Updates on Canine and Feline Mammary Tumors
Hope Veterinary Specialists
40 Three Tun Road
Malvern, PA 19355 USA
Cliffdoc2000@yahoo.com
Introduction
Mammary tumors occur in both dogs and cats, however, the incidence is higher in dogs than any other
species. They are the most third most common tumor in cats and account for 17% of all tumors in female
cats. In dogs, half of the mammary tumors are considered malignant, and half of these have metastasized
at the time of diagnosis. In contrast, nearly 90% of mammary tumors in cats are malignant.
In dogs, intact females have a seven-fold increased risk of developing mammary cancer compared to
neutered females and the age at which ovariohysterectomy is performed is proportional to the risk of
developing mammary cancer. The greatest reduction in risk occurs when an OVH is performed prior to
the first heat cycle, however a protective exists still exists if performed prior to 2.5 years of age. Like dogs,
ovariohysterectomy has a protective effect against the development of mammary tumors in cats. Intact
queens have a 7-fold higher risk of developing mammary cancer than spayed female cats and
ovariohysterectomy, regardless of age, results in a 40%-60% reduced risk of developing mammary
tumors compared to intact queens. The protective effects diminish over the first few years with a risk
reduction of 91%, 86% and 11% if the cat is spayed before 6 months, between 7–12 months, or 13–24
months, respectively. Interestingly, there appears to be little to no benefit noted after the first 24 months.
Obesity may be a factor in mammary neoplasia in dogs. In cats, eexogenous progestins and the
combination of estrogen-progestins are associated with a 3-fold risk of developing either benign or
malignant mammary tumors. Benign fibroepithelial hyperplasia may also be caused by administration of
sex steroids.
Clinical Signs, Diagnosis and Clinical Staging
Mammary tumors in dogs and cats have a similar morphologic appearance to dogs. Ulceration is more
common in cats and mammary tumors frequently adhere to the overlying skin but not the underlying
abdominal wall. In dogs the caudal glands appear to be more commonly affected while in catsthere is no
site or side predilection. Multiple gland involvement is observed in more than 50% of cases. Lung and
pleural metastasis is more common in cats and can be extensive, if present, and cause respiratory
insufficiency due to pleural carcinomatosis and effusion.
Diagnosis and clinical staging includes full bloodwork, 3 view chest radiographs and lymph node
evaluation. Fine needle aspiration of the regional lymph nodes is recommended because lymph node
metastasis has been documented in normal sized and unfixed nodes, and to confirm metastasis in
painful, enlarged, and fixed lymph nodes. In cats, lymph node metastasis is present histologically in 49%
cats with mammary tumors, but only 21% of lymph nodes are clinically palpable.
Treatment
Surgery
In dogs, the extent of surgery does not influence either survival or disease-free interval . The histologic
completeness of surgical margins is prognostic for survival so the most aggressive surgery needed to
achieve complete margins is recommended.
For cats, radical bilateral mastectomy is recommended, regardless of tumor size, for the treatment of cats
with mammary tumors because of their locally aggressive behaviour and better prognosis with more
aggressive surgical resection. Bilateral mastectomy can be performed in either one or two stages. More
recent evidence suggests unilateral mastectomy alone may provide a benefit over a local/regional
surgery.
Chemotherapy
Chemotherapy is recommended for dogs and cats with malignant and metastatic mammary tumors. The
role of chemotherapy is still unknown, despite understanding the poor outcome associated with this
disease and the high incidence of metastatic disease. Drugs commonly used in protocols include 5fluorouracil+cyclophosphamide, doxorubicin alone or with cyclophosphamide, carboplatin, mitoxantrone,
paclitaxel and docetaxel. The parent compound Paclitaxel is from the Taxane group of chemotherapy
agents with efficacy against breast, lung and ovarian cancer in human oncology. Use in veterinary
medicine is limited as a result side effects associated with its excipient Cremophor® EL. Oasmia
Pharmaceutical AB has managed to produce a water soluble formulation of Paclitaxel (Paccal® Vet), that
does not require premedication and abolish Cremophor® EL related side effects. A Phase I/II study on
multiple types of cancers and showed an overall response rate of 74 % with responses noted in patients
with mammary carcinoma, squamous cell carcinoma and mast cell tumor. The FDA has given Conditional
Approval (CA) for use in patients with nonresectable stage III, IV, or V mammary carcinoma in dogs that
have not received previous chemotherapy or radiotherapy; and resectable and nonresectable squamous
cell carcinoma in dogs that have not received previous chemotherapy or radiotherapy. Two trials are
ongoing further evaluating Paccal Vet-CA1 in a larger number of dogs with mammary and squamous cell
carcinoma.
Responses of nonresectable mammary tumors to metronomic therapy and tyrosine kinase inhibitors has
been reported
In cats, multiple retrospective studies have failed to demonstrate a prolonged survival vs surgery alone.
The most commonly used agent is doxorubicin alone (1 mg/kg to 30mg/m2 every 3 weeks) or in
combination with cyclophosphamide have been described for the adjunctive treatment of cats with
malignant mammary tumors. Clearly, further research and prospective trials are warranted to determine
the true role of chemotherapy in cats with mammary tumors.
Prognosis
Prognostic factors for dogs include:
Good
Tumor < 3cm
Well-circumscribed
Lymph node negative
Lymphoid infiltration
Estrogen/progesterone
receptor positive
Carcinoma
(Well differentiated,
complex, tubular)
Proliferation indices-low
Nonulcerated
No lymphatic invasion
No vascular invasions
Low vascularity
Spay within 2 yrs of surgery
Poor
Tumor >3cm
Invasive
Lymph node positive
No infiltration
Estrogen/Progesterone
receptor negative
Sarcoma
(Poorly differentiated
simple, solid, anaplastic)
Proliferation indices-High
Ulcerated
Lymphatic invasion
Vascular invasion
High vascularity
Spay time greater than 2 years of surgery
Prognostic factors for cats include:
Good
Younger cats
DSH
Lower stage (I <2cm)
No metastasis
More aggressive surgery
Poor
Older cats
Purebreed
Higher stage (2-3cm, >3cm)
Metastasis
Less aggressive surgery
No Lymphatic invasion
No vascular invasion
Low grade (well differentiated)
Low mitotic index
Lymphatic invasion
Vascular invasion
High grade (poorly differentiated)
High mitotic index
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