Michael S. Kent, MAS, DVM, DACVIM (Oncology), DACVR (Radiation Oncology)
Radiation therapy (RT) is an effective modality in treating a variety of cancers in companion animals. It has become more readily available with increasing numbers of academic centers and private practices offering RT. With the advent of newer technologies more cancers than ever can be treated with fewer side effects. The effectiveness of radiation therapy depends on many factors including the type of tumor, its location, whether it is localized or not and the quality of the radiation plan as well as the ability of a facility to carry out that plan in a reproducible manner. The most common machine used to treat veterinary patients with radiation therapy is a linear accelerator.1 Some facilities still use Cobalt-60 machines and fewer still use orthovoltage machines, although these are becoming increasingly rare and have the potential for worse side effects and are limited in the types and sizes of tumors that can be effectively treated. The rest of this session will review the more common tumor types treated with RT and the expected outcomes.
Squamous Cell Carcinoma
When treating squamous cell carcinoma location and species differences are key. While squamous cell carcinoma of the oral cavity in dogs is very responsive to treatment with a reported 1-year survival of over 75%, it is a relatively ineffective treatment for cats with oral squamous cell carcinoma.2 In contrast, squamous cell carcinoma on a cat's nasal planum is very responsive to radiation therapy and may be curative, while those on a dog's nasal planum are not. In addition superficial lesions in cats can be treated effectively with a strontium-90 probe, with 88% having a complete response and reported median survival times of over 3,000 days.3
Mast Cell Tumors
Most mast cell tumors can be effectively managed by surgery alone. If microscopic disease is left behind, RT can be used to treat the area. The reported outcomes are very good with 5-year survival rates of 86% reported for incompletely resected mast cell tumors treated with radiation therapy. Grade and stage are very important in determining prognosis, but even higher-grade tumors or those that have spread to the local lymph node can be effectively treated with RT as part of their treatment protocol. In one report in dogs with grade II mast cell tumors that had metastasized to the local lymph node the median disease-free survival was 1,240 days.4 Dogs with grade 3 mast cell tumors are also reported to do well after surgical resection and irradiation of the primary tumor area and local lymph node with a median survival of 28 months. It would be advisable to consider chemotherapy in these patients as well to try to treat potential metastatic disease.
Soft Tissue Sarcomas
Incompletely resected soft tissue sarcomas are commonly seen. While additional surgery may be possible and curative, radiation therapy is used when surgery will not result in a complete resection or will require limb amputation. With the advent of electron therapy it is also possible to treat tumors on the thorax or abdomen without causing damage to the underlying normal tissue. While outcome depends on grade, median disease free intervals of nearly 3 years and median survival of over 5 years have been reported.5,6
Due to their inherent nature infiltrative lipomas are difficult to resect. Radiation therapy can be used to stop progression of gross disease or to stop recurrence of incompletely resected infiltrative lipomas. In one study of 13 dogs with this disease median survival time was 90 months with only one dog being euthanized because of their infiltrative lipoma.7
Dogs with pituitary macroadenomas, whether or not they have pituitary dependent hyperadrenocorticism, often respond well to radiation therapy. While radiation won't necessarily control signs of Cushing's disease, it is often able to control neurological signs, improve quality of life, and extend survival compared to untreated dogs. In a recent study looking at 19 dogs treated with full course radiation therapy median survival was not reached and mean survival was 1405 days with 1, 2 & 3 year survivals of 93, 87 & 55%, respectively. Both the height of the tumor compared to the skull height and the area of the tumor compared to the brain area were prognostic for survival.8
For a more detailed description of nasal tumors please see the notes on nasal tumors in these proceedings. In dogs the most common tumor types include nasal carcinomas while sarcomas are less common. In cats the most common type of nasal tumor is lymphoma. Radiation therapy is very useful in the management of nasal tumors. In a recent study of cats with nasal lymphoma treated with radiation therapy and six-months of chemotherapy median survival was 955 days. The only prognostic variable identified was cribriform plate destruction.9 For dogs, median survivals for carcinomas range from 8-12 months and for sarcomas median survival ranges from 12-18 months.
The most commonly treated brain tumor in dogs is a meningioma. There are several studies which suggest that radiation therapy used alone or in conjunction with surgery can extend life. In one study dogs treated with surgery alone had a median survival of 7 months while those treated with RT and surgery had a median survival of 16.5 months. In another paper looking at dogs with brain masses causing neurological signs median survival was reported at 699 days with RT.10
Thyroid tumors are also considered to be responsive to radiation therapy. Surgery and radiation are often combined in the treatment of these tumors with reported median survivals of over 2 years. Even with large non-resectable tumors, outcome can be good. One study reported a median survival of over 24 months while another reported progression-free survival rates of 80% at 1 year and 72% at 3 years.11-14
Radiation therapy can be used to treat local disease in dogs and cats with oral melanoma. Most often this is done using a coarsely fractionated protocol with reported median survival rates of 6-12 months.15,16 Most dogs do not die from their local tumor but of metastatic disease. In cats with oral melanoma treated with radiation a case series of 6 cats reported a median survival of 146 days.17
Radiation can be used to help palliate the pain caused by primary bone tumors such as osteosarcoma in dogs. It can be very helpful in cases where owners do not want to amputate the affected limb but do want to try to control the dog's pain and extend life. Median survival has been reported at 4-6 months with about ¾ of the dogs having some relief of their pain.18,19 Attempts to definitively treat osteosarcoma with radiation as a definitive therapy have not proven to be as successful as current standards of care.20,21
Injection Site Sarcomas
Radiation therapy plays an important component in the treatment of injection site sarcomas in cats. Either in the preoperative or post-operative setting radiation extends both the progression free interval and overall survival. Median survival has been reported as high as two years when radiation is combined with surgery.22
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