Definition
External beam radiotherapy is a noninvasive method of irradiation through the skin. The source of radiation is at a distance (100 cm, approximately 3') located in the head of medical linear accelerator (Linac). This is the most commonly used irradiation modality.

Treatment planning
All steps of radiation therapy planning and treatment are done on-site. We are the only private practice hospital in the area under direct supervision by a veterinary radiation oncologist. The availability of a CT scanner, a Computerized Radiation Therapy Planning System and a Linear Accelerator in our facility provides the links needed to optimize the accuracy of radiation dose delivery to the tumor. This results in much more effective treatment and far fewer side-effects.
Before any treatment can commence, a treatment plan is developed to accurately determine the most appropriate way of treating your pet’s condition. CT scans are often needed to assess the tumor volume and accurately define the treatment area, and are required in order to plan the treatment of internal or diffuse tumors. Treatment of invasive or deep-seated tumors (brain, oral, nasal tumors or any intra-abdominal or intra-thoracic tumors) surrounded by radiosensitive organs necessitate CT-based, computer generated treatment planning to prevent damage to normal tissues.

The patient is positioned in the CT machine in the exact position to be used during radiation treatment so that the CT images can be used for radiation treatment planning.











These CT scan images are loaded into our in-house treatment planning computer, where the radiation oncologist and therapist process images to develop a treatment plan. The cancer and potential areas of cancer extension as well as normal tissues to be avoided are digitally outlined on each CT image.










Radiation beam angles and field shapes are then optimized for each patient by use of our radiation treatment planning software to match the region of high-dose radiation in the patient as closely as possible to the three-dimensional shape of the tumor. Distribution of the radiation dose is displayed on the treatment planning computer’s monitor in as many cross-sections and projections (beam's eye view) as needed to ensure that the tumor volume receives the full radiation dose but also that the adjacent critical tissues are spared. Once all treatment parameters are determined, the dose is calculated.






The availability of a CT-based Computerized Radiation Therapy Planning System and a Linear Accelerator in our facility provides the links needed to accurately deliver radiation to the tumor and optimize a muti-modality approach to cancer treatment. 

Treatment technique
Radiation therapy is a localized method of treatment, generally affecting only the part of the body that is receiving the treatment.

Our radiation unit is a Varian 6100 C Linear Accelerator (Linac). Treatments can involve a number of individual beams focused on the tumor from several different directions around the patient. From one to four beams may be used daily for each treatment. During treatment, the patient must remain perfectly still during the treatment so that the radiation only goes where it is needed. A short-acting anesthetic is given to immobilize the patient. There is a small but definite risk associated with repeated anesthesia; therefore, patients are monitored closely.














In order to accurately define the treatment area, the patient’s positioning is accurately assessed and adjusted, and lines drawn on the skin.  RT is delivered, with the patient placed on the treatment table in a customized immobilization device. The patient is positioned so that the center of the treatment field is aligned with a system of 3 laser beams to ensure treatment accuracy







Treatment
Curative treatments are given with a contemporary radiotherapy protocol of five treatments a week given Monday through Friday over 3-4 weeks. Palliative treatments used to control the growth of cancer or to relieve pain are given with fewer treatments, once, twice or 3-times a week.
Regular evaluations are performed during the treatment course to assess response to treatment and to monitor and treat (if necessary) side-effects. Although radiation treatment is completely painless, there may be some side effects that are specific to the area being treated and are usually minor and self-limiting.1  All of the side effects associated with radiation therapy are limited only to the area where the radiation is applied. "Radiation sickness" described in people, manifested by nausea, vomiting, and diarrhea, is not seen in dogs and cats undergoing radiation therapy.
Your veterinarian will work closely with you to keep your animal comfortable and healthy during radiation therapy. The quality of your pet’ life is always the first priority.

Indications
Radiation therapy plays an important role in treatment of localized benign and malignant tumors.1  It is indicated for the treatment of tumors that would have unacceptable functional and/or cosmetic side effects if they were surgically removed. It can also be used to eliminate residual disease left behind when surgery could not remove the entire tumor without creating serious functional or cosmetic deficits.
Radiation therapy is effective for control of localized cancers of the skin and subcutaneous tissues, the head and neck (oral and nasal cavity, ear canal, and thyroid), the central nervous system and bones. It is also used in the management of benign tumors, including adenomas of the pituitary (dogs and cats) and perianal glands (dogs) and oral epulides (gum tumors of dogs). Radiation therapy also plays an increasing role in the management of lymphoma in dogs. Whole-body irradiation in combination with standard chemotherapy has been shown to result in longer survival compared to chemotherapy alone.