Cancer in Animals

Radiation Therapy

  

Some basic concepts to consider before trying to understand radiation therapy is how tumors grow. First remember that cancer cells are not all that different from normal cells. Their cell cycle and means of growth and division are the same as normal cells- they have just lost their controls. The more aggressive the biology the more "out of control" the cells behave. A second concept important to therapy is how the tumor mass behaves as a whole. When a tumor is growing inside an animal it is important to realize that when it is very small and essentially undetectable it is a very metabolically active population of cells. Once the tumor becomes visible it is already starting to slow down in its growth and once it becomes a very large mass it is actually a very slow growing, metabolically inactive population. Radiation acts primarily 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 render it a better candidate for 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. Although radiation seems like a magical wizard it isn’t 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 is chosen based on 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 (or late responding tissues, those with little capacity for repair). Typical fractionation schemes at WSU are 3 Gy fractions, daily M-F, 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 in the cat) where the overall treatment time is shortened by treating twice a day. Also tumors deemed "incurable" due to size (the surgeons didn’t want to touch it) or metastatic potential (melanomas) can be treated with less aggressive, more course fractionated protocols because the patient is not expected to live long enough to experience the "late" side effects of radiation. At WSU these palliative protocols can be either 5 fractions of 6 Gy in a 10 day period, or 3 fractions of 8 Gy day 0,7,21- depending on the owners desires.

For some tumor types getting rid of whatever you can or "debulking" can be helpful prior to radiation for others the debulking process can just bring problems. This is why determination of tumor type and tumor imaging can be so important prior to decisions about whether surgery is indicated before starting radiation. Occasionally radiation is done prior to surgery to shrink a tumor to a size which, the surgeons can remove. Surrounding tissue sensitivity is something we usually can not change and it can at times be the dose limiting toxicity in the treatment of the tumor. Generally 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 wizards ability to cure. The goal of treatment planning is to deliver the highest possible dose to tumor, and the lowest dose to normal surrounding tissue. This may seem easy- but at times it can be quite difficult and even impossible.

Despite our best attempts at sparing normal tissues there are side effects to radiation therapy. These can generally be divided into early and late effects. Early effects happen within 3 months post-therapy, are expected, and will get better. They include hair loss, irritation of the skin caused by the cornified outer layer of the skin being absent for a time, mucositis, and conjunctivits. Symptomatic therapy and patience is generally the best treatment and care must be taken not to damage the tissues further (animal scratching, or human scrubbing). On rare occasion after radiation of nervous tissue acute edema will occur and must be treated with high doses of corticosteroids until the symptoms resolve. 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. These are serious side effects that may mean the treatment itself was done improperly.

Radiation in the post surgical patient is obviously most appropriate when local control is all that is necessary. If the tumor has been resected locally with clean margins, but will probably metastasize, chemotherapy is a better therapeutic option. Some other restrictions to radiation can include factors which make the patient unable be anesthetized (health concerns, or behavioral) or unable to tolerate the side effects of radiation. The individual tumors cell type sensitivity may play a roll but very little when speaking of a post surgery case. Surgery can often render an insensitive tumor, sensitive to radiation.

Common tumors for radiation therapy include

• Mast Cell Tumors
• Soft Tissue Sarcomas
• Oral Tumors- Acanthomaous Epulis, Squamous Cell Carcinoma, Fibrosarcoma, Melanoma
• Nasal Tumors
• Brain Tumors