Stephen Shaw, DVM, DACVIM (Oncology) Veterinary Medical Specialists (Campbell)
Radiation therapy (RT), along with surgery and chemotherapy, are the three main pillars of treatment for cancer in veterinary medicine today; however, unlike surgery and chemotherapy, the applications of RT often are not well understood since the equipment, licensing, and training required for administering this form of therapy typically limits its availability to certain specialty veterinary and human hospitals. There are a number of different modalities in which RT can be applied, such as with radiopharmaceuticals like iodine 131 for the treatment of feline hyperthyroidism, plesiotherapy with the application of a strontium probe to small superficial cancers, or most commonly, teletherapy (external beam radiation therapy) from a linear accelerator. Older forms of teletherapy with Cobalt-60 or orthovoltage machines are becoming less and less common.
Indications for external beam radiation therapy include the primary treatment of specific tumor types, adjuvant treatment in addition to surgery and/or chemotherapy for better tumor control, and for palliation to improve quality of life for advanced cancer cases.
Primary treatment for specific tumor types: The use of external beam radiation therapy for the primary treatment of tumors is recommended for several specific tumor types, namely nasal tumors of various histologies, as well as other tumors that cannot be surgically resected. Previous studies have shown that patients treated with rhinotomy plus RT have similar survival times to patients treated with RT alone. Patients treated with surgery alone had similar survival times to those who received no treatment, therefore making RT the most effective way of treating nasal tumors. Other tumors that can be effectively treated primarily with RT (if surgery is not feasible) include intracranial tumors such as pituitary macroadenomas and meningiomas. Round cell tumors are known to be exquisitely sensitive to RT, so localized round cell tumors for which surgery or chemotherapy are not feasible or effective will often shrink away with RT. A definitive course of RT consists of treatments given daily for a total of 16-21 total fractions.
Adjuvant treatment: In many cases, the complete local excision of a tumor is not feasible, so RT is recommended as adjuvant therapy to eliminate the residual disease left behind or to slow the regrowth of an excised tumor. Tumor types most frequently treated in this fashion include incompletely excised mast cell tumors and soft tissue sarcomas. Incompletely excised canine mast cell tumors that are treated with adjuvant RT have a less than 10% chance of local recurrence. Low to intermediate grade soft tissue sarcomas that are incompletely excised and treated with adjuvant RT have a tumor control rate of 75-85% for 3-5 years. Cats with injection site sarcomas treated with RT followed by surgery have disease free intervals of about 1-1.5 years with a median survival time of about 2 years.
Palliation: Palliative RT is most frequently recommended for terminal patients with advanced cancers. The aim of palliative RT is to shrink the tumor temporarily and provide relief of pain and discomfort associated with the tumor. Typically between 2 and 6 fractions are administered, as opposed to the 16-21 fractions given for definitive RT. Tumors that are commonly palliated include oral tumors, large soft tissue sarcomas, and osteosarcomas, but may include tumors of nearly any histology where surgery is not an option.
The side effects of external beam RT are typically divided into acute effects and late effects. Acute effects are the effects seen with rapidly dividing tissues such as skin and mucosa and include dry or moist desquamation and mucositis. These effects can be expected to occur to some degree usually 2-3 weeks after starting radiation, but are typically transient, lasting a few weeks. Late effects occur with slowly dividing tissue such as bone, nervous tissue, and other connective tissues and can occur anywhere from 6 months to several years later and include fibrosis, necrosis, and secondary tumor formation. Late effects generally have a very low incidence of occurrence, but can be relatively serious if they do occur. Definitive radiation protocols are generally associated with lower rates of late effects, whereas palliative protocols tend to be associated with fewer acute effects but a higher rate of late effects, which is why palliative protocols are generally reserved for advanced cancer cases where the patients are unlikely to ever be affected by late effects.