We report the case of a recurrent tumour with histological features of an osteosarcoma, and discuss the differential diagnosis. An 80 year old woman presented with a fungating tumour on the right calf, arising from an area of previous split skin grafting. There was also a new 1–2 cm diameter nodule at the medial ankle of the right leg. The patient had previously had a lesion removed from the same site, reported as a squamous cell carcinoma. The recurrent calf lesion was excised, as well as the skin nodule on the medial ankle of the right leg. Histological examination of both specimens showed similar features, with a malignant neoplasm having features of an osteosarcoma, with osteoid formation. No carcinomatous component was seen in the sections examined. Immunohistochemical studies for cytokeratin, S100, HMB45 and Melan‐A were negative in the malignant cells. Tumour giant cells were CD68 positive. The differential diagnoses included recurrent squamous cell carcinoma presenting as a carcinosarcoma with sarcomatous overgrowth, primary cutaneous osteosarcoma, metastatic osteosarcoma, malignant mixed dermal tumour, malignant peripheral nerve sheath tumour with an osteosarcomatous component and undifferentiated pleomorphic sarcoma with osteosarcoma‐like areas. Primary cutaneous carcinosarcomas, primary cutaneous osteosarcomas and metastatic osteosarcoma to the skin are all rare. Squamous cell carcinomas and basal cell carcinomas are the most common epithelial component of cutaneous carcinosarcomas, with osteosarcomas and undifferentiated pleomorphic sarcomas (malignant fibrous histiocytomas) the most commonly reported sarcomatous component. Extraskeletal osteosarcomas are also uncommon, with the skin a rarely reported primary site. Predisposing factors for extraskeletal osteosarcomas include past trauma, pre‐existing soft tissue calcification, ossification, myositis ossificans and intramuscular injection sites. This is an unusual presentation of what would appear to be a recurrent tumour, previously diagnosed as squamous cell carcinoma. The histological appearance raises a number of possibilities, requiring clinicopathological correlation.

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   |texte=   Recurrent cutaneous tumour with features of an osteosarcoma: Primary osteosarcoma or cutaneous carcinosarcoma?
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