T-cell lymphoma with significant intramuscular involvement with Mum-1 expression in a dog

Primary intramuscular lymphoma is rare in both humans and veterinary species. The limited literature shows that these tumors are often multifocal, with clinical signs often including limb edema and swelling and lameness. Signs are typically acute and rapidly progressive. A 10-year-old castrated male mixed breed dog with multifocal intramuscular masses was diagnosed as T-cell lymphoma with aberrant Mum-1 expression. He initially presented due to rapidly progressive right hindlimb edema. Achieving a definitive diagnosis was challenging and a combination of contrast computed tomography (CT), cytology, histopathology, immunohistochemistry (IHC), and PARR was necessary. Multifocal iso-attenuating, contrast-enhancing masses were identified diffusely throughout the musculature. Histopathology identified neoplastic lymphocytes that were immunohistochemically CD3-, CD20-, Pax5-, vimentin +, and Mum-1+. PARR identified clonal T-cell receptor gene arrangements. The dog had an initial rapid response to doxorubicin and L-asparaginase (L-SPAR), but progressive disease was noted at day 45, 11 days after the third dose of doxorubicin was administered. He was then treated with various rescue chemotherapy protocols and focal radiation therapy. Despite these aggressive therapies, his disease progressed and humane euthanasia was pursued 63 days after initial presentation due to poor quality of life. A necropsy was not performed. Primary intramuscular lymphoma or lymphoma with intramuscular involvement appears to be an aggressive subtype of lymphoma in dogs. Diagnosis can be difficult in these cases, and a multi-modal approach is often necessary. While these patients can respond well to chemotherapy and radiation therapy, the response is often short-lived, and prognosis remains guarded for these dogs.