Beyond the Usual Suspects: Recognizing Calcified Chondroid Mesenchymal Neoplasm in Hand Soft Tissue Tumors

A 69-year-old man presented for evaluation of a slow-growing, uncomfortable mass on his left index finger. The mass was clinically concerning for a tenosynovial giant cell tumor and was excised after magnetic resonance imaging showed a well-circumscribed, enhancing subcutaneous mass next to the flexor tendons of the left index finger. Pathological analysis of the excised mass revealed a multinodular, calcified chondroid mesenchymal neoplasm (CCMN), which is a rare soft tissue tumor that was recently described and is characterized by chondroid matrix production and recurrent FN1-receptor tyrosine kinase gene fusions. Molecular testing confirmed an FN1::FGFR1 fusion, supporting the diagnosis of CCMN. CCMN is an important entity for hand surgeons and pathologists to consider because it may present in the distal extremities, especially the fingers, and its behavior is indolent when compared with other chondroid soft tissue tumors that may necessitate more aggressive treatment. This case highlights the importance of integrating clinical, radiological, histopathologic, and molecular data to accurately diagnose CCMN and avoid misclassification as more aggressive neoplasms.