Nasal tumors are one of the most challenging tumors to treat in dogs and cats. These tumors usually fill up one or both sides of the entire nasal cavity and sinuses. In the skull of a dog or cat, this means that the tumor often wraps around the eyes and the brain and it usually lies just above the palate. Therefore, when treating these patients with radiation therapy in the past, we typically needed to include a great deal of the oral mucosa, the eyes and the brain. In most dogs, treating these areas with a definitive dose of radiation usually results in moderate to severe short-term side effects which can last for up to two weeks after the last radiation treatment. Most dogs, and some cats, treated in this manner are in a significant degree of pain or discomfort during this time. With recent advances in pain medications it is possible to get these patients through their treatment, but for many pet owners and their animals treating a nasal tumor has been a very difficult process and many animals may not get treated because of these side effects.
With the advent of newer radiation technologies this has changed. Intensity Modulated Radiation Therapy (IMRT) is a newer radiation technique that is becoming available in veterinary medicine to treat difficult tumors like nasal tumors. With IMRT the patient is positioned in a bite-block or similar positioning device for their radiation planning CT and every day for their radiation treatment. This is crucial with IMRT to minimize variation in their day to day set up. Using a three dimensional treatment planning computer radiation beams are aimed at the tumor from a number of different directions so that the radiation can approach from all sides. The radiation planning is done using what is known as inverse planning. Instead of telling the computer what beams to use for treatment, the radiation oncologist tells the computer what dose they want to deliver to the tumor as well as the maximum dose allowed to be delivered to the normal tissues. The treatment planning computer then determines how to deliver the best dose to the tumor while blocking the critical normal tissues. It does this by taking advantage of multiple leaves up in the head of the radiation machine which are used to deliver different doses of radiation to different parts of the field.
For dogs with nasal tumors this means that the radiation can be directed to the tumor and the dose to the normal tissues is minimal. Dogs who are being treated with IMRT usually have small areas of ulceration in their mouths and the skin on their nose, but most of the normal tissues have only mild changes. Also, although there is limited data so far, the risk of long term side effects such as cataracts will likely be low.
Dr. John Farrelly - The Veterinary Cancer Center