A Contouring Strategy and Reference Atlases for the Full Abdominopelvic Bowel Bag on Treatment Planning and Cone Beam Computed Tomography Images

PURPOSE: To establish a practical contouring strategy with reference atlases for the abdominopelvic bowel bag on treatment planning computed tomography (TPCT) and cone beam computed tomography (CBCT) images. METHODS AND MATERIALS: A scoping literature review was done to evaluate the existing definitions and contouring guidelines for bowel bag and small bowel planning-at-risk volume–like structures. A comprehensive definition was proposed for the abdominopelvic bowel bag that expanded the Radiation Therapy Oncology Group Pelvic Normal Tissue Consensus definition. Seven patients with TPCT and first-treatment-day CBCT images were selected from an institutional database to represent a range of normal anatomy and CBCT image quality. The TPCT and CBCT images were contoured using the proposed definition. During contouring, the Radiation Therapy Oncology Group definition's list of inclusion and exclusion structures was expanded. For areas with limited visibility of the bowel bag on either TPCT or CBCT, a set of operational definitions was developed based on consistently visible reference structures. RESULTS: A literature review showed that previously existing bowel bag definitions predominantly focused on the pelvic region and did not provide a complete and practical description of the full abdominopelvic contour relative to structures consistently visible in all radiation therapy images. The proposed contouring strategy had 4 components: a definition, a list of inclusion and exclusion structures, 15 tabulated operational definitions, and a set of atlases. The bowel bag was defined as the peritoneal cavity and retroperitoneal duodenum and ascending and descending colon, as visualized at the time of image acquisition. The operational definitions formalized the location of the peritoneal fascial planes through a simple look-up table. The proposed contouring strategy and reference atlases were successfully used on both TPCT and CBCT images. CONCLUSIONS: This study produced a practical contouring strategy and reference atlases to enable reproducible delineation of the full bowel bag on TPCT and CBCT images. The strategy is a necessary first step toward consensus contouring with reduced observer variability, which is a prerequisite for evaluation of cumulative dose and its correlation with toxic effects, adaptive planning strategies, and automated contouring potential.