Therapeutic Modalities: Radiation Therapy

Owing to the equipment, shielding, and expertise requirements of radiation therapy, its use is typically limited to veterinary specialty facilities.

Goals of Radiation Therapy

The classic paradigm of RT categorized radiation as either definitive intent (aka curative intent) or palliative intent.

Definitive-intent RT involves dose-intense treatment courses with the goal and expectation of providing prolonged tumor control and survival. With these goals in mind, moderate, acute, and temporarytoxicity is expected and considered tolerable. In contrast, palliative-intent RT involves less dose-intense treatments intended to minimize toxicity, as the primary goal is improvement or extension of quality of life, usually over the short to medium term (e.g., 3–9 months) (Table 5.5).

Historically, definitive-intent RT has involved “finely fractionated” courses of treatment where the total radiation dose is divided into many (e.g., 10–20) total treatments at relatively low doses per treatment and high cumulative radiation doses. In contrast, palliative-intent RT was often delivered in more “coarsely fractionated” courses of 1–5 treatments with a higher dose per treatment.

Despite this useful framework, current radiation practices have blurred the distinctions between definitive intent and palliative intent for several reasons:

• Although curative outcomes are common for localized tumors that are treated with a combination of surgery (to remove all gross disease) and RT (to sterilize residual microscopic disease), many other tumors treated primarily with definitive-intent radiation have high rates of eventual recurrence. In addition, palliative-intent RT can be intensified with the goal of providing more durable palliation or local tumor control.
• Innovation in clinical practice and publication of various studies have introduced treatment courses that fall between finely and coarsely fractionated protocols that improve convenience, response rates, or toxicity rates compared with previous protocols.
• Newer technologies (see Box) have collectively allowed for significant reduction in volume and severity of radiation toxicity and the introduction of definitive RT protocols delivered in short (coarsely fractionated) treatment courses (Table 5.5). In certain clinical situations (e.g., brain tumors, nasal tumors), this has made it possible to give much higher doses per treatment without an unacceptable increase in toxicity to surrounding tissues (i.e., stereotactic RT [SRT]). For example, 15 years ago, nearly all brain tumors were treated with 16–21 courses of radiation, but today, most brain tumors treated at facilities with capable equipment receive 1–5 radiation treatments with similar outcomes for response, survival, and toxicity. In these cases, this has allowed prescription of high radiation doses in short courses that are definitive intent rather than palliative intent. ¹

TABLE 5.5 Common Fractionated Radiation Therapy Protocols

Download PDF

*In most veterinary settings.
EOD, every other day; SRT, stereotactic radiation therapy.

Advances in Radiation Technology Therapy

3D treatment planning-Volumetric imaging (e.g. CT, MRI) and radiation treatment plans and calculates the radiation does delivered anywhere within the scanned volume.

Intensity-modulated radiation therapy-Radiation beams are shaped and controlled by a computer to precisely conform the radiation dose to the shape of a target. This allows a decreased radiation dose and reduces radiation side effects to surround normal tissue.

Stereotactic (body) radiation therapy (SRT/ SBRT)-Often used to deliver high radiation doses targeted only to the visible tumor in shortened treatment protocols. Minimal radiation is delivered to the margins outside the target.

On-board imaging (OBI)-Diagnostic quality digital imaging incorporated onto a linear accelerator (most commonly cone-beam CT) allows precise verification and adjustment of radiation delivery settings and patient positioning immediately before treatment.

Normal Tissue Response and Radiation Side Effects

Normal tissue responses to radiation are generally limited to the tissues near the target/tumor and are divided between acute effects and late effects.

Acute effects are seen in tissues with rapidly dividing cells (skin, mucosa, bone marrow) between 2 weeks and 2–3 months from the start of a treatment course. Examples of acute effects include radiation dermatitis (dry or moist desquamation), or mucositis. These effects are expected (although severity varies) with definitive doses of radiation and typically resolve over a few weeks with supportive care. Accordingly, acute effects are not usually dose limiting but can impact quality of life during and for a few weeks after the treatment period.

Late effects typically develop many months to years after radiation. Mild late effects may be cosmetic such as leukotrichia (whitening of fur) or permanent alopecia. Other late effects can significantly impact function (e.g., cataract). Severe late effects such as necrosis (e.g., bone, brain), ulceration, and new cancers caused by radiation are potentially irreversible and life threatening. Radiation is often dose limited because of the desire to keep the incidence of severe late effects below 5% of treated patients. Definitive radiation courses have been traditionally delivered over many treatment fractions because giving the same total dose over more treatments is preferentially sparing of late-responding tissues like brain and bone compared with cancer cells and acute-responding tissues.

Tumor-Specific Radiation Therapy Considerations

A complete listing of the tumors that are treatable with radiation and their associated goals and outcomes is beyond the scope of these guidelines. Table 5.6 summarizes typical clinical scenarios when radiation is considered, with their associated treatment goals and examples. Generally, most localized tumors can be targeted with radiation. Response rates and durations vary by tumor type and disease stage and grade. Tumors that can be cytoreduced to microscopic disease before radiation have the highest rates of long-term control.

TABLE 5.6 Considerations for Radiation Therapy and Treatment Goals

Download PDF

The 2026 AAHA Oncology Guidelines for Dogs and Cats are generously supported by CareCredit, Hill’s Pet Nutrition, Merck Animal Health, and Zoetis.