Because this talk is using a lot of unfamiliar words, the outline of this summary is a bit different. Definitions are at the beginning. If there is any confusion during the seminar hopefully referring to this will help you understand. The goal of this talk is not to make you a radiation oncologist, but rather to help you understand the decision making process we go through when deciding how best to treat a patient. It is also to help you realize that radiation therapy has come a long way! Traditional radiation, while very effective, was a pretty scary concept to most owners because of the side effects they would have to deal with. The goal here is to help you understand radiation a little better so that you can educate your clients regarding their options.

Definitions

Fraction (fx) - a treatment of radiation; traditionally an animal will get 15–20 "fractions" of radiation delivered daily on a Monday to Friday basis over 3 to 4 weeks.

Gray (Gy) - a unit of measurement of radiation dose; traditional radiation treatments deliver 1.5–2.7 Gy per fraction/treatment; with stereotactic radiation we can give up to 20 Gy/fx.

Isocenter - the center of the tumor as defined by the contour drawn by the radiation oncologist; this is where we center the radiation beams.

Hypo-fractionation - this involves a reduction in the number of radiation treatments given to a patient; in order to keep the same "biologically effective dose" (i.e., expect the same biological effect with a radiation prescription that is delivered in less time) we need to give more dose per treatment (i.e., increase the Gy/fx). This increases the risk and severity of late effects.

Clinical set ups - no CT is used; radiation field is based on radiographs, palpation and anatomic knowledge; uses hand calculations (i.e., calculator) in order to determine how much radiation to deliver; generally uses parallel opposed fields (top and bottom or both sides); normal tissue is not spared; can be palliative or definitive protocol.

Palliative radiation - the goal is to provide relieve without creating side effects; delivered in a shorter period of time with reduced number of fractions.

Definitive radiation - curative intent; works best in the face of microscopic disease (i.e., after surgical removal of most of the mass) because it is a game of numbers! If there are less cancer cells to kill we have a better chance of getting them all. Delivers small doses daily to limit side effects (especially late effects).

Multi-leaf collimators (MLC's) - thin tungsten leaves (0.25–1 cm in width) on the linear accelerator that each have a motor to move them across a radiation field; allows modification of the radiation to make it more conformal.

Image guided radiation therapy (IGRT) - advanced imaging such as CT's are used to create more precise radiation plans and to be sure that radiation is being delivered where we want it to be. If you want to spare normal tissue you have to know where it is! Prior to delivering doses of radiation we have to be sure that the tumor and the normal tissues are where they were during the original planning CT. This involves imaging while the patient is set up in the radiation therapy room, using imaging equipment found on the linear accelerator. The image quality is poor for diagnostics, but good enough to verify positioning.

Intensity modulated radiation therapy (IMRT) - a form of IGRT that uses MLC's to make the radiation dose more conformal. This has been shown to reduce side effects without affecting tumor control.

Stereotactic radiation therapy (SRT) - An extreme form of hypofractionation and IMRT where very large dose/fx is delivered to a tumor. Only possible because we can avoid normal tissues to prevent side effects. Instead of 15–20 fractions in a definitive treatment we give 1–5 fractions.

Introduction

When we are deciding on a radiation prescription for a patient, we must be conscious that every single case is treated as an individual. Which radiation protocol we use is based upon several factors, including:

 Curative intent - Is this a tumor that we can "cure" if we get aggressive? Is there metastatic disease which would limit life expectancy even if we got excellent local tumor control? Are the money, expected side effects and complications worth moving forward with a definitive protocol?

 Palliative - Can radiation provide a considerable amount of relief for this patient? Radiation works very well to reduce inflammation, intratumoral pressure. For example, in dogs with osteosarcoma, up to 90% of canine patients without pathologic fractures will get 3–4 months of pain relief with a palliative radiation protocol.

 Cost restrictions - radiation is expensive! Most pet owners can't afford it, or can't justify the cost for the expected survival. It is an incredibly valuable tool, but most veterinary patients afflicted with cancer will die of their disease at some point in time, and owners have to be well educated on what to expect so they can make informed decisions regarding the cost:benefit of moving forward. Other treatment (chemotherapy, surgery, supportive medications) may still be needed after radiation.

 Time restrictions - traditional radiation protocols can take up to 4 weeks to deliver. At Colorado State University a lot of clients come from far away and staying in town for that long is just not feasible.

 Prognosis - this goes back to deciding whether or not to use a definitive, curative intent protocol. In a dog with a poor prognosis, a full 4 week protocol may not be the best idea. Palliative options could be explored instead, which radiation can play a role in.

 Side effects - educating the clients on expected side effects is very important. They will often have to be willing to do at home care to prevent acute effects from becoming quality-of-life limiting. They will also have to be prepared for how dramatic these effects could look. More importantly they have to be aware of possible late radiation effects, know what to look for, know the chances of them happening and be prepared that if they happen, euthanasia may have to be considered.

Where Are We Now?

Remember that cancer cells are dividing so quickly they don't have time for repair. The dose that can be delivered on a daily basis depends on the normal tissues surrounding a tumor.

For example, we might deliver a higher daily dose to a tumor on a limb than we would to a tumor in the brain, because skin and bone can tolerate more radiation and recover than the brain can. Plus, radiation damage to skin and bone is not necessarily life threatening, whereas radiation effects of the brain may be. The more dose we can deliver, either daily or in total, means a better chance to kill all the tumor cells. Our goal is 100% control. Anything less means that the tumor could eventually regrow. The difference in dose between what will kill a tumor and what will cause life threatening radiation effects is called the therapeutic index. In an ideal world the therapeutic index would be high, meaning that the amount of radiation required to get control of a cancer is much, much lower than that causing acute and/or late radiation effects (ARE and LRE).

So how do we get enough dose to a tumor to kill it while limiting the side effects to the normal tissues? There are two ways to avoid bad side effects. Traditionally this is done by delivering small daily doses of radiation. There is no sparing of normal tissues surrounding the tumor. Instead we are relying on their innate ability to repair themselves at checkpoints between doses, which a tumor cell won't do.

More recently technological advances have allowed for sparing of normal tissues. Utilizing a CT we can visualize normal tissues and spare them by means of image guided radiation (IGRT). Originally this was done with metallic wedges and blocks that could attenuate a radiation beam. These devices were non-specific but very effective. These days most linear accelerators are equipped with multi-leaf collimators (MLC's), which are thin plates of a metal called tungsten that slides in and out of a radiation field to protect normal tissues.

Using MLC's we can spare normal tissues. This means that when we deliver radiation we can get really good tumor control with limited side effects. This has been shown to be true in veterinary medicine, especially in published reports on treatment of nasal tumors.

Using these MLC's we can even increase daily doses of radiation to unheard of levels, by protecting normal tissues. This type of radiation is called stereotactic radiation, or SRT.

In conclusion, when a radiation oncologist decides on a radiation protocol for a patient, the treatment is very individualized. The goal of the owner and the comfort of the patient is of utmost importance. Other considerations include cost and time restrictions, prognosis for the patient's particular type of tumor, and potential side effects.