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Mammary
Gland Tumor
Dr.
Gail Mason, DVM, MA, DACVIM
Kathi
Smith, RVT, Internal Medicine & Oncology Technician
Cancer of the mammary gland in dogs and cats
is quite common. Carcinomas are the most common
type in both species, and approximately 50%
of all canine mammary tumors are malignant.
Most tumors appear in dogs between 8-10 years
of age (similar to cats). The exact etiology
of mammary cancer is unknown. It is known that
spaying a cat or dog before her first estrus
(heat) cycle dramatically decreases the risk
of mammary tumors (0-5% risk vs. 26%).
It
is general consensus among veterinary oncologists
that no mammary lump should ever be left in
place and "observed." Needle biopsies
are frequently inconclusive and misleading in
this case. Biopsy with wide excision is the
primary recommendation. These results help determine
the best treatment plan for the patient.
Tumor
size in both dogs and cats has been found to
be prognostic. Dogs with tumors less than 5
cm in diameter (cats less than 2 cm) have significantly
larger survival times than animals with larger
tumors.
Disease
Staging
As
with other cancers, it is important to assess
the extent of cancer in a particular patient.
Complete staging usually requires:
1. Complete blood count
2. Serum chemistry panel
3. Urinalysis
4. Chest radiographs (x-rays)
5. Abdominal radiographs
6. +/- ultrasonography
7. Cytology of regional lymph nodes
8. Review and assessment of biopsy results.
This
includes complete vs. incomplete resection borders
as well as cell types and degree of tumor invasiveness.
Treatment
The
traditional method of treatment for mammary
tumors in dogs and cats is surgery. Treatment
by local excision with wide and deep (at least
2 cm) margins is advised. This means that the
surgeon needs to take some healthy tissue from
around the tumor to ensure the best chance for
complete removal. Remember, even though the
incision may be quite large, it heals in the
same amount of time as a "short one."
There appears to be no advantage of a bilateral
chain masectomy (over local excision) unless
multiple mammary glands are involved. Local
and wide excision for tumors that are less than
5 cm (2 cm for cats) may be curative.
As
in human medicine, chemotherapy can be considered
for patients that have a high risk for recurrent
or metastatic disease. These patients would
include:
1. those in which complete tumor resection is
not possible
2. those having sarcomas vs. carcinomas
3. those having large and/or multiple tumors
4. those whose biopsy results indicate aggressive
disease
5. those with local or distant metstasis
Chemotherapeutic
agents that have been used as adjunct therapy
include doxorubicin, cyclophosphamide, mitoxantrone
and cisplatin. Most dogs can be treated without
serious side effects and owner acceptance of
costs and potential for complications is generally
good. Overall, anti-estrogen drugs (e.g. Tamoxifen
¨) have failed to show clinical benefit in veterinary
patients. Radiation therapy has not yet been
evaluated as a treatment modality in animals
but may hold promise as it does in humans.
Survival
times for patients with invasive carcinomas
are difficult to predict. Published medical
reports generally state ranges of 6.5-30 months
for patients on adjuvant chemotherapy.
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