Assessment of transposed ovarian movement: how much of a safety margin should be added during pelvic radiotherapy?

The purpose of this study was to analyze transposed ovarian movement. Data from 27 patients who underwent ovarian transposition after surgical treatment for uterine cancer were retrospectively analyzed. Computed tomography (CT) images including transposed ovaries were superimposed on other CT images acquired at different times, and were matched on bony structures. Differences in ovarian position between the CT images were measured. The planning organ at risk volume (PRV) margins were calculated from the formula of the 90% reference intervals (RIs) and the 95% RI, which were defined as mean ± 1.65 standard deviation (SD) and mean ± 1.96 SD, respectively. The 90% RI in the cranial, caudal, anterior, posterior, left and right directions were 1.5, 1.5, 1.4, 1.0, 1.7 and 0.9 cm, respectively. The 95% RI in the corresponding directions were 1.5, 2.0, 1.7, 1.2, 1.9 and 1.2 cm, respectively. These data suggest that bilateral ovaries need a PRV margin of ∼2 cm in all directions. The present study suggests that a transposed ovary needs the same PRV margin as a normal ovary (∼2 cm). Even after transposition, ovaries should be kept away from the radiation field to take into consideration the degree of ovarian movement.