Abstract

Osteosarcoma is a rare malignant bone tumor with a worldwide incidence of 3.4 million cases per year. The majority of osteosarcoma cases occur in association with bone; however, rare cases of extraskeletal osteosarcoma have been reported in the literature. We report the case of an 88 year-old man with a 2 cm bleeding mass on the pretibial aspect of his right leg. A shave biopsy of the lesion revealed pleomorphic spindle cells arranged in fascicles. Immunohistochemistry revealed that the cells were weakly positive for SMA and negative for S-100, Melan-A, and cytokeratins, leading to a diagnosis of primary cutaneous leiomyosarcoma. Microscopic evaluation of his wide local excision revealed a dermal spindle cell neoplasm without subcutaneous involvement. Brisk mitotic figures (23/hpf) were noted. Immunostains showed that the tumor cells were negative for CD31 and desmin, and diffusely positive for CD10, findings interpreted as pleomorphic dermal sarcoma. The patient remained free of disease for 6 months until he developed a new nodule on his right leg. Excision of this lesion revealed a spindle cell lesion with foci of lacy osteoid matrix formation. The new lesion showed diffuse positivity with CD10 and SATB2, and negative for SMA, desmin, Sox10, S100, HHV8, and p40, leading to a diagnosis of extraskeletal osteosarcoma. Additional review of the previous excision revealed an equivocal focus suspicious for possible neoplastic bone formation. SATB2 showed diffuse immunoreactivity in the tumor cells indicating an osteoblastic line of differentiation; consistent with extraskeletal cutaneous osteosarcoma. We report a very rare occurrence of superficial extraskeletal osteosarcoma, highlighting the importance of including it in the differential diagnosis of cutaneous spindle cell neoplasms. This case also emphasizes the importance of interpreting CD10 positivity in the right context as it could represent a pitfall leading to the misdiagnosis of extraskeletal osteosarcoma.

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