Primary Mediastinal Lymphoma in Dogs

Approximately 22 to 36% of all canine lymphoma cases are reported to have some degree of mediastinal involvement. Primary mediastinal lymphoma, however, is not well documented in veterinary literature.

Researchers at the University of California, Davis recently performed a retrospective study to examine factors related to primary mediastinal lymphoma in dogs, including patient signalment, clinical presentation, prognostic factors, and treatment outcome.

Analysis

Veterinary records from the UC Davis Veterinary Medical Teaching Hospital were retrospectively identified. Inclusion criteria included a definitive diagnosis of mediastinal lymphoma via microscopic examination of mediastinal mass or pleural effusion samples.

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Cases were assigned with a T- or B-cell phenotype and staged using thoracic radiography, complete blood count, and a chemistry panel. When available, tumor samples were characterized morphologically. Treatment response, which was assessed using thoracic radiography, was categorized as complete or partial—indicated by a 30% or greater decrease in target lesion diameter.

Results

Twenty-five male and 17 female dogs representing 20 breeds met the inclusion criteria for the study. The mean age of participants was 7.8 years and mean weight was 33 kg. A diagnosis of primary mediastinal lymphoma was confirmed via cytology (37 dogs) or histopathology (5 dogs) using needle‐core biopsies collected with ultrasound guidance. Eighty-five percent of all cases were categorized as substage b based on the presence of significant clinical signs at diagnosis.

Twenty-eight cases were classified as T-cell phenotype and 1 case was B-cell phenotype; lineage was either not evaluated or not determinable for the remaining 13 cases. Tumor samples, which were available for 32 dogs, were classified as lymphoblastic (18), large cell (13), or large, granular lymphocyte type (1) lymphoma.

Clinical signs recorded during initial workup included:

• lethargy (71%)
• anorexia (59%)
• polyuria/polydipsia (42%)
• coughing (34%)
• dyspnea or tachypnea (24%)
• vomiting (24%)

Common blood chemistry abnormalities included hypercalcemia (68%), which was most prevelant in dogs with lymphoblastic lymphoma and azotemia (33%). Mean total and ionized calcium values in hypercalcemic patients were 15.45 mg/dL and 1.9 mm/L, respectively. The most common abnormal finding on radiography was pleural effusion (45%), while 19% of abdominal ultrasound examinations revealed lymphoma had spread to the spleen, liver, or intra-abdominal lymph nodes.

Chemotherapy protocol records were available for 36 dogs. Twenty-seven dogs were treated with the CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone) protocol, which resulted in 69% complete response, 27% partial response, and 4% progressive disease. Restaging diagnostics were not performed consistently enough to gauge response rates to other therapies.

Pleural effusion was the only examined factor that correlated with decreased median overall survival (70 vs. 183 days), while the use of CHOP chemotherapy increased median overall survival from 67 days to 194 days.

Take-home Message

The study concluded that primary mediastinal lymphoma in dogs is most commonly a variant of high-grade, T-cell lymphoma. While prognosis is generally poor, the authors found that the use of multi-agent CHOP chemotherapy significantly increased overall survival in canine patients.

Dr. Stilwell received her DVM from Auburn University, followed by a MS in Fisheries and Aquatic Sciences and a PhD in Veterinary Medical Sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.