Perineural invasion (PNI) occurs in 2% to 6% of cutaneous head and neck basal and squamous cell carcinomas (SCCs) and is associated with mid-face location, recurrent tumors, high histologic grade, and increasing tumor size. Patients may be asymptomatic with PNI appreciated on pathologic examination of the surgical specimen (microscopic), or may present with cranial nerve (CN) deficits (clinical). The V and VII CNs are most commonly involved. Magnetic resonance imaging (MRI) may be obtained to detect and define the extent of PNI; computed tomography (CT) or ultrasound-guided fine needle aspiration cytology (UGFNAC) may assist with detecting or excluding regional lymph node metastases. Patients with apparently resectable cancers undergo surgery, usually followed by postoperative radiotherapy (RT). Patients with unresectable cancers are treated with definitive RT. Moreover, RT may be considered if significant functional or cosmetic impairment is expected after surgical treatment. The 5-year outcomes after treatment for clinically unsuspected microscopic compared with clinical PNI are: local control, 80% and 55%; cause-specific survival, 75% and 65%; and overall survival, 55% and 50%, respectively. The incidence of grade ≥ 3 complications is higher after treatment for clinical PNI versus microscopic PNI; approximately 35% compared with 15%, respectively. Proton beam RT may be used to reduce the risk of late complications by reducing RT dose to the visual apparatus and central nervous system (CNS).
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