Peri-Implant Gingival Undifferentiated SWI/SNF Complex-Deficient Tumor with Molecularly Confirmed Biallelic SMARCA4 Inactivation: Diagnostic Pitfalls and Genomic Characterization.

Background and Clinical Significance: SWI/SNF chromatin remodeling complex-deficient malignancies constitute an aggressive group of undifferentiated tumors defined by inactivation of core subunits including SMARCA4 (BRG1) or SMARCB1 (INI1). In the head and neck, these tumors predominate in the sinonasal tract; oral cavity presentations are exceedingly rare, with reported cases predominantly representing metastatic disease. Peri-implant gingival masses in clinical practice are overwhelmingly reactive, but their occasional malignant nature mandates timely biopsy and thorough pathologic workup. We report the first comprehensively molecularly characterized case of a peri-implant gingival SWI/SNF complex-deficient tumor with confirmed biallelic SMARCA4 inactivation. Case Presentation: A 75-year-old man presented with a one-week history of a rapidly enlarging exophytic erythematous peri-implant gingival mass in the right posterior mandible (region 44-47). Incisional biopsy demonstrated an undifferentiated high-grade tumor with epithelioid, plasmablastoid, and focally rhabdoid morphology with necrosis. Immunohistochemistry showed complete loss of BRG1 (SMARCA4) with retained INI1 (SMARCB1), EMA positivity, Ki-67 of approximately 100%, and negativity across all lineage-specific markers (hematolymphoid, epithelial, melanocytic, endothelial, squamous). Comprehensive next-generation sequencing (Oncomine Comprehensive Assay Plus) confirmed biallelic SMARCA4 inactivation via a truncating nonsense mutation (p.Trp1346Ter; VAF 73.85%) combined with copy number loss, establishing the molecular mechanism underlying BRG1 protein loss. Co-occurring alterations included homozygous CDKN2A/CDKN2B deletion, MTAP loss (9p21.3), clonal TP53 and KEAP1 mutations, and intermediate-high tumor mutational burden (13.3 mutations/Mb) with microsatellite stability. The patient initiated carboplatin-paclitaxel and achieved a partial response at one month with further shrinkage by four months. This case illustrates a rare oral cavity manifestation of SWI/SNF complex deficiency arising in a peri-implant location, with a diagnostic workup that required integration of immunohistochemistry and molecular profiling for definitive characterization. The MTAP deletion co-occurring with homozygous CDKN2A/B loss identifies a potentially actionable synthetic lethal vulnerability to MAT2A and PRMT5 inhibitors currently under clinical investigation. An occult primary site could not be fully excluded due to absence of a dedicated staging workup. Conclusions: Rapidly enlarging peri-implant gingival masses should prompt timely biopsy and SWI/SNF marker testing when histology is high-grade and lineage-ambiguous. NGS-based molecular profiling confirms diagnosis, elucidates mechanism, and reveals actionable targets in this rare tumor class.