C80-13 Primary Mediastinal Osteosarcoma: A Rare and Aggressive Tumor in an Unusual Location

Background Mediastinal osteosarcoma ( MO) is an exceptionally rare and aggressive malignancy arising in a location devoid of osseous structures. Unlike conventional osteosarcoma, which originates in the metaphyseal regions of long bones in younger individuals, mediastinal osteosarcoma develops within soft tissues, leading to a poorly understood pathogenesis and unpredictable clinical behavior. Owing to its nonspecific symptoms and imaging overlap with other mediastinal tumors, early diagnosis is challenging. To date, around ten well-documented cases have been reported. Case Presentation A 41-year-old man with ulcerative colitis presented with progressive exertional dyspnea of three months’ duration. Laboratory tests were unremarkable except for elevated alkaline phosphatase. Computed tomography revealed a 10 × 8.8 cm calcified anterior mediastinal mass with multiple calcified pulmonary nodules and lymphadenopathy (Figure 1) . Robotic bronchoscopy of a minimally calcified right middle lobe nodule revealed a high-grade sarcomatoid malignancy forming malignant osteoid. Tumor cells expressed SATB2 (Special AT-rich sequence-binding protein 2) and were negative for epithelial markers (Pancytokeratin, TTF-1, Napsin A, and P40). The findings favored a diagnosis of high-grade extraskeletal osteosarcoma. PET/CT demonstrated distant metastases to the brain and the absence of any primary bone lesions. The case was discussed in a multidisciplinary tumor board, and combined chemotherapy with cisplatin and doxorubicin was initiated, along with radiotherapy targeting the intracranial lesion. Discussion Extraskeletal osteosarcoma (ESOS) accounts for 1-2% of soft tissue sarcomas and rarely occurs in the mediastinum, with ten mediastinal osteosarcoma cases reported in the literature. The mean age at diagnosis of MO is around the fifth decade, and most patients are men. Symptoms are nonspecific and typically reflect mass effect, such as chest pain, cough, or dyspnea. Imaging often reveals large, heterogeneously calcified mediastinal lesions. Such CT findings should raise suspicion for osteosarcoma. Diagnosis requires histologic demonstration of malignant osteoid and exclusion of other sarcomas or metastatic tumors. Immunohistochemistry typically shows SATB2 and vimentin positivity with the absence of epithelial markers. Treatment options include surgical resection when feasible, supplemented with platinum- or anthracycline-based chemotherapy and, in select cases, radiotherapy. However, due to frequent metastasis and limited resectability, prognosis remains poor, with a median survival of approximately four months. Conclusion Mediastinal osteosarcoma should be considered in the differential diagnosis of calcified mediastinal masses. Accurate diagnosis depends on adequate tissue sampling and immunohistochemical profiling. Continued reporting of such rare cases is essential to improve understanding of clinical behavior and optimize therapeutic strategies of these tumors. This abstract is funded by: None