Transient Leukemia in an Adult Dog With Cutaneous T-Cell Lymphoma

A 5-year-old neutered female Beagle was presented with lethargy, vomiting, and reduced appetite. Physical examination revealed mild hyperthermia and a cutaneous thickness on the right thigh. Hematology showed marked leukocytosis and moderate thrombocytopenia, consisting of 80% of atypical circulating cells, initially suggesting acute undifferentiated leukemia. Two weeks later, repeated hematology revealed unremarkable results. Cytology of the skin lesion showed a round cell neoplasia with features similar to the atypical circulating cells. Following the development of multiple cutaneous nodules and recurrence of clinical signs, further diagnostics, including flow cytometry, histopathology, and immunohistochemistry (IHC) of the skin nodules, as well as PCR for antigen receptor rearrangement (PARR) and immunocytochemistry (ICC) from the initial blood smear, confirmed a neoplastic T-cell proliferation consistent with cutaneous T-cell lymphoma with a probable transient leukemic phase. Despite chemotherapy, remission was short-lived, and the patient relapsed, ultimately leading to euthanasia. This case highlights a rare instance of transient leukemia likely originating from a primary cutaneous T-cell lymphoma, emphasizing the need for comprehensive diagnostic workups, combining hematology, biochemistry, cytology, flow cytometry, and immunophenotyping to avoid misclassification in hematologic malignancies.