Deriving optimal planning organ at risk volume margins in prostate external beam radiotherapy

Planning organ-at-risk volume (PRV) margins can be applied to the bladder and rectum in prostate external beam radiation therapy (EBRT), in order to incorporate the uncertainties resulting from their inter-fraction motion. For each of a total of 16 patients, the bladder and rectum were delineated on CBCT images for five treatment fractions in addition to the planning CT image set. The bladder and rectum boundary displacements across the images were measured and the frequency and size of organ boundary displacements were evaluated. Subsequently, PRV margins were created to cover a specific percentage of organ boundary motion for a specified percentage of the population. In this investigation, two bladder PRV margins were generated to deal with two bladder conditions of low and high-volume variation among fractions. A combined PRV margin was also generated for the rectum by separating the rectum into three parts and deriving independent PRV margins for each segment. Outward coverage and effectiveness metrics allowed evaluation of the margins.