Lucy the One Eyed Wonder Dog
Nancy Gustafson, DVM, MS, DACVR
Lucy is a 12.8 year old female spayed black Labrador Retriever. She had a small mass removed from the outside of her left thigh by her referring veterinarian in the spring of 2006. The pathologist reported that the mass was a grade 2 mast cell tumor (MCT) with clean, but scant margins. The cancer grew back later that year and was removed again in November 2006. The pathology report from the second surgery revealed a grade 2 MCT, with tumor cells extending to the deep margin. This means that the 2nd surgery was able to remove an adequate margin of normal tissue around the edges of the tumor, but not underneath the tumor. A 3rd surgery to obtain deeper margins was not possible due to the location of the mass on Lucy’s leg. Lucy was then referred to our Oncology service on 11/27/06 to discuss treatment options and prognosis for her grade 2 MCT.
Mast cell tumors are one of the most common skin tumors in dogs. They frequently occur on the head and extremities, which can make complete surgical removal much more challenging. Mast cells are normal white blood cells that help fight infections and allergies. They contain histamine and heparin, which are released when the cells die. These substances can make mast cell tumors appear itchy, red, and fluctuant in size and appearance, and can also cause gastrointestinal upset. We typically use anti-histamines and antacids to prevent or minimize these symptoms. Mast cell tumors have potential to spread to other organs (metastasis). The preferred organs are liver, spleen and lymph nodes. The grade of the tumor is determined by a variety of characteristics when looking at a piece of the tumor under the microscope. It helps determine the probability of metastasis as well as the probability of regrowth when the margins are not clean. Stage of disease characterizes the disease location (extent of disease). Physical exam (to assess the regional lymph nodes) and abdominal ultrasound (to assess spleen, liver and lymph nodes inside of the abdomen) are typically the standard tests to determine stage in mast cell tumors. Occasionally chest x-rays are indicated to assess the lymph nodes inside of the chest.
Mast cell tumors can be “cured” when surgery can completely remove the tumor. Complete removal is generally accepted to mean that at least a five mm margin of normal tissue is seen between the cut surface and the closet tumor cell. The tumor will regrow when surgery does not result in wide clean margins. Radiation therapy after surgery can still result in an excellent outcome. Mast cells are extremely sensitive to ionizing radiation, and studies have shown that in dogs treated with surgery followed by full course radiation, 95-99% were disease free at one year, and 85-95% were disease free at two years. Chemotherapy is typically only considered when the tumor is a high grade or metastasis is present.
Lucy was healthy, other than her tumor. All of her peripheral lymph nodes were normal size and consistency on physical exam. Staging tests (blood work and an abdominal ultrasound) were performed by her veterinarian to make sure she did not have cancer in her spleen, liver or lymph nodes inside of her abdomen. Lucy staged cleanly, therefore the only concern was the residual tumor cells in her left thigh. Her grade 2 MCT would have a greater than 50% chance of recurring because the margins were not clean. Adjuvant radiation therapy was discussed as the best chance for long-term (>2 year) tumor control, since another surgery was not an option for Lucy.
Radiation therapy is very well tolerated in dogs and the administration of the dose itself is not painful. Dogs develop a transient “burn” to the skin towards the end of treatment, which heals within 2-4 weeks and is managed with combination pain medications and a burn cream. The hair is shaved in the radiation field prior to the burn to make it easier to clean and manage. Elizabethan or “Bite Not” collars are used to prevent self-trauma, which worsens the side effect. The radiation dose is limited by the long-term side effects, which are permanent. These effects are limited to hair loss and a hair color. A short, light plane of anesthesia is required in all pets to achieve exact patient positioning for the daily treatments.
Lucy’s owner elected to pursue full course radiation therapy, which was started the day after her consultation (end of November 2006). She received 15 daily (Monday-Friday) fractions of radiation. The irradiated area included her surgical scar and a margin of surrounding normal tissue. Lucy handled the daily anesthesia well. Lucy, like all of our radiation patients, was extensively monitored while under anesthesia. She woke up very quickly from each treatment, ate her breakfast and proceeded normally with the rest of her day. Lucy was at the hospital for about 45 minutes every day while receiving radiation therapy. She started to develop redness to her skin at the 10th treatment. The burn started soon after she finished the radiation therapy. She was given oral pain medications and her owner cleaned her burn and applied a soothing cream daily. She wore a BiteNot collar while the burn was healing, which was healed within three weeks. Months later, Lucy’s skin became pigmented (black) and the hair around the edges of the radiation field grew in white instead of black. The hair in the center of the radiation field did not regrow. These changes were purely cosmetic and did not affect Lucy’s health or disposition.
Lucy was presented for recheck exams monthly for three months then every three months for the first year. Today (2.5 years after completing radiation), Lucy visits her veterinarian routinely and still does not have any tumor recurrence.