A Radiation Oncologist's Guide to Contouring the Hippocampus

Objectives: Limiting the dose to the hippocampus will likely decrease cognitive problems from brain radiotherapy. The purpose of this article is to present a step-by-step guide to contouring the hippocampus on axial images as would be done in the standard process of planning conformal radiotherapy for brain radiotherapy. Materials and Methods: We explain how to contour the hippocampus on axial T1 echo sequence magnetic resonance images with intravenous gadolinium. Results: Step 1: Find the slice where the temporal horn-meaning the most anterior portion of the lateral ventricle-is well visualized; the hippocampus is the gray matter inside the curve of the temporal horn. Step 2: Contour inferiorly from the level of the temporal horn; below the level of temporal horn there is no visible boundary between the hippocampus and amygdala. You have to guess where this boundary is by extrapolating from more superior slices. The caudal most portion of the hippocampus is at the level of the pons and pituitary gland. Step 3: Contour superiorly from the level of the temporal horn; above the level of the temporal horn the hippocampus is the strip of gray matter bounded by cerebrospinal fluid in the lateral ventricle and ambient cistern. The cranial-most extent of the hippocampus is at the level of the splenium of the corpus callosum. Conclusions: This article presents a step-by-step guide to contouring the hippocampus on axial magnetic resonance images for radiation therapy treatment planning.