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Abstract

A 58-year-old female presented with a growing mass on the left index finger. Excision revealed a 1.4 cm mass, grossly exhibiting a glistening, yellow-white cut surface. Microscopic examination revealed a dermal, infiltrative proliferation of uniform oval-to-round cells arranged in both retiform and nested patterns, with focal hobnailing. Cytologic atypia and increased mitotic activity were not appreciated. Immunohistochemical studies revealed diffuse, strong positivity for ERG and CD31, confirming vascular differentiation. Only patchy weak positivity was seen for S100, synaptophysin, and CD99, and the lesion lacked expression for CD34, desmin, muscle specific actin, SOX10, HMB45, P63, AE1/AE3, CK7, CEA, beta-catenin (non-nuclear), GATA3, chromogranin, CAMTA1, and TFE3. YAP1 expression was retained, and Ki-67 demonstrated low proliferative rate. The presence of both retiform and nested morphologies among cytologically bland neoplastic endothelial cells supported the diagnosis of composite hemangioendothelioma (CHE). CHE is an uncommon vascular tumor with low-to-intermediate malignant potential, featuring a mix of morphologic patterns. Given the one- to two-cell thick linear clusters coursing through the stroma and around adnexal structures, mimicking the infiltrative/morpheaform pattern of basal cell carcinomas, this entity may be mistaken for an aggressive malignancy. The monomorphic cells, lack of cytologic atypia, and low mitotic/proliferative activity should instead hint toward a more banal diagnosis. This case aims to raise awareness of CHE as a potential diagnosis among acral vascular lesions. Although metastasis is rare, CHE can be locally aggressive, and incomplete excision carries a risk of local recurrence. Ongoing surveillance is essential to guide timely intervention and prevent progression.