Carcinosarcoma Arising in an Eccrine Spiradenoma and Presenting With Metastasis: Case Report and Literature Review.

Eccrine spiradenomas are benign sweat gland neoplasms that rarely undergo malignant transformation. Carcinosarcoma arising from an eccrine spiradenoma is exceptionally rare. A 41-year-old male presented with a rapidly growing neck/shoulder mass, progressive numbness, spasticity, and weakness. Further workup additionally revealed an epidural mass with spinal cord compression. Both masses were excised and predominantly showed morphologic features of high-grade osteosarcoma, with overtly malignant spindled cells producing lace-like osteoid. However, a single section from the upper back mass contained a roughly 2 mm focus of conventional eccrine spiradenoma, with an adjacent small focus having features of a poorly differentiated non-small cell carcinoma. The final diagnosis was that of a high-grade carcinosarcoma with heterologous osteosarcomatous differentiation, arising from a pre-existing eccrine spiradenoma, with metastasis to the T4-5 epidural region. The patient experienced rapid regrowth of the spinal mass and underwent radiotherapy but had unresectable metastatic disease at 2 months follow-up. We describe what is to our knowledge only the 21st example of carcinosarcoma arising from eccrine spiradenoma, mimicking metastatic osteosarcoma. Awareness of this very rare entity, careful sampling, close microscopic examination, and, in selected cases, ancillary immunohistochemistry are the keys to making this challenging diagnosis.