Primary Mediastinal Osteosarcoma: A Rare and Aggressive Tumour in an Unusual Location.

Introduction: Mediastinal osteosarcoma 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 tissue, leading to a poorly understood pathogenesis and unpredictable clinical behaviour. Owing to its nonspecific symptoms and imaging overlap with other mediastinal tumours, early diagnosis is challenging. To date, around ten well-documented cases have been reported. Case description: A 41-year-old man presented with progressive dyspnoea of 3 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. Robotic bronchoscopy of a minimally calcified right middle lobe nodule revealed a high-grade sarcomatoid malignancy forming malignant osteoid. Tumour 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 favoured a diagnosis of extraskeletal osteosarcoma. Positron emission tomography/computed tomography scan demonstrated distant metastases to the brain and the absence of any primary bone lesions. The case was discussed in a multidisciplinary tumour board, and combined chemotherapy with Cisplatin and Doxorubicin was initiated, along with radiotherapy targeting the intracranial lesion, followed by Regorafenib, a kinase inhibitor. 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 behaviour and optimize therapeutic strategies for these tumours. LEARNING POINTS: Primary mediastinal osteosarcoma is a rare disease.Calcifications within a mediastinal tumour should raise suspicion of osteosarcoma.Due to the rarity of mediastinal osteosarcoma, there is no consensus on how to treat this tumour.