Increase in minimal residual disease in peripheral blood before clinical relapse in dogs with lymphoma that achieved complete remission after chemotherapy

Background: We developed previously a minimal residual disease (MRD) monitoring system in dogs with lymphoma by exploring a highly sensitive real‐time PCR system. Objectives: To identify the change in MRD before clinical relapse in dogs with lymphoma that achieved complete remission after chemotherapy. Animals: Twenty dogs with multicentric high‐grade B‐cell lymphoma. Methods: MRD levels in peripheral blood mononuclear cells (PBMCs) were measured by real‐time PCR amplifying the rearranged immunoglobulin heavy chain gene. MRD measurement and clinical assessment were performed every 2–4 weeks for 28–601 days after completion of chemotherapy. An increase in MRD was defined as an increase by more than 0.5, calculated by log₁₀copy number of MRD per 10⁵ PBMCs], based on the uncertainty level observed in a canine lymphoma cell line. Results: During the follow‐up period, 15 dogs relapsed in 28–320 days (median, 120 days) after completion of chemotherapy. An increase in MRD was detected 2 weeks or more before relapse in 14 of the 15 dogs, but an increase in MRD before relapse could not be detected in the remaining 1 dog. The time from increased MRD to clinical relapse was 0–63 days (median, 42 days). In contrast, no increase in MRD was detected in 5 dogs that did not experience clinical relapse. Conclusion and Clinical Importance: An increase in MRD can be detected before clinical relapse in dogs with lymphoma. Application of early reinduction therapy based on an increase in MRD before clinical relapse may improve treatment outcome in canine lymphoma.