladder and rectum were manually delineated, while contours of lymph nodes were transferred from the planning CT after bone‐based rigid image registration. The daily prostate displacements were obtained using dual‐image registrations: alignment to the bone and alignment to the prostate. For each segment of the initial plan, an in‐house program automatically shifted MLC leaf pairs that were collimated to the prostate according to measured prostate displacements. The MLC shifted plans were calculated based on shifted MLCs that applied to corresponding KV‐CBCT. The conventional iso‐center shifted plans were created based on original MLC shapes with shifted iso‐centers according to prostate contour‐based (iso‐shift‐contour plans) or bone‐based registration (iso‐shift‐bone plans). The dose to 95% (D95) of prostate and lymph nodes, and dose to 5% (D5) of rectum and bladder, were compared for all verification plans. Results: Using D95 within 3% of the prescription dose as a criterion, 97%, 97.6%, and 95% of 124 fractions met the criterion for prostate in the MLC‐shift, iso‐shift‐contour, and iso‐shift‐bone plans, respectively. Accordingly, 96%, 91%, and 97.6 of 124 fractions met the criterion for lymph nodes. For bladder, D5 of the bladder < 102% of the planned dose was observed in 67%, 56%, and 70% of the fractions, and D5 of the rectum <105% of the planned dose was observed in 85%, 88%, and 86% of the fractions, respectively. Conclusions: MLC‐shift method is an effective strategy for concurrent treatment of two independent moving targets, especially for patients with large organ motion.