Multiple myeloma in a 9-year-old Labrador: Radiation oncology perspective
Multiple myeloma is often associated with lytic and painful bone lesions.1 As plasma cells are lymphocytes (and are therefore particularly sensitive to radiation therapy), painful bony lesions due to multiple myeloma may be palliated with external beam radiation therapy.2 Solitary extramedullary plasmacytomas (EMPs), which can occur in the oral cavity and may not be amenable to complete surgical excision as well as solitary osseous plasmacytomas (SOP) can also be successfully treated with radiation therapy.3,4
Radiation therapy can be very effective in achieving local control of these lesions. The difficulty in dogs affected by multiple lytic lesions is to pick and choose which to irradiate. Are we picking the most painful lesions? Are we missing any painful lesions and wasting our radiation dose? Will the dog develop new lesions during or shortly after we treat it with radiation therapy? All those questions are difficult to answer and require extensive discussion with motivated clients who are willing to try palliative radiation therapy for painful bone lesions.
Dogs suffering from solitary EMP or SOP may be better candidates for radiation therapy. I have successfully treated two dogs with oral EMPs with definitive radiation therapy alone, and both achieved long-standing (i.e. years) complete remission. As the risk for developing multiple myeloma with EMP is low, a definitive course of radiation therapy is warranted for those cases to minimize the risk for long-term side effects, such as bone necrosis, and to achieve durable remissions. This protocol usually consists of three to four weeks of daily (Monday through Friday) treatments with a total dose of 45 Gy or higher.
These protocols are associated with acute side effects, such as hair loss, dry to moist desquamation of the skin and mucositis of the oral cavity, which usually start during the last week of radiation therapy and are mostly mild and self-limiting. I start patients undergoing a definitive course of radiation therapy on a low dose of daily prednisone (0.5 mg/kg orally every 24 hours) and an appropriate antibiotic about halfway through the protocol to minimize acute side effects. Pain medications and locally administered oral solutions containing lidocaine, diphenhydramine and sucralfate are prescribed as needed. Feeding soft foods is recommended. Despite mild to moderate mucositis, most patients continue to eat when offered small meatballs of wet food.
In dogs with SOP (which have a higher risk of developing systemic multiple myeloma), a palliative course of radiation therapy to control bone pain may be the better choice. At UT, painful appendicular bone lesions are treated with 8 Gy per fraction, two days in a row, as described by Knapp-Hoch et al. in 2009.5 The advantage of this protocol is that the total dose is low enough that it can be repeated in the future if the patient responds favorably and later relapses.
As discussed above, plasma cells are very sensitive to radiation, and in extremely lytic lesions—especially vertebral lesions—I always caution that there may be a high risk of developing a pathologic fracture if the tumor cells are effectively killed by the radiation therapy. Because of the risk of rapid tumor cell apoptosis with a high dose per fraction, I may recommend a different protocol consisting of a lower dose per fraction to treat extensive lytic lesions or bone lesions in areas with a high risk for developing late side effects, such as close to the spinal cord. In a report of two dogs irradiated for vertebral plasma cell tumors, one developed radiation-induced myelopathy four months later.6 Other authors recommend surgical repair of a pathologic fracture followed by radiation therapy.7
Matus RE, Leifer CE, MacEwen EG, et al. Prognostic factors for multiple myeloma in the dog. J Am Vet Med Assoc 1986;188(11):1288-1291.
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Meis JM, Butler JJ, Osborne BM, et al. Solitary plasmacytomas of bone and extramedullary plasmacytomas. A clinicopathologic and immunohistochemical study. Cancer 1987;59(8):1475-1485.
MacEwen EG, Patnaik AK, Hurvitz AI, et al. Nonsecretory multiple myeloma in two dogs. J Am Vet Med Assoc 1984;184(10):1283-1286.
Knapp-Hoch HM, Fidel JL, Sellon RK, et al. An expedited palliative radiation protocol for lytic or proliferative lesions of appendicular bone in dogs. J Am Anim Hosp Assoc 2009;45(1):24-32.
Rusbridge C, Wheeler SJ, Lamb CR, et al. Vertebral plasma cell tumors in 8 dogs. J Vet Intern Med. 1999;13(2):126-133.
Withrow S, Vail D, Page R. Myeloma-related disorders. In: Vail D, ed. Withrow and MacEwen's small animal clinical oncology. 5th ed. St. Louis: Elsevier, 2013.