Cutaneous epitheliotropic lymphoma with marked nuclear pleomorphism in a dog.

importanceCutaneous epitheliotropic lymphoma (CEL) is a rare neoplastic disease in dogs, for which cases of atypical immunophenotypes or anaplastic morphological features have rarely been documented. Herein, we describe a rare case of CEL exhibiting marked cellular pleomorphism, complicating the cytological diagnoses, which included amelanotic melanoma. CASE PRESENTATION: A 12-year-old spayed female Golden Retriever presented with a rapidly growing oral mass, with an additional cutaneous lesion on the dorsum. Hematologic and biochemical analyses were unremarkable. Computed tomography revealed heterogeneously enhanced lymph nodes in the head and thorax. Fine needle aspirates from both lesions revealed individualized round cells, some of which exhibited multinucleation and giant cell morphology. Cytologic findings were suggestive of malignant round cell tumors, including lymphoma and amelanotic melanoma. Histopathologic examination confirmed the diagnosis of CEL. Flow cytometry revealed a CD45⁺CD21⁻CD3⁻CD5⁺CD4⁻CD8⁻CD25⁻ immunophenotype. Polymerase chain reaction for antigen receptor rearrangement revealed clonality in the T-cell receptor γδ region. The dog did not respond to lomustine and eventually showed rapid clinical deterioration shortly before euthanasia. CONCLUSIONS AND RELEVANCE: Here, we report atypical clinicopathological features observed in the case of CEL. These findings have potential biological, diagnostic, and prognostic relevance, emphasizing the need for further studies to elucidate their clinical significance and to expand the understanding of this disease.