Response and outcome following radiation therapy of macroscopic canine plasma cell tumours

Thirty dogs with macroscopic plasma cell tumours (PCTs) were treated with radiation therapy (RT). Twelve patients were treated with palliative-intent prescriptions (range, 4-10 Gy/fraction (median, 7 Gy/fraction) for a total dose of 20 to 35 Gy (median total dose 30 Gy). Eighteen patients received definitive-intent prescriptions (range, 3.0-4.2 Gy/fraction (median, 3 Gy/fraction) for a total dose of 42 to 54 Gy (median total dose 48 Gy). Involved sites included the oral cavity, skin, multiple myeloma (MM)-associated lytic bone lesions, bone (solitary osseous plasmacytoma; SOP), nasal cavity, larynx, retrobulbar space, lymph node and rectum. Ninety-five percent of evaluable dogs had a complete (CR; 16/22) or partial response (PR; 5/22). Patients with MM experienced significant analgesia. The median progression-free survival (PFS) was 611 days (range: 36-2001 days). Events in the non-MM cases included in-field progression (5/26, 19%) and disseminated disease (5/26, 19%). The median survival time (MST) for all dogs was 697 days (range: 71-2075 days), and when only non-MM cases were considered, MST was 771 days (range: 71-2075 days). Fourteen patients were alive without disease progression or had died of unrelated causes. Achievement of a PR was associated with an inferior PFS and MST as compared with CR. Palliative-intent RT was associated with inferior MST as compared with definitive-intent RT. RT is a useful therapeutic modality for PCTs and tumour responses are often complete and durable, with protracted survivals. The optimal radiation dose and schedule are yet to be defined.