Lyndsay Kubicek, DVM DACVR (Radiation Oncology)
When we describe radiation therapy in veterinary medicine one of the first considerations is the role in which we use radiation therapy. Traditional fractionated radiation therapy is used in settings in which our goals are long term control, or otherwise called definitive or curative intent radiation therapy. However there are times when long-term cancer control is not attainable and we are looking to keep patients comfortable. This is where the role of palliative radiation is used in veterinary medicine.
A patient with a mast cell tumor of the muzzle which is an example of a case for which we performed palliative radiation therapy
Palliative radiation therapy is used to control the clinical signs and discomfort associated with many types of tumors that cannot be treated by other techniques such as surgery. The main goal of palliative radiation is to maintain a good quality of life for cancer patients. It is important to stress that the goals of palliative therapy are to provide pain relief, and not to increase survival time or cure the cancer.
The dosing regimen for palliative radiation involves delivering larger doses per fraction typically once weekly for 3-4 doses, however other protocols exists that involve daily dosing for 5 consecutive days, twice daily for 2 consecutive days or twice weekly for 2 weeks among others. Pending dosing, response and time between therapies, palliative radiation therapy may be repeated, however the risk of delayed side effects can increase with subsequent palliative protocols. Prior to deciding to deliver a second course, a serious discussion of the pros and cons must be had with the owner.
There are multiple tumor types that have been reported to have a clinical benefit from palliative radiation. Palliative radiation therapy has proven particularly effective in alleviating pain associated with tumors that are arising from, or invading bone such as osteosarcoma and boney metastasis. The mechanism of action regarding pain relief is poorly understood, even in human radiation therapy. The majority, approximately two thirds of patients, have moderate to significant improvement in clinical signs associated with pain. Such benefits can last for a few weeks to several months based on the individual patient. Clinical improvement may occur as quickly as several days after the first treatment, or it may take a few weeks following the last dose of therapy before a decrease in clinical signs are seen.
Palliative radiation therapy aims to achieve relief of pain and other clinical signs associated with nasal tumors such as the incidence of epistaxis associated with nasal tumors, which improves quality of life.1 In 56 dogs treated with four weekly fractions, 73% had complete resolution of epistaxis while 27% had partial resolution.1 Currently there is no standardization of palliative regimens for canine nasal tumors. Reports describe using either a 4–5 biweekly or a 3–4 weekly, or 10 daily prescription. Another study showed a 100% response rate in 38 dogs with a median duration of response of 10 months and median overall survival of 10 months.2 In 48 other dogs treated with various fractionation schemes, the majority being 3 fractions given on day 0, 7, 21, the overall response rate was 91.6% with 66% complete resolution of clinical signs and a median response duration of 4 month.3 Finally, in two other studies of 12 and 56 dogs using a protocol delivered 4 weekly doses, median survival was 14.5 months and 7 months respectively with 95% response rates.1, 4
Palliative radiation has been used for thyroid tumors where surgery or definitive therapy is not elected. A study of 13 dogs with invasive thyroid carcinoma treated with palliative intent radiation therapy consisting of 4 treatments once a week, found that all dogs had a reduction of primary tumor size or cessation of tumor growth with a median survival time of 22 months.5 The concern with palliative radiation for tumors in the thyroid region is late term side effects namely esophageal stricture and trachealis. In general palliative radiation for such tumors is reserved for patients with diagnosed metastatic disease.
Other tumor types that have been reported to benefit from palliative radiation include oral tumors, anal sac tumors, gross disease sarcomas, prostate tumors, retroperitoneal hemanagiosarcoma and lymphoma.6,7 As we do see a benefit with the use of palliative radiation in multiple tumor types and locations, I do recommend consulting with a radiation oncologist when faced with a patient with cancer and owners who may be interested in radiation therapy.
If you would like further information please call the oncology service at 617-541-5136.
- Mellanby RJ, Stevenson RK, Herrtage ME, et al. Long-term outcome of 56 dogs with nasal tumours treated with four doses of radiation at intervals of seven days. Vet Rec 2002;151:253–257.
- Buchholz J, Hagen R, Leo C, et al. 3D conformal radiation ther- apy for palliative treatment of canine nasal tumors. Vet Radiol Ultrasound 2009;50:679–683.
- Gieger T, Rassnick K, Siegel S, et al. Palliation of clinical signs in 48 dogs with nasal carcinomas treated with coarse-fraction radiation therapy. J Am Anim Hosp Assoc 2008;44:116–123.
- Morris JS, Dunn KJ, Dobson JM, et al. Effects of radiotherapy alone and surgery and radiotherapy on survival of dogs with nasal tumors. J Small Anim Pract 1994;35:567–573.
- Brearley MJ, Hayes AM, Murphy S. Hypofractionated radiation therapy for invasive thyroid carcinoma in dogs: A retrospective analysis of survival. J Small Anim Pract 1999;40:206–210.
- McDonald C, Looper J, Greene S. Response Rate and Duration Associated with a 4Gy 5 Fraction Palliative Radiation Protocol. Vet Radiol Ultrasound 2012;53:358–364.
- Hillers KR, Lana SE, Fuller CR. Effects of Palliative Radiation Therapy on Nonsplenic Hemangiosarcoma in Dogs. J Am Anim Hosp Assoc 2007;43:187-192.