Calcified Chondroid Mesenchymal Neoplasm with an FN1-FGFR2 Translocation, Intralesional Fat, Diffuse CD34 Positivity, and a Lack of Giant Cells: A Case Report with Uncommon Findings

Summary: Calcified chondroid mesenchymal neoplasm (CCMN) is a term for an emerging group of soft tissue tumors that display cartilaginous matrix formation and harbor translocations involving the FN1 gene. These tumors are rare and behave indolently but can be mistaken for malignant neoplasms. Current literature indicates that these tumors frequently contain giant cells, and infrequently contain intralesional fat or stain for CD34. Henceforth, we present a case of CCMN with uncommon findings causing a soft tissue finger mass in a 74-year-old male with some erosion into the underlying bone. Microscopically, the mass was composed of a multinodular chondroid neoplasm with areas of abundant coarse calcifications. Tumor cells were polygonal to stellate with eosinophilic cytoplasm, and interspersed adipocytes were present within the lesion. Giant cells were absent. Immunohistochemically, the tumor was diffusely positive for CD34, as well as ERG and S100. Next-generation sequencing confirmed the presence of an FN1-FGFR2 translocation and the diagnosis of CCMN. While this case showed many features that have been previously described in the literature, this case also showed some aspects that are uncommon based on the literature and could add to diagnostic difficulty. These include CD34 staining, which in conjunction with ERG staining could raise the possibility of epithelioid hemangioendothelioma, and interspersed adipocytes, which could heighten concern for an infiltrating malignant neoplasm like chondrosarcoma. The findings in this presented case help continue to expand the knowledge of an emerging subcutaneous pathologic entity, and highlight some uncommon features that should not dissuade one from making a diagnosis of CCMN.