Bidirectional trigeminal perineural spread of adenoid cystic carcinoma via the pterygopalatine fossa.

Adenoid cystic carcinoma (ACC) is characterized by a strong propensity for perineural invasion (PNI), which complicates skull base evaluation and treatment planning. We report a case of buccal mucosal ACC demonstrating simultaneous proximal extension to the cavernous sinus and distal orbital involvement following infiltration of the pterygopalatine fossa (PPF). Contrast-enhanced magnetic resonance imaging (MRI) demonstrated continuous tumor propagation from the posterior superior alveolar nerve to the maxillary nerve (V2), with bidirectional spread originating from the PPF. Positron emission tomography-computed tomography (PET-CT) confirmed metabolically active disease along both proximal and distal neural segments. This PPF-centered dissemination pattern has important implications for staging accuracy, radiotherapy target delineation, and surveillance strategies in neurotropic head and neck malignancies.