Investigation of an adaptive treatment regime for prostate radiation therapy.

Purpose: Changes in body contour during the course of radiation therapy can compromise the accuracy of treatment delivery. In prostate radiation therapy these changes can result from the daily patient positioning as well as the deformation of the body shape between treatment days and simulation. This study investigates the feasibility of using an online correction method to account for interfraction body contour changes prior to daily delivery of prostate radiation therapy based on cone beam computed tomography (CBCT) imaging.

Methods And Materials: This is a retrospective study where 5 prostate patients who received intensity modulated radiation therapy were selected. Patients were selected from a cohort who demonstrated repeated minor (1-2 cm) variations in body contour between planning CT and daily CBCT. Pretreatment CBCT images were used to calculate the "treated" dose and investigate the deviations from prescription caused by changes in body contour. A simple tissue maximum ratio correction was then applied to each field to calculate the "adapted" dose, which accounts for physical changes in path length on the central axis. A comparison was made between the treated and adapted scenarios.

Results: For this cohort, 22 of the 64 analyzed fractions showed body contour changes ≥1 cm. The treated doses showed deviations from prescribed dose ranging from -2.4% to 1.3% per fraction. After the plan was adapted, 4 of the 5 cases showed an improvement (P < .05) in target dose for the entirety of treatment. The 1 patient not showing improvement had negligible dose change due to contour variation.

Conclusions: This online adaptive strategy is a promising approach to account for changes in body contour during prostate intensity modulated radiation therapy.