Radiation oncology perspective
Nathan Lee, DVM, DACVR (radiation oncology)
Nathan Lee, DVM, DACVR (radiation oncology)
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Local recurrence is seen in 20% to 50% of dogs with anal sac adenocarcinomas treated with surgery alone,1-4 so postoperative definitive radiation therapy should be considered in any patient with a primary tumor > 2.5 cm in diameter.
There is no standard of care for radiation therapy of anal sac adenocarcinoma. At The University of Tennessee, our current
definitive radiation protocol consists of daily treatments, Monday through Friday, for a total of 24 treatments.
With definitive radiation therapy, acute side effects to the tissues in the radiation field begin to develop during the third
week of treatment and typically consist of moist desquamation and mucositis in the perianal area, colitis, tenesmus, and perineal
discomfort. While these effects can appear severe and require proper client education for management, they are self-limiting
and usually resolve within 10 to 20 days after radiation therapy is complete.
Late side effects associated with perianal and pelvic radiation are usually seen six months to several years after therapy
and may include chronic colitis, rectal or colonic strictures, rectal perforation, pelvic limb edema, secondary tumor formation,
and perineal pain. The incidence of late effects is significantly reduced if fraction sizes are reduced to < 2.7 Gy and the
total dose is limited to 54 Gy.10,11
Patients with large primary anal sac adenocarcinomas with or without metastasis to the sublumbar lymph nodes may respond to
palliative radiation. I have seen dogs receiving a dose of radiation once a week for three to four treatments show a significant
decrease in tumor burden, and their quality of life is improved life for about one to one and a half years. It should be stressed,
though, that palliative radiation therapy should only be used in patients with marked bulky disease. Palliative radiation
therapy using large doses per fraction also significantly increases the risk for late side effects from radiation therapy.10,11
The Clinical Rounds team is from the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, The University
of Tennessee, Knoxville, TN 37996.
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