Progression of primary mediastinal T-cell lymphoma to a multicentric form in a young dog.

Canine lymphoma is the most common hematopoietic malignancy, but the primary mediastinal form is rare. Progression from this form to systemic multicentric lymphoma has not been clearly documented in veterinary medicine, and optimal treatment strategies remain uncertain. This report describes a case of primary mediastinal T-cell lymphoma in a young dog that progressed to multicentric disease and was managed with chemotherapy and radiation therapy. A 2-year-old castrated male Shetland sheepdog was referred for evaluation of a cranial mediastinal mass detected on thoracic radiographs. On physical examination, bradycardia was noted, with all peripheral lymph nodes within normal limits. Laboratory abnormalities included severe hypercalcemia, elevated symmetric dimethylarginine, and the presence of large lymphocytes on blood smear. Cytology, polymerase chain reaction for antigen receptor rearrangements, and flow cytometry confirmed CD4+ T-cell mediastinal lymphoma. Initial treatment with the 25-week L-CHOP protocol achieved complete remission, but relapse occurred at Week 8, prompting radiation therapy to the mediastinal and submandibular masses. These lesions regressed but generalized peripheral lymphadenomegaly and a splenic honeycomb pattern developed, indicating progression to multicentric lymphoma. Based on ex vivo drug sensitivity testing, lomustine was initiated as rescue chemotherapy, achieving a second complete remission. Nevertheless, relapse occurred 38 d after the initial lomustine administration, and the dog ultimately died. Key clinical message: This case highlights the fact that progression from primary mediastinal to multicentric lymphoma may be associated with a poor prognosis in dogs. Radiation therapy demonstrated potential efficacy and warrants further investigation as a treatment option for canine mediastinal lymphoma.