Pediatric SMARCB1 (INI1)–Deficient Parapharyngeal Myoepithelial Carcinoma: A Stair-Step Mandibulotomy Approach and Case Report

Background: SMARCB1 (INI-1)–deficient myoepithelial carcinoma of soft tissue is an exceptionally rare and aggressive pediatric neoplasm. Although epithelial in origin, these tumors display sarcoma-like behavior and often arise in the head and neck, where complete surgical excision can be technically challenging. Here, we present a complex case where multidisciplinary management was necessary for a complete tumor resection utilizing a novel mandibulotomy approach to help achieve an R0 tumor resection. Case Description: A 3-year-old female presented with progressive left neck swelling, with imaging demonstrating a 5.6 cm retropharyngeal mass displacing the carotid space. An open biopsy revealed a SMARCB1 (INI-1)–deficient myoepithelial carcinoma, confirmed through molecular testing and secondary review at an outside research hospital. PET-CT showed no evidence of metastatic disease. The patient underwent 6 cycles of modified CHOP-based chemotherapy (vincristine, cyclophosphamide, doxorubicin) with a near-complete metabolic response. Definitive surgical management included radical tumor resection requiring a stair-step mandibulotomy with selective neck dissection and carotid artery resection following preoperative embolization. Tracheostomy and gastrostomy were performed concurrently. Facial, vagus, and hypoglossal nerves were preserved. Postoperatively, the patient had a left-sided stroke requiring physiotherapy and speech-language therapy, but now has almost complete recovery. Adjuvant proton beam radiotherapy was added to the current chemotherapy. The patient continues with follow-up, demonstrating good outcomes and tolerating the treatment. Conclusion: This case illustrates the diagnostic and surgical complexities of SMARCB1-deficient myoepithelial carcinoma in the pediatric parapharyngeal space. Early multidisciplinary collaboration and tailored multimodal therapy are critical to optimizing outcomes while preserving neurovascular function.