A Rare Malignant Case of a Primary Pseudomyogenic Haemangioendothelioma of the Bone.

Pseudomyogenic haemangioendotheliomas (PMH) are exceedingly rare, mostly occurring in soft tissue, with malignant cases even more uncommon. In this report, we present a case of a 28-year-old male initially suspected of having a fibroblastic osteosarcoma of the right femur, which was then correctly diagnosed as a primary pseudomyogenic hemangioendothelioma of the bone with synchronous metastases to other skeletal segments. Molecular analysis through targeted RNA sequencing confirmed the correct diagnosis, revealing a fusion transcript ACTB::FOSB. To our knowledge, this is one of the few reported cases of suffering from multiple pathological fractures. The rapid skeletal progression and the onset of distant metastases in this case is highly unusual considering the typically indolent clinical course commonly reported in the literature for this tumor.