An epithelioid hemangioendothelioma with a novel RREB1::TFE3 gene fusion.

Epithelioid hemangioendothelioma (EHE) is a rare vascular tumor of intermediate malignant potential. Most cases are characterized by a WWTR1::CAMTA1 gene fusion, with YAP1::TFE3 being the second most common. We present a case of EHE in a 78-year-old woman with an RREB1::TFE3 gene fusion, which, to our knowledge, has not previously been described. The tumor was detected incidentally as a subpectoral soft tissue mass during imaging for a simultaneously diagnosed metastatic pulmonary adenocarcinoma. Aside from this, the patient's presentation was consistent with common clinical findings in EHE. Histologically, however, the tumor partly revealed an uncommon morphology: on one hand, solid areas of monomorphic epithelioid tumor cells with abundant eosinophilic cytoplasm, and on the other, regions with considerable and unusual pleomorphism and multinucleation, thus not clearly aligning with the prototypical phenotypes of the classical molecular subtypes. In summary, this case expands the molecular spectrum of EHE and supports further investigation into its molecular heterogeneity.