Case Report: Epitheliotropic cutaneous presentation of multicentric diffuse large B-cell lymphoma in a dog.

Diffuse large B-cell lymphoma (DLBCL) is the most common histological subtype of canine multicentric lymphoma, whereas epitheliotropism is classically associated with cutaneous T-cell lymphoma. A 9-year-old neutered male Chihuahua was diagnosed with stage IVb multicentric lymphoma and achieved complete remission with a UW-25 L-CHOP protocol. However, an upper lip mass, cytologically consistent with lymphoma, developed during remission. Electrochemotherapy was performed for local control and resulted in marked reduction of the lesion. The dog later developed recurrent peripheral lymphadenopathy and multiple progressive cutaneous masses despite various chemotherapy protocols and was euthanized at week 59 because of progressive clinical deterioration. Histopathology confirmed diffuse large cell lymphoma in the submandibular lymph node, whereas the cutaneous lesions showed neoplastic lymphoid infiltration with multifocal epidermal invasion, mimicking epitheliotropic T-cell lymphoma. Immunohistochemistry (IHC) confirmed diffuse CD20 expression in both sites, with CD3 labeling limited to scattered background lymphocytes. These findings supported the interpretation of secondary cutaneous involvement of DLBCL with a highly unusual epitheliotropic pattern and emphasize the importance of IHC for accurate immunophenotypic classification of cutaneous lymphoma lesions.