Aggressive Mesenchymal Chondrosarcoma of the Rib With Early Skeletal Metastases: A Diagnostic Challenge.

Mesenchymal chondrosarcoma (MCS) is a rare, aggressive subtype of chondrosarcoma characterized by biphasic histology, often misdiagnosed due to its rarity and histological resemblance to other small round cell tumors. It predominantly affects adolescents and young adults. We report a 27-year-old male presenting with a progressively enlarging, painless mass in the right inframammary region, initially attributed to muscular strain. Over one month, the mass rapidly increased in size, accompanied by mild pain exacerbated by movement. Physical examination revealed a firm, non-tender mass approximately 10 x 5 cm in diameter between the sixth and seventh ribs. Contrast-enhanced CT revealed a large expansile lytic lesion of the right sixth rib (10 × 5.5 cm) with cortical destruction, pathological fracture and soft tissue invasion into surrounding musculature. Core needle biopsy revealed a biphasic tumor comprising undifferentiated mesenchymal cells and islands of well-differentiated hyaline cartilage. Immunohistochemical staining showed positivity for vimentin, CD99 and S100 protein with a high Ki-67 proliferation index (30-40%). These findings were consistent with MSC. 18-Fluorodeoxyglucose-positron emission tomography (FDG-PET) scan detected widespread skeletal metastases involving the spine, humerus, scapula, ribs, pelvis and femur. A multidisciplinary team initiated neoadjuvant anthracycline-based chemotherapy to reduce tumor burden and manage systemic disease, with plans for potential surgical intervention upon reassessment. Given the typically poor prognosis associated with MCS, it should be considered in the differential diagnosis of bone tumors in young adults presenting with atypical musculoskeletal masses. Prompt diagnostic workup, including advanced imaging and comprehensive histopathological and immunohistochemical analysis, is essential for accurate diagnosis. A multidisciplinary approach to management is imperative to address the aggressive nature of MCS. Early detection and intervention remain key factors in enhancing survival rates and quality of life for patients afflicted with this formidable malignancy.