Updated: Jun 11
Cancer is a widely varied and intractable illness that ends millions of lives and causes untold suffering.
It's no surprise that a huge amount of time, effort, and money is dedicated to finding ways to identify and treat it in all its forms, in humans and in animals. New therapies are being developed every year.
It's well known that radiation can be effective at treating cancer, and many different methods have been developed to apply that radiation. If you've heard about radiation causing cancer, well, that's true too, so why is radiation therapy safe and effective? We'll discuss that in a moment, as well as a relatively novel therapy called RadioGel (or IsoPet), how it works, and what it means for pets with cancer.
Note that this is all a very brief and superficial overview. It's sufficient for explaining the concepts behind IsoPet and the PRnT mechanism, but it's much more nuanced than what we're expressing here.
Not All Radiation is the Same
For a brief overview, it's worth talking about the different kinds of radiation. In broad strokes, radiation comes in two forms: ionizing and non-ionizing. Non-ionizing radiation can move molecules around but doesn't strip electrons from them.
It's all around you every day; microwaves, radio waves, and even visible light are all forms of non-ionizing radiation.
Ionizing radiation can strip electrons from atoms, leaving them ionized, hence the name. Ions tend to be more dangerous for a variety of reasons, primarily because they naturally want to replace those electrons and can strip them from other molecules nearby. This applies equally to things like water, metal, or your very DNA.
Ionizing radiation comes in three forms: alpha, beta, and gamma. Alpha particles are very energetic and very common but don't travel very far before decaying. They can't even really penetrate the skin, though they're still very dangerous if ingested or inhaled. They're very much an "all or nothing" kind of radioactive particle, where exposure to the skin is near-meaningless, but exposure internally is extremely damaging.
Beta particles can travel further and penetrate more than alpha particles, but because of that, the damage they cause is more spread out. However, they can still be stopped by such barriers as "clothing" and "aluminum," making them still relatively harmless compared to the third kind of radiation unless ingested.
Gamma radiation, in contrast, is not a particle but a photon with enough energy to be ionizing. They aren't stopped by skin or clothing and can penetrate feet of concrete or inches of lead. They can pass entirely through your body and cause damage the whole way through.
How Radiation Causes (And Cures) Cancer
Radiation, when uncontrolled, can be extremely damaging or even deadly. When carefully controlled, however, it can be therapeutic. It all comes down to the kind of radiation, the dosage, and how it's administered.
Radiation causes cancer because it damages the molecules that makeup DNA. In some small fraction of the DNA it damages, that damage is not enough to kill the DNA, but just mutate it into a self-replicating form which, when it grows enough, becomes a cancerous tumor. This happens all the time, but usually, the body will detect and destroy the damaged cells. Sometimes, though, they evade detection, causing problems.
Radiation can also be used to further destroy the cells, killing them entirely and preventing them from replicating. That's how radiation treats cancer: by killing it at the DNA level. This process is called "Mitotic Catastrophe."
"At high doses, radiation therapy kills cancer cells or slows their growth by damaging their DNA. Cancer cells whose DNA is damaged beyond repair stop dividing or die. When the damaged cells die, they are broken down and removed by the body. Radiation therapy does not kill cancer cells right away. It takes days or weeks of treatment before DNA is damaged enough for cancer cells to die. Then, cancer cells keep dying for weeks or months after radiation therapy ends." – Cancer.gov
Since radiation is such a huge health hazard, any radiation treatment must be carefully controlled.
There are a ton of different kinds of radiation therapy, using all different kinds of radiation, but the one we're most interested in today is called Brachytherapy.
Brachytherapy is a short-term therapy using a radiation source that decays very quickly (that is, days rather than months or years) and doesn't penetrate very far. The radiation source is injected into a tumor, where it broadcasts its radiation around to the tumor cells around it, killing them and preventing the tumor from growing. Since it's unable to penetrate very far, the radiation won't do much damage beyond the borders of the cancer itself.
Traditional Brachytherapy uses large needles to deposit radioactive "seeds" or "ribbons" that sit in place, giving off radiation to the tumor surrounding it until they decay to the point of safety.
What is PRnT and RadioGel?
PRnT stands for Precision Radionuclide Therapy. RadioGel is the trademark of Vivos, Inc. and is a form of PRnT using Yttrium-90 phosphate.
Unlike most brachytherapy courses, RadioGel uses a liquid/gel rather than a solid seed injected into a tumor.
The gel is a mixture of water and a polymer to give it a stable form, and the yttrium-90 gives off beta radiation into the surrounding tumor tissues while not penetrating to nearby organs or other tissues. It's generally used to treat tumors in locations that cannot be surgically removed but may respond well to radiation treatments.
This gel, at room temperature, is a liquid that can easily be injected into a tumor and fill the spaces between tumor cells. Then, as it warms up to body temperature, it thickens into a gel to remain in place, emitting its beta radiation to the surrounding tumor but not spreading elsewhere in the body.
RadioGel is the version of PRnT used in treating human cancers. But cancers aren't limited to humans; any animal can develop cancer, including our pets. IsoPet is the FDA-approved version of RadioGel used on dogs, cats, and horses.
Functionally, RadioGel and IsoPet work the same way. The differences lie in the dosage as much as anything, and that's broadly determined by the size and type of tumor being treated anyway. Currently, IsoPet is approved to treat feline and canine sarcomas.
What Are the Benefits of IsoPet?
There is a wide range of different cancer treatments for pets. These can range from surgical removal to chemotherapy to more novel treatments like FidoCure or Vidium. In some cases, however, the available treatments don't work, and something like IsoPet may be a good option. So, what makes it a viable option, better than other brachytherapies or other cancer therapies in general?
It's less damaging to inject. One of the downsides to traditional Brachytherapy is that the radioactive seeds are relatively large, and their short range means numerous seeds need to be injected into a tumor to properly distribute the radiation throughout the tumor.
This requires many large-gauge needles.
IsoPet uses much smaller particles of yttrium-90 in a liquid suspension, so fewer, smaller needles are necessary.
The radiation is well-contained within the tumor. This has been verified by studies performed at multiple universities, the results of which can be found here. The unique gel carrier is effective at keeping the radiation localized and preventing it from spreading throughout the body of the patient.
It's naturally absorbed and excreted safely. The primary benefit of using yttrium-90 phosphate is that it has a half-life of only 2.7 days, as opposed to other radioactive agents, such as:
Cesium-131, with a half-life of 9.7 days
Iodine-125, with a half-life of 60 days
The patient's body absorbs the hydrogel and the particles of yttrium, but it does so slowly; by the time any significant portion of it has been absorbed by the body, it has decayed beyond the point of still being radioactive, so it won't damage surrounding tissues or cause more issues than it solves.
Moreover, with seed- or ribbon-based Brachytherapy, the seeds are usually left in the body, but their migration can cause problems down the line. With IsoPet, the gen is absorbed and removed naturally, and it's completely biodegradable.
Similarly, with many forms of radiation treatment, the radiation is carried out through bodily fluids and can be a hazard to others nearby. Not so with IsoPet; there's no need to take special precautions or keep your pet isolated.
It can be effective against normally radiation-resistant diseases. While the extent of the treatment's efficacy remains to be seen, Vivos claims it can work on normally radiation-resistant tumors.
It's quite safe to handle and administer with no special precautions. With most forms of radiation treatment, the tools, materials, machines, and chemicals used are all radioactive. The vet administering the therapy will likely need special shielding to protect themselves from their tools; pet parents will likewise need to take precautions. With IsoPet, there are only minimal special precautions. Short-range beta emitters are easily blocked by the very tools used to inject them and a set of gloves the vet should be wearing anyway, and there's very little risk to anyone administering or caring for the patient.
Radioactive waste generated by the procedure decays rapidly and is safe to discard afterward. Similar to the above, radioactive waste (syringes, swaps, packaging, and so forth) is only radioactive for a relatively short amount of time. Since the half-life for the active isotope is only 2.7 days, after ~40 days, the waste materials have negligible radioactivity and can be discarded just like any other medical waste or trash.
It can be administered as an outpatient procedure with no special equipment necessary. Vets offering IsoPet can administer the treatment in their clinics and do not need to send the patient to specialty facilities or otherwise to a third-party location. It's a relatively small investment to be able to offer IsoPet as a treatment option, and a large portion of that investment is in the training necessary to administer it.
It's potentially cheaper than other brachytherapy options. Since the yttrium is easier to manufacture and safer to handle, it's also generally cheaper. The exact costs can vary, however.
Are There Drawbacks to IsoPet?
Right now, the greatest potential drawbacks of IsoPet are relatively light.
First and foremost, it's a relatively new methodology and only recently received approval. As such, it's still relatively rare and untested.
There may be unforeseen side effects down the line, or it may be less effective than it initially appears.
There's a lot of uncertainty, and relatively few trials have been performed. While it has received FDA approval, that's not necessarily a guarantee of efficacy or of long-term safety. Initial trials are promising, however.
For now, IsoPet is limited to certain sarcomas in cats and dogs, so it's fairly limited in what it can treat. However, it does become another weapon in the veterinarian's arsenal against pet cancers, and that's always good to have.
Is IsoPet a Good Option for Your Practice?
Every vet practice is different, and the range of options you can offer, both practically and financially, is unique to you. IsoPet is a novel offering that shows promise but hasn't yet truly hit the mainstream. It's entirely possible that it will be a good tool to have on hand, but it also may be outdone by other treatment options.
Truthfully, not only is each practice different, each patient is different. As a vet, you deal with canine cancers but also all manner of other ailments in your patients, from parasites to injuries to emergency care.
You're not expected to be an expert in all things cancer. You can leave that to the oncologists.
That's where we come in.
When you have a challenging or tricky patient, and you're not sure what the best course of action is, I encourage you to call Hope Vet Specialty! Veterinarians all across the country have us on speed dial. Our trained staff includes experts in a variety of fields, so whether you need advice for dermatology, oncology, internal medicine, neurology, or critical care, we can offer advice and help identify treatment options you may not know were available or effective. Just contact us to get started, and we'll be ready for you whenever you need us.