APC-Mutated MUC4-Positive Fibroblastoma With Cytoplasmic β-Catenin: A Novel Variant Expanding Its Immunophenotypic Spectrum

MUC4-positive fibroblastoma is a newly characterized fibroblastic neoplasm, typically presenting as a bland spindle cell tumor with diffuse cytoplasmic MUC4 expression, nuclear β‑catenin positivity, and APC gene alterations. We report a novel case in a 34-year-old woman demonstrating diffuse cytoplasmic MUC4 expression but negative cytoplasmic β‑catenin staining, with molecular confirmation of biallelic APC inactivation. This finding broadens the known immunophenotypic spectrum of MUC4-positive fibroblastoma and challenges current assumptions regarding β‑catenin localization in APC-altered soft tissue tumors. Additional clinical, radiologic, and long-term follow-up data may help refine our understanding of the biological behavior and diagnostic boundaries of this emerging entity.