143 Comparative evaluation of coplanar techniques for prostate conformal radiotherapy

Abstract

The aim of this study was to assess the relative merits of 4 different coplanar treatment techniques for conformal radiotherapy of the prostate. Twelve subjects with T1/T2 prostate cancer underwent a pelvic CT scan supine with the bladder full. The prostate and base of seminal vesicles were outlined with a margin of 10 mm. to give the planning target volume (PTV). The rectum, bladder and femoral heads were outlined as organs at risk (OARs). Isocentric plans using 3, 4, 6 & 8 fields with conformal blocks were generated and beam weights selected using a simulated annealing optimisation algorithm. The volume of each OAR within arbitrary dose bins of > 50% and > 80% were calculated for each plan and rectal normal tissue complication probabilities computed to determine the dose that would give a 5% complication rate (NTCP0.05). The optimised 6 & 8 field plans resulted in significantly smaller volumes of the femoral heads within both the > 50% and > 80% dose bins compared with the 3 & 4 field plans. There was a small but consistent advantage for the optimised 4 field plans with respect to the volume of rectum within both the > 50% and > 80% dose bins. The isocentric dose necessary to give the rectal NTCP0.05 was up to 4 Gy higher for the 4 field plans versus the next best technique. The optimised 6 field plans consistently resulted in a smaller volume of bladder within both the > 50% and > 80% dose bins. We conclude that there is no universally optimal coplanar beam arrangement for conformal radiotherapy of the prostate in this sample of 12 subjects.