Radiation therapy is a localized, tailored cancer treatment modality. The goal is either durable, long-term tumor control (definitive intent) or alleviation of pain and discomfort (palliative intent). An understanding of the patient\u2019s entire clinical picture, as well as the clinical biology of different cancers, is essential for decision making. This article highlights some of the essential elements for clinical decision making in veterinary radiation oncology, with case examples to give general practitioners a better understanding of what types of cases may be appropriate for radiation therapy.<\/p>\n
Take-Home Points<\/strong><\/p>\n <\/div><\/div><\/p>\n Radiation therapy (RT) is not a well-established subject in veterinary school, meaning that many general practitioners may be hesitant to refer patients for RT. This article outlines the clinical indications, mechanisms of action, types of treatment, decision-making factors, and side effects associated with RT and provides a few brief case examples.<\/span><\/p>\n A fundamental tenet of oncologic treatment is \u201clocal disease requires local treatment(s).\u201d RT delivers radiation to a targeted area; therefore, patients with microscopic or macroscopic locally aggressive\/invasive tumors can be good candidates for RT. RT can be used as a sole treatment or can be combined with surgery, chemotherapy<\/a>, or immunotherapy.<\/span><\/p>\n RT is performed with either definitive or palliative intent. Definitive intent has the goal of achieving durable long-term control of the patient\u2019s tumor. Palliative-intent has the goals of relieving clinical signs associated with the tumor and decreasing the size of the tumor while avoiding treatment-related side effects, without as much emphasis on prolonged survival. Palliative RT can work well for 3 to 6 months, depending on the patient\u2019s condition and cancer type. Most RT in veterinary medicine is palliative.<\/span><\/p>\n Patients should be extensively staged<\/a> prior to initiating RT. Imaging of both the abdomen and chest with modalities such as computed tomography (CT), abdominal ultrasonography, and thoracic radiography is important to detect metastasis as well as concurrent cancers. Because RT is expensive and time consuming, complete staging helps to confirm if the benefit, risk, and cost of treatment are likely to be worthwhile.<\/span><\/p>\n If, after staging as well as histological or cytological confirmation of cancer, the patient is deemed a good candidate for RT, a CT study (CT simulation) is performed. In many cases, if CT is used as a staging modality, then both the staging study and the CT simulation are performed at the same time. During this simulation, the patient is positioned as it would be for RT. In some cases (e.g., 20 fractions prescribed), a vascular access port may be implanted at this time to aid in anesthesia during RT.<\/span><\/p>\n After the CT simulation is complete, a treatment plan is made. A computer program uses the CT study to simulate the beam angles and distribution of the RT dose in the patient, and this simulation is quality assured by the radiation oncologist and a medical physicist. Under nonemergent circumstances, this entire process takes approximately 1 week to produce a final RT plan.<\/span><\/p>\n In some cases, RT can be administered without a CT simulation, and dose calculations can be made using measurements of the thickness of the anatomic area being treated. This can be done to save on cost; for patients with tumors that are very superficial or in a distal extremity; or in cases with an emergency need for RT, such as tumors that block the airway (e.g., tracheal tumors, laryngeal tumors).<\/span><\/p>\n Once the RT treatment plan is ready, the patient is treated. Most RT patients can be treated as outpatients and are dropped off in the morning and recovered\/discharged by late morning. If a vascular access port has not been placed, an IV catheter is placed, locked with heparin, and maintained for the week. The IV catheter is removed on the weekends or if there are problems maintaining it during the week.<\/span><\/p>\n RT works by damaging the DNA of cancer cells to cause cell death. Normal cells can also sustain reversible or irreversible DNA damage from radiation, but cancer cells have deficiencies in DNA repair compared with normal cells, which creates a therapeutic window that RT can exploit.1<\/sup><\/span><\/p>\n Fractionated therapy (breaking up a large dose into multiple small doses) allows normal tissues that are included in the treatment field to be treated to relatively high doses. Radiation is measured in Grays (Gy), and the RT prescription is described as the number of fractions (treatment sessions) times the dose per fraction (e.g., 3 Gy \u00d7 19 or 19 \u00d7 3 Gy). Each fractional dose is small enough to allow slowly proliferating normal tissues to recover from radiation injury and minimize the risk of late radiation side effects. Conventional fractionation delivers the total RT dose over a period of several days or weeks, depending on the type of RT being used. RT prescriptions may be described as hyperfractionated or hypofractionated. These terms refer to the relative number of fractions and relative dose of administered RT. In veterinary medicine, an RT prescription of 2.5\u00a0Gy \u00d7 20 fractions would be hyperfractionated, while 10 Gy \u00d7 3 fractions administered on consecutive weekdays would be an example of hypofractionation.<\/span><\/p>\n Technology has developed such that large, ablative doses of radiation can be administered to a grossly visible tumor in a short period of time. Beam shaping and image guidance are used to avoid normal tissues as much as possible while only treating the grossly visible tumor, allowing the same dose that would be administered over the course of 10 days using traditional fractionation to be administered over 3\u00a0consecutive weekdays. This type of radiation therapy may be referred to as stereotactic radiation therapy (SRT), stereotactic body radiotherapy (SBRT), stereotactic ablative radiotherapy (SABR), or stereotactic radiosurgery (SRS). While DNA damage is an important player in this modality in terms of biological effect, immunomodulation caused by release of vast amounts of tumor antigen and vascular damage to the tumor microenvironment may also account for clinical responses to SRT.2<\/sup><\/span><\/p>\n Side effects of RT in normal tissues may be late or acute. Late radiation side effects are irreversible effects that may occur in slowly proliferating tissues such as bone, smooth muscle, or the central nervous system many months to years after RT. Characteristic lesions attributed to late radiation side effects include bone necrosis, strictures, fibrosis, and fistula formation. The incidence of late radiation side effects in sensitive normal tissues increases exponentially with increasing dose per fraction, as in hypofractionated RT prescriptions (e.g., 8 Gy \u00d7 4, 10 Gy \u00d7 3). The potential for these side effects is minimized by careful treatment planning as well as using technology such as image guidance and specialized treatment planning to avoid normal tissues.<\/span><\/p>\n Acute side effects of RT occur in rapidly proliferating tissues such as the skin, oral mucosa, or gastrointestinal tract during or shortly after treatment. Acute side effects are reversible, and their occurrence depends on the anatomic area and volume of normal tissue irradiated. Characteristic sunburn-like lesions in an irradiated area of skin (desquamation) are a commonly observed acute side effect.<\/span><\/p>\n Acute side effects are more likely to occur with hyperfractionated treatments and treatments administered to superficial areas such as a distal extremity or the gums\/lips and mouth. These side effects, although uncomfortable for a few weeks, are completely reversible. The patient is managed with multimodal pain medications such as a nonsteroidal anti-inflammatory drug<\/a>, gabapentin, and amantadine. If the mouth is affected, nerve blocks can also be used. <\/span><\/p>\n Clients can be assured that with appropriate pain management, most patients recover from acute side effects without complications.<\/span><\/p>\n The decision of which type of RT would be best for each patient is fundamentally based on the underlying clinical biology of the cancer present. Different cancers have different sensitivities to RT, and this affects the treatment strategy pursued as well as the indication for multimodal treatment (e.g., surgery followed by RT).<\/span><\/p>\n Because normal cells can repair DNA damage more efficiently than tumor cells, durable local control may be possible depending on the type of cancer being treated.1<\/sup> This strategy works well in situations where microscopic cancer is left behind as part of a multimodal treatment strategy with surgery. This radiation prescription, called conventionally fractionated definitive RT, typically consists of 10 to 20\u00a0fractions administered on consecutive weekdays.<\/span><\/p>\n\n
Overview of Veterinary Radiation Therapy<\/h2>\n
Intent of Treatment<\/h3>\n
Cancer Staging and Treatment Simulation<\/h3>\n
Treatment Planning<\/h3>\n
Veterinary Radiation Therapy Mechanisms of Action<\/h2>\n
Side Effects of Veterinary Radiation Therapy<\/h2>\n
Types of Veterinary Radiation Therapy<\/h2>\n
Conventionally Fractionated Definitive Radiation Therapy<\/h3>\n
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