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There are many applications of radiation therapy in veterinary medicine. In some cases (such as regional lymphoma), radiotherapy can be used as a stand-alone treatment, usually in combination with surgery and chemotherapy, and radiotherapy is used for pain control. Because it is a regional treatment, its use in common tumors is limited.
Thanks to radiotherapy, many advances have been made in cancer treatment in recent years. Radiation therapy has many uses in veterinary medicine. In some cases (like regional lymphoma) radiotherapy can be applied as the only treatment option but it can also be used along with surgical operations, chemotherapy and pain management. Since it is a locoregional treatment, its use is limited in invasive tumors.
Radiation therapy is the most common conjoint therapy in veterinary medicine. In cases when the tumor cannot be completely removed surgically or when another surgical intervention is not possible, in order to clear the remaining malignant parts of mast cell tumors or soft tissue sarcoma, radiation is used. In those cases, radiotherapy should begin 10-14 days after the operation. There can be delays depending on the healing time of the surgical wound. In order to prevent high grade tumors from multiplying, radiotherapy should begin 7-10 days after the operation as soon as the surgical wound heals. At the same time, if the tumor is too large to be removed surgically or if it is a deep tumor, radiotherapy is used to decrease the tumor size to make it suitable for surgical operation. If the tumor cannot be removed completely or there is risk of growth until the radiotherapy begins, one dose of radiotherapy can be applied during the operation. Treatment should continue with external radiotherapy.
In cases of bone tumors or metastases, there is no option for surgical intervention. Radiotherapy can be used to decrease the tumor size and manage the patient’s pain. Radiotherapy is the only option for patients with a brain tumor too large to be removed with a surgical operation. 
The main targets of radiotherapy are the water molecules in cells where hydroxyl radicals are produced which harm the DNA. The hypoxic cells that are deprived of oxygen are resistant to radiation. Most cells in a tumor have low levels of oxygen.
Radiation therapy is usually applied in weekly periods and in multiple doses. The purpose is to provide the time for recovery from radiation for the healthy cells and give the maximum amount of harm to slow regenerating tumor cells. In addition, oxygen deprived hypoxic cells have even lower levels of oxygen after a single dose which helps the cell receive more oxygen and become more sensitive to radiation. The response of tumors to radiation depends on the growth fraction of the tumor, the oxygen level, cell kinetics, and cell loss ratio. That is why the diagnosis of the tissue is of utmost importance before beginning radiotherapy. The sensitivity of the healthy tissues should be taken into consideration while determining the administered dose.
The complications of radiation therapy can be categorized in two ways. One is the acute complications that emerge during the treatment. The most common complications include skin rashes and regional inflammations. This usually takes place in the epithelia. If the complications are serious, they need to be taken under immediate control since the patient’s condition might delay the treatment. When treated properly, the acute reactions are expected to heal in 2-4 weeks. It is difficult for radiation burns in joints to heal but a full recovery will take place with the appropriate treatment. Symptoms for poisoning can emerge after months, even years. In addition, another late complication is leucotrichia, loss of pigment in the hair. Leucotrichia is not a health hazard. Late complications which are serious vary according to the area where the radiation is administered. Bone necrosis might appear 5-7 years after the radiotherapy treatment of mast cell tumors or soft tissue sarcomas which were not completely removed surgically. Rarely, secondary tumors might appear. 
Radiation might be administered in different ways. Teletherapy uses an external source for radiation. Orthovoltage, Cobalt-60 and linear accelerators are devices used in external beam radiotherapy. In brachytherapy, the radioactive source is administered directly into the tissue but it is applied rarely in veterinary medicine since implant use is difficult in animals and a quarantine is required. Another method is to systematically administer radioactive isotopes. 
The tumor volume should be determined before radiotherapy treatment so that the treatment can be organized accordingly. Otherwise treatment is bound to fail. In order to expose all neoplastic tissues to radiation, the region where the therapy is applied to should be larger than the tumor’s size. Radiation is administered not only on visible tumors but also on normal tissue borders. The regional lymph nodes are also included in this area. The location of the remnants of the malignant tumors which could not be completely removed during an operation cannot be determined by looking at the surgical wound. For the right treatment, surgical and pathological reports should be examined and methods such as hempclips and radiography should be used. CT (Computed Tomography) and MRI provides the most precise result about the tumor. It is essential for the radiation oncologist and the surgeon to be in consultation with each other. 
Thanks to radiotherapy, many advances have been made in cancer treatment in recent years. Do not hesitate to contact your veterinary for more information.
© The contents of the Ada Veterinary Oncology website are prepared by Ada Veterinary Polyclinic. Can not be used without permission.
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