SMARCA4-Deficient Undifferentiated Tumor Presenting as a Tonsillar Mass With Widespread Metastases: A Rare Head and Neck Manifestation and Complete Response to Chemoimmunotherapy.

SWI/SNF-related, matrix-associated, actin-dependent regulator of chromatin, subfamily A (SMARCA4)-deficient undifferentiated tumors (SMARCA4-UTs) are rare, aggressive malignancies typically arising in the thoracic cavity. We present a case of a 73-year-old male who initially presented with symptoms consistent with acute tonsillitis but rapidly developed odynophagia, dysphagia, cervical lymphadenopathy, and profound weight loss. Imaging revealed a left tonsillar mass with extensive metastases to lymph nodes, lungs, bones, and mediastinum. A biopsy of the tonsillar mass confirmed a SMARCA4-deficient undifferentiated tumor. The patient underwent systemic treatment with carboplatin, paclitaxel, and pembrolizumab, complicated by refeeding syndrome, perforated appendicitis, and pulmonary embolism. Despite this, he achieved complete remission after six cycles and continues on maintenance pembrolizumab. This case illustrates the potential for durable response with chemoimmunotherapy in this rare and aggressive malignancy, even when presenting in atypical anatomic locations.