Novel Pediatric Presentation of Pseudomyogenic Hemangioendothelioma: Diagnostic Pitfalls and Considerations of an Atypical Case.

Pseudomyogenic hemangioendothelioma (PMH) is a rare, often multicentric neoplasm occurring primarily in soft tissue. Fewer than 50 cases of isolated bone lesions are described, with nearly all restricted to bone(s) of 1 region of the body. Two cases are described with diffuse bone involvement without soft tissue lesions, which has never been described in pediatric patients. We describe a unique case of PMH presenting as diffuse, polyostotic bone lesions in a 16-year-old. Neoplastic cells showed very weak, patchy staining for essential diagnostic stains, highlighting an important diagnostic pitfall. Additionally, the tumor showed uncharacteristic prominent histiocytic infiltrate without the usual associated neutrophils. Molecular sequencing detected ACTB::FOSB fusion transcript, confirming the diagnosis. This is the first described cases of diffuse, polyostotic PMH in a pediatric patient without soft tissue involvement. Furthermore, this is the first described case of histocyte-rich PMH. Overall, the case illustrates several unique features and diagnostic pitfalls important for pediatric pathologists to recognize in PMH. A high index of suspicion is necessary to perform the appropriate targeted molecular testing that might be needed to confirm the diagnosis.