A Radiation Oncologist's Guide to Contouring the Hypothalamus

Abstract

Objectives

The hypothalamus is a complex structure involved in maintaining homeostasis in the body. Radiation induces potential hypothalamic damage leading to dysfunction and hormonal endocrinopathies that have vast clinical manifestations and significant long term health complications. Thus, limiting dose to the hypothalamus could have significant implications on treatment morbidity and long-term quality of life; however, Identifying the hypothalamus on CT images is challenging due to its intricate structure and unclear anatomic boundaries. We present a hypothalamus contouring atlas using MR images tailored for radiation oncologists during radiation treatment planning.

Methods

Imaging was obtained on a 1.5 Tesla Philips MRI scanner with 3 mm slice thickness using T1 MPRAGE sequences with IV gadolinium. Contouring was completed in MIM Maestro v7.2.3 based on anatomy visible on axial and sagittal images consistent with standard practice for delineating organs at risk.

Results

The first step begins with identification of the anterior (AC) and posterior (PC) commissure. Utilize a 5 mm brush to contour the lateral surface of the third ventricle between the AC and PC. Next, continue contouring inferiorly, following the third ventricle anteriorly. The inferior border is defined by the optic chiasm (OC), infundibulum, tuber cinereum, and mammillary bodies (MB). Stop contouring one slice above the level of the OC/MB. Then, assess the posterior margin, defined by an imaginary line connecting the MB to the PC. Finally, assess the anterior margin. To ensure inclusion of the preoptic hypothalamus contours should extend rostrally to an imaginary line between the OC and the AC.

Conclusion

Accurately defining the hypothalamus is critical for dosimetric precision, minimizing exposure to radiation, and improved long term outcomes. The presented guide provides a practical approach for contouring the hypothalamus on axial MRI imaging for radiation treatment planning.