Case report highlights modified melphalan approach in canine EMP

A case report published July 20, 2026, in Frontiers in Veterinary Science details an uncommon and clinically important turn in a dog with colorectal extramedullary plasmacytoma: after complete surgical excision of a solitary rectal mass, the patient developed multifocal hepatic plasma cell tumors five months later and was then managed with a modified pulse-dose melphalan protocol plus prednisone. Despite recurrent hematologic toxicity, the dog achieved imaging-based regression of liver lesions and survived more than 19 months after hepatic disease was recognized. (frontiersin.org)

That matters because canine extramedullary plasmacytomas are often framed as low-risk tumors, especially in cutaneous and oral sites. In the new report’s introduction, the author notes that extramedullary plasmacytoma accounts for about 2.4% of all canine tumors, with most cases arising in the skin, followed by the oral cavity, and only about 4% in the rectum or colon. The paper also points to published recurrence rates ranging from 1.7% to 31.5%, and nodal or distant metastasis reported in 0.8% to 10% of cases, suggesting that at least some non-cutaneous tumors may be more biologically aggressive than many clinicians expect. (frontiersin.org)

In this case, the dog’s rectal mass was excised with clean margins, but liver lesions were identified months later. The report says there was no evidence meeting standard criteria for multiple myeloma-related disease: skeletal radiographs did not show osteolytic lesions, calcium remained normal, and serum and urine protein concentrations were within normal limits, although bone marrow aspiration and protein electrophoresis were not completed. That leaves some diagnostic uncertainty, which the author acknowledges, but the clinical picture supported suspected disseminated extramedullary plasmacytoma rather than classic multiple myeloma. (frontiersin.org)

Treatment began with prednisone at 0.72 mg/kg once daily and melphalan at 7 mg/m² once daily for five consecutive days in a 21-day cycle. Fourteen days later, the dog developed severe neutropenia and thrombocytopenia, and melphalan was stopped while supportive outpatient care continued. After counts recovered, the clinician restarted melphalan on the same 21-day cycle but shortened dosing to four consecutive days. That version was reportedly tolerated for 15 cycles, with serial abdominal ultrasonography every three months showing progressive reduction in the size and number of hepatic lesions. Later, recurrent pancytopenia prompted another interruption, and melphalan was eventually reintroduced at three treatment days per cycle. The paper reports that stopping melphalan was followed by clinical deterioration within about four weeks, while restarting at the lower-intensity schedule again produced improvement, supporting continued drug responsiveness. (frontiersin.org)

The author presents the modified schedule less as proof of efficacy than as a practical dosing strategy for a fragile patient. The paper explicitly states that, as a single case report, the findings are preliminary and do not establish efficacy. Still, the report argues that stepwise shortening of melphalan exposure within a fixed cycle may offer a workable way to preserve treatment benefit while allowing bone marrow recovery in dogs with disseminated or aggressive extramedullary plasmacytoma. The protocol was also chosen in part to reduce chemotherapy handling in the home, an issue that can matter for pet parents and care teams alike. (frontiersin.org)

There does appear to be a broader clinical rationale for using melphalan here. In a published canine cutaneous plasmacytosis series, melphalan and prednisone produced an overall response rate of 73.7%, supporting the drug’s established role across canine plasma cell neoplasms, even though cutaneous plasmacytosis is a different disease setting. The Frontiers report likewise notes that melphalan is generally considered the chemotherapeutic drug of choice for plasma cell neoplasms in dogs, particularly multiple myeloma-related disorders, but that evidence for metastatic or visceral extramedullary plasmacytomas remains limited to a small number of case reports. (academic.oup.com)

Why it matters: For veterinary professionals, this report is a reminder not to overgeneralize from the usually favorable behavior of cutaneous plasmacytomas when dealing with colorectal or visceral presentations. It also highlights a real-world management question in oncology practice: when a drug appears active but marrow toxicity becomes cumulative, dose intensity may need to give way to dose sustainability. The case supports frequent CBC monitoring, careful client communication about adverse effects and handling, and a lower threshold for systemic staging and follow-up imaging in dogs with non-cutaneous extramedullary plasmacytoma. The author notes that CT would likely be more sensitive than ultrasonography for staging and monitoring, an important point for referral settings weighing cost, access, and diagnostic yield. (frontiersin.org)

What to watch: The next step is validation. Clinicians will want to see whether additional reports, or ideally retrospective multi-center data, can identify which colorectal or other non-cutaneous extramedullary plasmacytomas are most likely to disseminate, and whether modified pulse-dose melphalan schedules can reproducibly extend control without unacceptable hematologic toxicity. For now, this publication offers a useful signal, not a new standard of care. (frontiersin.org)