Primary Kaposiform hemangioendothelioma of the tibia: report of a case with a previously undescribed GNAQ mutation.

Primary occurrence of Kaposiform hemangioendothelioma (KHE) in bone is extremely rare. We report the radiologic, histopathologic and molecular features of a case of KHE affecting the tibia of an adult male, who presented with pain in the left knee initially associated with previous trauma. Radiographs showed an eccentric well-defined lytic lesion of the tibial epiphysis. On MRI, the lesion showed hypo to isointensity on T1-weighted images and hyperintensity on STIR sequences. A CT-guided biopsy was pathologically inconclusive. MRI performed 11 months later showed lesion progression with cortical erosion and soft tissue extension. Extended curettage was performed and histologically the lesion consisted of irregular nodules permeating bone trabeculae, composed of glomeruloid structures with densely packed capillary-sized vessels lined by flat endothelial cells without nuclear atypia. A second component of bland spindle cells arranged in fascicles with slit-like spaces resembling Kaposi sarcoma was present, together with intermixed dilated lymphatic channels and multiple nodular lymphoid aggregates. Neoplastic cells were positive for CD31 and CD34, whereas podoplanin was positive in the lymphatic component. Using digital droplet PCR, we identified a previously unreported GNAQ T96S mutation, whereas other GNAQ mutations were not detected. Despite its rarity, recognition of intraosseous KHE is crucial because it allows for prompt curative surgical treatment and avoids potential complications.