Planning organ at risk volume margins for organ motion of the intestine

Background and purposeTo account for internal organ motion and set-up uncertainties around organs at risk (OR) in radiotherapy (RT), the ICRU report no 62 introduced the planning organ at risk volume (PRV). In the present study, we have quantified PRV margins for the intestine, which is an important OR in pelvic RT.Materials and methodsThe present study was based on intestine contours outlined in a total of 149 CT scans of 20 male bladder cancer patients (20 planning scans, 129 during treatment). From these data, we created location probability maps of the intestine for each patient. A commercial treatment planning system was used to add 3D isotropic intestine PRV margins (from 5 to 30 mm, in intervals of 5 mm) around the intestine planning outline. We then derived the fraction of patients for which a given PRV encompassed various degrees of intestine motion (85%, 90% and 95% of volumes with different probabilities of intestinal occupancy). As a measure of the specificity of the PRV, we also derived the fraction of the PRV containing volumes with zero probability of intestinal occupancy.ResultsIsotropic margins of up to 30 mm are required to account for all intestine motion in 90% of the patients, while isotropic margins of 5–10 mm will encompass 85–95% of the volumes having a probability of intestinal occupancy of ⩾75% in the same fraction of patients. Intestine PRVs are not very specific and will also include volumes where the intestine will rarely or never be located.ConclusionsLarge intestinal motion was found, but isotropic PRV margins of 5–10 mm will include the major part of volumes with a large probability of intestinal occupancy in most patients.