Mediastinal Lymphoma in 70 Dogs Treated With Lomustine or Anthracycline‐Based Multi‐Agent Chemotherapy

Primary mediastinal lymphoma is rare in dogs and literature exploring this disease is limited. Therefore, the aim of this study was to describe presentation, treatment and outcome in a large cohort of dogs with mediastinal lymphoma and explore prognostic factors including chemotherapy protocol. This retrospective multi-institute study included 70 dogs with primary mediastinal lymphoma treated with lomustine-based (LOP/LOPP) or anthracycline-based (CHOP/CEOP) chemotherapy. Most immunophenotyped cases were of T-cell lineage (95.6%). The majority were substage b (90%) and hypercalcaemia was noted in 69.1% of dogs. Clinical and objective response rates to chemotherapy were 92.7% and 97.9%, respectively, with 76.6% of dogs achieving a complete response. Median progression free survival (PFS) was 132 days (95% CI 83-181), and median overall survival time (OST) was 223 days (95% CI 175-271). The 6-month, 1-year, and 2-year survival rates were 55.7%, 22.9%, and 15.7%, respectively. On multivariable analysis, factors associated with longer PFS included hypercalcaemia (p = 0.041), chemotherapy-induced neutropenia (p = 0.014) and CD4+/CD8- immunophenotype (p = 0.004). Neutropenia at diagnosis was associated with shorter PFS (p = 0.015) and OST (p = 0.004). Other factors associated with shorter OST included granular morphology (p = 0.023) and CD4+/CD8+ immunophenotype (p = 0.004). Chemotherapy-induced neutropenia was associated with improved OST (p = 0.042). Differences in outcome between anthracycline- or lomustine-based chemotherapy protocols were not statistically significant. Overall, the prognosis for primary mediastinal lymphoma in dogs is poor to fair when treated with multi-agent chemotherapy. This is the second study associating hypercalcaemia with improved PFS in dogs with non-indolent T-cell lymphoma. Results also suggest prognostic significance of specific CD4/CD8 expression patterns.