Optimal coverage of peritoneal surface in whole abdominal radiation for ovarian neoplasms

Patterns of failure in ovarian carcinoma include early seeding of the entire peritoneal cavity. Inability to encompass the anatomic extent of the peritoneal cavity is a possible factor leading to relapse. However, little has been published regarding technical advances in optimal coverage of the peritoneal surface in whole abdominal radiation. In the Department of Radiation Oncology at the University of Michigan, 21 consecutive patients were analyzed prospectively in regard to adequate coverage of peritoneum in the treatment of advanced ovarian carcinoma. Simulation and focused blocks were designed to treat the whole abdomen. CT treatment planning studies were obtained with the entire peritoneum identified as the target volume. Simulator designed blocks were projected over the CT scans throughout the treatment volume. Dose volume histograms were used to calculate the amount of target volume missed for each treatment plan. All treatment plans demonstrated different degrees of volume miss, ranging from 1 cm3 to 837.3 cm3 with a median of 137.9 cm3 overall. Volume missed directly correlated with increasing patient weight and flatter pelvic shape, but poorly with AP separation. This was especially evident for patients requiring treatment at extended distances in both the supine and prone positions. We conclude that bony landmarks are poor guidelines in designing pelvic blocks, especially in heavy patients and patients requiring treatment in both prone and supine positions. CT treatment planning is helpful to ensure optimal peritoneal coverage.