ONLINE REPOSITIONING DURING TREATMENT OF THE PROSTATE - A STUDY OF POTENTIAL LIMITS AND GAINS

Purpose: With on-line portal imaging devices and image registration to ols, the verification of radiation field position Prior to each treatm ent becomes technically feasible. In this paper, we analyze the impact of pre-treatment verification and field position adjustment on target coverage and normal tissue sparing. Methods and Materials: Port films were compared with corresponding simulation films to determine the ma gnitude of setup variations in patients treated for prostate cancer. F rom these data, an analytic function was determined between geometric coverage of the target and field margin size. A paradigm for on-line p atient repositioning was employed to generate a new relationship betwe en margin and target coverage. Margins were selected for the situation s of normal treatment and on-line repositioning to ensure target cover age. Dose-volume histograms were generated for a typical prostate trea tment using these margins. Results: On-line repositioning, when setup errors exceed 1 cm, results in a 6 mm reduction in margin, suggesting that 10% of the volume of bladder and rectum may be spared of high dos e. Conclusion: The use of on-line imaging and image registration to gu ide adjustment of patient setup may lead to a eduction in the volume o f normal tissues irradiated, and possibly improve the probability of c omplication-free survival in future treatments.