Development of a contouring guide for three different types of optic chiasm: A practical approach

Sparing of the organs at risk ( OAR s) is a crucial task in daily radiotherapy practice. Irradiation of the optic chiasm ( OC ) results in radiation‐induced optic neuropathy ( RION ). The structure of the OC is complex, and OC morphology can vary in axial images. Therefore, a standard atlas can result in inaccurate descriptions of OC morphology in different patients. The aim of our study was to provide a guide based on computed tomography ( CT ) for the delineation of different types of OC . Methods Thirty‐six patients were selected to participate in our study. These patients underwent CT analysis of the brain, head and neck regions in a supine position. Axial images 3 mm in thickness were obtained at 3‐mm intervals. A magnetic resonance imaging ( MRI ) study was also performed using the same set‐up. The OC was then delineated. The contours were revised by three neuroradiologists and nine radiation oncologists with > 5 years of expertise. Results Three types of OC were distinguished by magnetic resonance ( MR ). The location and boundaries of normal, prefixed and postfixed chiasms were developed with a CT ‐based atlas. Discrepancies were observed in the delineation of the prefixed and postfixed OC . Conclusions Our guide allows improved definitions of the anatomical boundaries for different types of OC . Our experience could provide useful information for radiation oncologists in daily practice.