Hippocampal-Sparing Radiotherapy in Primary Sinonasal and Cutaneous Head and Neck Malignancies: A Feasibility Study

Abstract

Patients with primary sinonasal and cutaneous head and neck (H&N) malignancies often receive meaningful hippocampal doses, but the hippocampus is not a classic avoidance structure in radiation planning of these primary sites. This series characterizes the feasibility and tradeoffs of hippocampal-sparing radiotherapy (HSRT) for patients with primary sinonasal and cutaneous H&N malignancies. We retrospectively identified patients at a single institution treated definitively for primary sinonasal or cutaneous malignancies of the H&N. Each patient received (chemo)radiation and all received clinically-significant radiation dose to one or both hippocampi. We created new HSRT plans for each patient with intensity-modulated radiotherapy using original target and organ-at-risk (OAR) volumes. Hippocampi were contoured based on Radiation Therapy Oncology Group guidelines. Absolute and relative differences in radiation dose to the hippocampi, planning target volumes (PTV), and OARs were recorded. We used paired-samples t-tests to compare hippocampal and PTV dosimetric measures with and without HSRT. Thirty-seven patients were included (22 sinonasal, 11 cutaneous H&N, and 4 parotid primary tumors). Median prescription dose was 6600cGy (range: 5000-7440cGy). The most common fractionation regimens were 200cGy/fraction daily (51%, 19/37 patients) and 120cGy/fraction twice daily (41%, 15/37 patients). There were significant decreases in hippocampal Dmax and D100% using HSRT without compromising PTV coverage (Table 1). HSRT resulted in a relative increase of mean lacrimal gland dose by an average of 3.8%, optic chiasm Dmax by 1.3%, and whole brain Dmax of 1.2%. However, other OAR doses were lower with HSRT, including parotid gland mean dose, lens Dmax, optic nerve Dmax, cochlea mean dose, brainstem Dmax, and whole brain mean dose. HSRT is feasible and results in meaningful radiation dose reduction to the hippocampi without reducing PTV coverage or increasing dose to other OARs. The hippocampi should be regularly included as avoidance structures when treating primary sinonasal and cutaneous H&N tumors with radiation. We suggest target hippocampal constraints of Dmax < 1600cGy and D100% < 500cGy when feasible (without compromising PTV coverage). The clinical significance of HSRT in patients with primary H&N tumors should be investigated prospectively.