Gamma Knife Boost For Head And Neck Cancers With Skull Base Extension: A Case Series

Many Head and Neck Cancers (HNC) have a predilection for perineural spread which can track proximally and lead to skull base involvement. In cases of skull base involvement, including gross/residual disease after surgery, it can be challenging to deliver tumoricidal radiotherapy dose without exceeding nearby critical structure constraints despite the adoption of highly conformal approaches such as IMRT and proton. The use of GammaKnife stereotactic radiosurgery as a boost (GK-boost) to IMRT may further improve sparing of these critical structures while escalating dose to disease in the skull base. In this case series we report the clinical outcomes of patients receiving a GK-boost to the skull base as a component of their overall IMRT plan. Patients with HNC and gross/residual skull base disease after surgery treated with conventionally fractionated radiation therapy (IMRT/IMPT) plus a GK-boost between March 2014 and September 2019 were retrospectively reviewed. Five patients had squamous cell carcinomas (SCC) from a skin primary, one patient had a SCC of the oral cavity, and one patient had an adenoid cystic carcinoma. Treatment toxicity, survival, and local control were evaluated. Of the patients evaluated, 4 received adjuvant radiotherapy after surgery and 3 were treated with definitive radiotherapy. The median dose for IMRT was 66 Gy in 30-33 Fx. The median GK-boost dose was 8-11 Gy in a single fraction. The majority of patients received the GK boost after a median interval of 9 days post IMRT. The boost was delivered to the trigeminal nerve tract in five patients and the facial nerve tract in two patients. Median follow-up was 33 months. There were no local failures. There was 1 death and 1 regional recurrence in the same patient. This patient had a regional recurrence 11.2 months after completing treatment in the left orbit outside the prescribed PTV, which received 30 Gy or less. There were no G3 or higher acute toxicity associated with GK boost. There were 2 patients that developed asymptomatic temporal lobe radionecrosis. GammaKnife boost to the skull base is well tolerated and provides a durable local control benefit. An ongoing dosimetric comparative study will assess the extent of normal tissue dose sparing. A larger prospective study is needed to validate these clinical findings.