In soft tissue pathology, MUC4 is considered a sensitive and specific immunohistochemical marker for low-grade fibromyxoid sarcoma (LGFMS) and sclerosing epithelioid fibrosarcoma (SEF), which are characterized by FUS/EWSR1 :: CREB3L2/1 fusions. Recently, MUC4-positive fibroblastoma has been proposed as a novel entity, and we herein describe 7 cases that align with this disease concept. These tumors occurred in 7 female patients aged 14 to 60 years, and they were located in the neck (2 cases), temple, arm, chest wall, pharynx, and thigh, with 5 being deep-seated. All tumors were surgically removed. No patients experienced recurrence during follow-up periods of 2 to 113 months. The well-circumscribed tumors comprised hypocellular fibrous tissue, populated by nonatypical spindle cells. Myxoid stroma was absent. Common features included thin-walled patent vessels, extremely long vessels, and mast cells. In some cases, fat entrapment/overgrowth was conspicuous. All tumors tested positive for MUC4 and nuclear β-catenin expression. The tumors were molecularly investigated with fluorescence in situ hybridization, RNA sequencing, DNA panel sequencing, DNA methylation analysis, and/or nanopore sequencing. Genetic analysis showed the absence of FUS/EWSR1 fusions in all 7 cases. All 5 tested tumors harbored APC alterations, with 3 having inactivating mutations and 2 showing copy number loss. DNA methylation profiles of 2 tumors did not match those of any references, including LGFMS or SEF, as indicated by t-SNE. Overall, our study supports the recent proposal of MUC4-positive fibroblastoma as a distinct entity and further delineates its phenotypic and molecular characteristics. These tumors should be distinguished from LGFMS, SEF, desmoid fibromatosis, and other fibrous or fibroadipose tumors.
Yoshida et al. (Mon,) studied this question.