6 Months of Radiation Therapy at SARC: A Brief Review of Australia’s Only Veterinary Deep Radiation Service

Six months after installation of Australia’s frist veterinary deep radiation therapy facility, we take a look at some of our practical experiences with treatment. Our brief review takes a look at the common applications for radiation therapy, as well as a review of a practical case presentation and the treatment strategies available. Deep radiation therapy is only recently available in Australia for companion animals and is an exciting progression in the treatment options for cancer in cats and dogs.

Figures (to right from top):
Figure 1 - Typical radiation therapy for treatment of an incomplete excision of a soft tissue sarcoma.

Figure 2 - Treatment of a brain tumour in the olfactory lobe through a single portal.

Figure 3 - Treatment of a brain tumour in the olfactory lobe through three portals.

The first veterinary radiation therapy unit in the southern hemisphere capable of administering deep therapy has now been running for 6 months at SARC. We have administered hundreds of treatments in dogs and cats and are finding an ever-expanding list of appropriate cases for treatment. Just as importantly, we are also building a better practical appreciation of the types and severity of early side effects that patients are experiencing.

As envisaged, the most common case presentation we are seeing is for radiation of a surgical site where dirty margins were seen on biopsy following removal of a mast cell tumour or soft tissue sarcoma. In this setting, the literature suggests that 80% of patients will be tumour free 5 years after radiotherapy. This can be viewed as curative in most patients.

The typical treatment protocol (figure 1) involves administration of 15 daily treatments for a total of 48-55 Grey (Gy - the unit of radiotherapy) via a single portal or beam. The beam is directed perpendicularly to the surface of the skin. The beam loses 50% of the dose at 6 cm of tissue depth. Treating through a single portal would result in significant (and unacceptable) side-effects and is one of the major limiting side effects of kilovoltage radiation therapy. Single portal protocols limit effective treatments to less than 3 cm below the skin.

Overcoming single beam limitations

The alternative is to treat through multiple portals or beams. Figure 2 shows the treatment of a brain tumour through a single portal. In this patient, the tumour is 6 cm below the skin surface. The optimal dose for brain tumours is 54 Gy delivered in 12-16 treatments. This dose can be easily deilivered through a multiple portal technique (in this case three portals - figure 3); two lateral opposed beams and one dorsal beam. This simple technique allows any one area of skin to receive a safe and acceptable amount of only 36 Gy while delivering the required amount to the tumour site.

A case in point: Zac, the 10 year old boxer

Zac presented having 3-6 seizures per day. He was completely collapsed because of his brain tumour and his antiseizure medications. He was treated with 54 Gy and has completely recovered. He is 1 month out from radiation therapy and is completely normal with no seizure activity. Needless to say, his owners are thrilled. Thanks to a multiple portal protocol his skin has suffered no visible side effects. A CT scan of the brain was repeated after treatment and showed a 78% decrease in tumour volume. The median survival time for patients with brain tumours treated with orthovoltage radiation therapy in this fashion is 20 months.

Further improvements

A device is currently under development which can repeatedly rotate the head through a 180 degree arc during treatment. This device allows very accurate treatment of brain tumours to avoid errors in beam orientation, reduce exposure of normal brain tissue and further reduce skin side-effects. A working prototype will be available at SARC within weeks.

A note on radiation side effects

Radiation side effects occur only in the radiation field. There are no systemic side effects. Radiation side effects are classified as acute and late. Acute effects are due to inflammation in the affected tissue and temporary cessation of skin mitosis. Such effects are quite tolerable and are typically similar to a superficial burn. So far, only one patient has had required protocol modification due to acute side-effects. No patients have required treatment to be modified or discontinued due to side effects in the skin.

Late effects are more serious and involve necrosis of bone, muscle and skin. They can occur months to years after treatment. Their incidence is related to the size of the fractions of radiation therapy. The use of daily doses of radiation as it is administered at SARC reduces the likelihood of late side effects, in contrast to weekly administration. The typical treatment for late side effects is excision of the affected tissue.

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