Stereotactic Radiosurgery for Retreatment of Gross Perineural Invasion in Recurrent Cutaneous Squamous Cell Carcinoma of the Head and Neck

Objectives: To report outcomes, failure patterns, and toxicity after stereotactic radiosurgery (SRS) for recurrent head and neck cutaneous squamous cell carcinoma with gross perineural invasion (GPNI). Methods: Ten patients who received SRS as part of retreatment for recurrent head and neck cutaneous squamous cell carcinoma with GPNI were included. All patients exhibited clinical and radiologic evidence of GPNI before SRS. Previous treatments included surgery alone in 3 patients and surgery with adjuvant external beam radiotherapy (EBRT) in 7 patients. Retreatment included SRS alone in 2 and EBRT boosted with SRS in 8 patients. Magnetic resonance images were obtained every 3 to 6 months after SRS to track failure patterns. Results: At a median 22-month follow-up, the 2-year progression-free and overall survival rates were 20% and 50%, respectively. Seven patients exhibited local failures, all of which occurred outside both SRS and EBRT fields. Five local failures occurred in previously clinically uninvolved cranial nerves (CNs). CN disease spreads through 3 distinct patterns: among different branches of CN V; between CNs V and VII; and between VI and CNs III, IV, and/or VI. Five patients experienced side effects potentially attributable to radiation. Conclusions: Although there is excellent in-field control with this approach, the rate of out-of-field failures remains unacceptably high. We found that the majority of failures occurred in previously clinically uninvolved CNs often just outside treatment fields. Novel treatment strategies targeting this mode of perineural spread are needed.