LUCY
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.