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Radiotherapy to Treat Pet Cancer

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Radiogtherapy to treat pet cancer at Animal Hospital Postojna.

Radiation is a one of three pillars of veterinary oncological treatments beside surgery and chemotherapy. It can treat larger areas of tumor tissue compared to surgery.  Whole body is treated with chemotherapy and is thus appropriate for pet tumors that metastasize. Veterinary chemotherapy alone rarely has the ability to cure or control pet tumors alone and can not be used as substitute for radiation therapy. More and more dog and cat tumors nowadays are treated with so called multimodality treatment with important participation of radiotherapy. The percentage of human patients where radiotherapy is single or essential part of multimodality treatment in developed countries approaches 60%. The percentage is far smaller in veterinary oncology as there are only about 20 institutions with modern radiotherapy unit in Europe.   Here are some pet tumors that can be successfully treated with radiation therapy:
mast cell tumors, soft tissue sarcomas, nasal tumors, brain tumors, oral tumors – melanoma, squamous cell carcinoma, acanthomaous epulis, fibrosarcoma, squamous cell carcimomas of head and neck, adenocarcinoma of paraanal sacs, localized lymphomas.
It is crucial to be familiar how tumors grow before trying to understand radiation therapy.

Cancer cells are somehow not so much different from normal cells. Their cell cycle, growth and division are the same as in normal cells- they have just lost their controls. The aggressiveness of the tumor is dictated by this  "out of control"  cell behavior. Very important principle for therapy is how the tumor mass behaves as a whole. When a tumor starts to grow, it is very small and almost undetectable, but with metabolically active population of cells. As it becomes visible it is slowing down its growth and once it becomes a very large mass it is actually a very slow growing, metabolically inactive population. Radiation is most effective on rapidly dividing cell populations so tumors should be treated when they are as small as possible. We can not treat a tumor before we know it exists but many times we can remove a portion of the tumor and thereby making him more vulnerable to radiation therapy.

Radiation therapy works by the deposition of energy on or near DNA. Because the DNA is damaged cells die when they try to divide. Unfortunately  radiation is not as selective as we would like. Normal and cancer cells are killed, and only a constant proportion of cells are killed with each dose. Most radiation protocols involve several small fractions of the total dose of radiation -or fractionation- instead of one large dose. The attempt is to kill the maximum number of tumor cells and yet allow time for repair and repopulation to occur in the normal cells so that the normal cell population survives.

A total dose of radiation is determined on the base what is needed to kill the tumor, a time interval is chosen based on the tumor and the normal surrounding tissues, and the fraction size is chosen based on the tissue in the field that has the least ability to cope with large doses of radiation.

Classical schemes  are 3 Gy/ fractions, daily, 5- 6 days a week, to a total of 18-20 fractions. Some variations would include treatment of a tumor which repopulates to quickly (e.g. oral squamous cell carcinoma) where the overall treatment time is shortened by treating twice a day. Tumors considered incurable due to size or due to high metastatic potential (melanomas) can be treated with less aggressive, so called course fractionated protocols because the patient’s life expectance is not so long to see the "late" side effects of radiation. These palliative protocols consist by example  5-6 fractions of 6 Gy twice weekly, or 4 fractions of 8-9 Gy on weekly intervals.

Treating pets with cancer at Animal Hospital Postojna. Treating pets with cancer at Animal Hospital Postojna.

Occasionally radiation is done prior to surgery to reduce the volume of a tumor to a size which is operable. In principle radiation doses have to be adapted to spare neurologic tissue and bone (fraction size and total dose), but eyes, the oral cavity, and internal structures can also limit the radiation ability to cure. Planning, the process which consists of imaging and calculations, has the aim to  deliver the highest possible dose to tumor, and the lowest dose to normal surrounding tissue.

Even with most accurate and precise planning to spare normal tissues there are side effects to radiation therapy. These can generally be divided into early and late effects. Early effects happen after few weeks  post-therapy, are expected, and resolve relatively soon. They include hair loss, irritation of the skin, mucositis, and conjunctivits. Symptomatic treatment helps getting patient through this period. Late effects occur months to years after radiation and will not get better. Acceptable side affects include alopecia and hyperpigmentation of the skin and cataracts. Less acceptable effects would be nervous tissue atrophy or necrosis, bone necrosis, and skin fibrosis.

Less known, but important fact is that radiation can be very effective in pain management, typical example is extremely painful tumor – osteosarcoma, where radiation has double effect, less important antitumoral and very important analgetic. Many non-tumoral osteoarthritis and some autoimmune disases can be successfully managed with low doses of radiation with hardly any side effect.
It is important to know that animal patients have to be lightly anesthetized to be put exactly in same postion at each radiotherapy. Every patient is thoroughly examined before commencing radiotherapy to detect any anesthestic risks and we use adapted anesthetic protocols which enable animal to recover within minutes after anesthesia. We hardly found anesthesia to a a limited factor in radiotherapy.

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