Therapeutic modalities: radiation therapy
In simple terms, radiation therapy utilizes ionizing radiation to kill cancer cells. The linear accelerator is the standard device for administering radiation therapy, and functions by accelerating electrons at relativistic speeds.17 High-energy photons have excellent penetrability and skin-sparing effect. Electron emissions range in energy from 6–30 megaelectronvolts, have a rapid dosage fall-off, and are useful for superficial tumors where critical structures are located beneath the treated area.
Goals of radiation therapy
The goal of definitive or curative radiation therapy is eradication of all viable tumor cells within the patient. Its intent is to cure the patient whenever possible and to prolong survival as long as possible.18 Palliative radiation is playing a larger role in veterinary oncology as owners increasingly seek to improve quality of life, decrease pain, and minimize hospitalization of their pets rather than achieving a cure. Most palliative protocols use lower total radiation doses and a higher dose-per-fraction to accomplish these goals.
Preoperative radiation therapy has potential advantages over postoperative radiation. These include treatment of well oxygenated tissue rather than scars, decreased tumor seeding, a smaller treatment volume, and, in some situations, less aggressive surgery. Potential disadvantages include increased wound complications and delayed surgical extirpation. Preoperative radiation is not used in every situation. The decision to do so is based on tumor location, surgeon preference, and risk of wound complication.
Pet radiation therapy centers
Pet radiology centers are available to veterinarians who wish to refer their oncology patients for radiotherapy. In addition to other resources, the Veterinary Cancer Society provides an online list (vetcancersociety.org) of veterinary radiation therapy centers, including contact information, in 30 states throughout the United States.
Normal tissue response
Within the first few wk after the start of radiation, acute effects are typically seen in normal tissues such as bone marrow, epidermis, gastrointestinal cells, and mucosa as well as in neoplastic cells. Factors affecting acute response to radiation in normal tissue include total dose, overall treatment time (dose intensity), and volume of tissue irradiated. Acute effects in healthy tissue are to be expected and will occur if curative doses are administered, but will resolve with time and supportive care. Acute side effects should not be considered dose-limiting although they can temporarily affect the patient’s quality of life. Late effects of radiation are seen in slowly proliferating normal tissue. These effects are related to damage to the vascular and connective (stromal) tissue in non- or slowly-proliferating tissue such as the brain, spinal cord, muscle, bone, kidney, and lung. Damage is often progressive and nonreversible, thus limiting the dose that can be given. Tissue destruction is related to dose, treatment volume, and dose-perfraction, and can be limited through the use of fractionated radiation therapy.
Patients with tumors in complex anatomical locations (e.g., head, neck, body wall) may require CT imaging for planning purposes prior to radiation. Patients treated with palliative courses of radiation may not require computer-based planning depending on tumor size and location. Hemoclips placed at surgery aid in delineating the tumor bed.19 Patient positioning during radiotherapy should attempt to exactly duplicate the patient position at the time of CT.
Tumor-specific radiation considerations
A variety of cancers are responsive to radiation therapy. These include brain tumors, nasal tumors, oral tumors, and tumors of the extremities and body. Brain tumor treatment may consist of radiation alone or combined with surgery.20,21 The brain tumors reported to favorably respond to radiation include meningioma, schwannoma, choroid plexus tumors, astrocytoma, glioma, and pituitary macroadenomas and adenocarcinomas. 20,21 All nasal tumors appear to respond to radiation. Specifically, canine and feline lymphoma, sarcomas, and carcinomas of the nasal cavity respond favorably to radiation. Canine oral tumors, specifically acanthomatous epulis, squamous cell carcinoma, fibrosarcoma, and melanoma, respond to radiation. Canine soft tissue sarcomas, lymphoma, mast cell tumors, ceruminous gland tumors, thyroid carcinomas, bladder tumors, prostate tumors, perianal adenomas, and apocrine gland anal sac adenocarcinomas also respond to radiation, as does localized lymphoma. Radiation is commonly used for palliation in osteosarcomas in dogs.21 Unfortunately, not all cancers respond well to radiation. One such example is a large soft tissue sarcoma.21
3-D conformal radiation therapy allows the beam to be tightly shaped to the tumor and allows sparing of normal tissues.22 Intensity modulated radiation therapy allows the beam collimator to move during treatment, allowing the tumor to be irradiated at different angles and distances during a single treatment. State of art radiation therapy currently includes stereotactic radiosurgery and stereotactic body radiation therapy. These methods involve more sophisticated technology and delivery of single or several fractions of high-dose radiation therapy with a narrow margin. Long-term studies are sparse in veterinary medicine, but these technologies offer the promise of higher doses to tumors, lower doses to normal structures, and fewer dosage fractions.