Because feline mammary tumors generally behave aggressively, identifying HER2 overexpression in these tumors may account for
the poor prognosis associated with this tumor type. Recent investigations have demonstrated HER2 expression to be low (25%)
in benign mammary tumors and absent in nontumor samples.38 In contrast, 41% to 90% of feline mammary carcinomas express HER2.22,23,38
In addition to its identification in mammary carcinomas, HER2 expression appears prognostic for survival time. Queens with
greater HER2 expression treated with surgery alone had a shorter median survival time (14.6 months) than HER2-negative queens
(18.7 months). Interestingly, HER2 was not correlated with histologic subtype, tumor grade, or lymphatic invasion, suggesting
that HER2 expression may serve as a prognostic variable independent of other histologic criteria in cats with mammary tumors.23
CLINICAL SIGNS
Figure 2. A geriatric female cat suffering from painful and ulcerative bilateral mammary gland carcinoma. (Photo courtesy
of Craig Clifford.)
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Cats with mammary neoplasia often present with multiple tumors, and bilateral mammary gland involvement may be identified
in up to 40% of patients at diagnosis.4,8 Bleeding and ulceration of the affected mammary glands may be observed in 18% to 25% of cats with large or long-standing
tumors (Figure 2).4,7,14 Pet owners may note nonspecific signs such as weight loss, inappetence, and lethargy. Less commonly, exercise intolerance,
dyspnea, or cyanosis caused by diffuse pulmonary metastasis may be the presenting complaint.15
DIAGNOSIS AND STAGING
Figure 3. A young pregnant queen with benign fibroepithelial hyperplasia, presenting with massive bilateral mammary gland
enlargement, pain, and ulceration. This condition is often mistaken for mammary cancer.
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A palpable mass that is not freely movable or attached to underlying structures and is associated with the mammary chain is
highly suggestive of a mammary tumor; however, your differential diagnoses should include mastitis and mammary fibroepithelial
hyperplasia, a benign condition in young, cycling females or animals receiving progestin therapy (Figure 3). Mammary fibroepithelial hyperplasia can result in marked nonpainful enlargement of all mammary tissue and is treated effectively
with ovariohysterectomy or discontinuation of progestin therapy.
Figure 4. Cytologic examination of pleural effusion collected from a female cat with mammary adenocarcinoma. Carcinoma cells
are characterized by ballooning cytoplasm (Wrights-Giemsa stain; 500X).
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Obtain a thorough history, including information regarding the duration of clinical signs, rate of growth, spay status, age
at ovariohysterectomy, and the use of progestins. During the physical examination, identify the tumor size and location, number
of affected glands, evidence of tumor ulceration and fixation to underlying tissues, lymph node enlargement, and evidence
of distant metastasis.21
Make a definitive diagnosis with fine-needle aspiration biopsy and cytology (Figure 4) or incisional biopsy. Cytologic examination may yield false negative results, so perform a histologic examination to confirm
malignancy.
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