Mammary Gland Tumors and Mastectomy

Mammary gland tumors (MGTs) are the most common neoplasms in intact female dogs and the third most common in cats. They arise from the epithelial and mesenchymal components of the mammary gland, with a wide spectrum of histologic types ranging from benign adenomas to highly malignant carcinomas. Mastectomy, the surgical removal of one or more mammary glands, is the primary treatment for localized disease. The surgical approach varies from simple lumpectomy to radical unilateral or bilateral mastectomy, depending on tumor size, number, location, and histologic grade. In dogs, approximately 50% of MGTs are malignant, whereas in cats, over 85% are malignant, with a high propensity for local invasion and pulmonary metastasis. The disease is hormonally influenced, with estrogen and progesterone receptors playing a role in tumorigenesis. Early ovariohysterectomy (OHE) significantly reduces the risk of developing MGTs. Surgical excision remains the cornerstone of therapy, with adjuvant chemotherapy or radiation reserved for high-grade or metastatic disease. The prognosis is highly variable, influenced by tumor size, histologic grade, lymph node involvement, and surgical margins.

The exact etiology of mammary gland tumors is multifactorial, involving genetic, hormonal, and environmental factors. Hormonal influence is paramount: prolonged exposure to ovarian hormones, particularly progesterone, stimulates mammary epithelial proliferation and increases the risk of neoplastic transformation. In dogs, OHE before the first estrus reduces the risk of MGTs to 0.5%, after one estrus to 8%, and after two estrus cycles to 26%, highlighting the protective effect of early spaying. In cats, OHE before 6 months of age reduces the risk by 91%, and before 1 year by 86%. Genetic predisposition is evident in certain breeds, with mutations in tumor suppressor genes (e.g., p53, BRCA1/BRCA2 homologs) and overexpression of oncogenes (e.g., HER2, c-erbB-2) implicated. Chronic inflammation, obesity, and dietary factors (high fat intake) may also contribute. Viral etiologies have been suggested in mice (mouse mammary tumor virus) but not confirmed in dogs or cats. Iatrogenic causes include prolonged use of progestin-based contraceptives, which increase risk. Environmental carcinogens, such as exposure to tobacco smoke or pesticides, may play a minor role. The molecular mechanisms involve dysregulation of cell cycle, apoptosis, and angiogenesis, with estrogen and progesterone receptors driving proliferation in hormone-sensitive tumors.

Mammary gland tumors are the most common neoplasms in intact female dogs, with an incidence of 3.4% in the general population, rising to 50-70% in certain breeds. The median age at diagnosis is 10-11 years in dogs and 10-12 years in cats. Breeds at increased risk in dogs include Poodles, English Springer Spaniels, Brittany Spaniels, Cocker Spaniels, German Shepherds, and Doberman Pinschers. Mixed breeds are also commonly affected. In cats, Siamese and Domestic Shorthair breeds are overrepresented. Sex predilection is strong: females are overwhelmingly affected, with rare cases in males (less than 1% of all MGTs). Intact females have a 7-fold higher risk compared to spayed females. The incidence is higher in dogs living in regions with less routine spaying. In working dogs, such as police or military dogs, the risk may be lower due to early spaying protocols. The prevalence of malignancy varies: in dogs, about 50% are malignant, while in cats, over 85% are malignant. Metastasis to regional lymph nodes (inguinal and axillary) occurs in 25-50% of malignant cases, and pulmonary metastasis in 10-25% at diagnosis. The disease is a significant cause of morbidity and mortality in geriatric female dogs and cats.

Mammary gland tumors arise from the epithelial cells lining the ducts and alveoli, as well as from myoepithelial cells and stromal fibroblasts. The pathogenesis involves a stepwise progression from hyperplasia to benign neoplasia to invasive carcinoma, driven by cumulative genetic and epigenetic alterations. Hormonal stimulation, particularly by progesterone, induces proliferation of mammary epithelium, increasing the likelihood of mutations. In dogs, malignant tumors often arise from pre-existing benign lesions, whereas in cats, de novo malignant transformation is more common. Histologically, tumors are classified into simple (one cell type) and complex (multiple cell types) types. Simple carcinomas include tubular, papillary, solid, and anaplastic subtypes. Complex carcinomas contain both epithelial and myoepithelial components. Inflammatory mammary carcinoma is a highly aggressive variant characterized by rapid growth, edema, erythema, and pain, with poor prognosis. Tumor invasion into lymphatic and blood vessels leads to regional lymph node metastasis (inguinal, axillary) and hematogenous spread to lungs, liver, and bones. The tumor microenvironment, including inflammatory cells, fibroblasts, and angiogenic factors, promotes tumor progression. Overexpression of HER2 and epidermal growth factor receptors correlates with aggressive behavior. Hormone receptor status influences response to endocrine therapy, though such therapy is less established in veterinary medicine. The systemic effects include paraneoplastic syndromes such as hypercalcemia, though rare.

Intrinsic factors include age (older animals), sex (females), breed (purebred dogs, Siamese cats), genetic predisposition (family history), hormonal status (intact females, early estrus), obesity (adipose tissue produces estrogen), and immune function. Extrinsic factors include early OHE (protective), use of progestin contraceptives, dietary high fat intake, and environmental carcinogens. Prior mammary gland disease, such as benign hyperplasia, may increase risk. In dogs, the number of estrus cycles before OHE is a critical factor. In cats, OHE before 6 months of age is highly protective. Body condition score >7 (on 9-point scale) is associated with increased risk. Exposure to tobacco smoke has been linked to increased risk in cats. Management practices, such as delayed spaying for breeding purposes, increase risk. Working dogs may have lower risk due to early spaying. The presence of other neoplasms, such as ovarian tumors, may be associated. Genetic mutations in BRCA1 and BRCA2 have been identified in some breeds, increasing susceptibility.

Clinical signs vary depending on tumor size, number, location, and malignancy. Common presentation includes palpable masses in the mammary chain, which may be single or multiple, firm or cystic, and may be fixed to underlying tissues if invasive. The caudal glands (inguinal) are most commonly affected in dogs, while in cats, all glands are equally affected. Signs of malignancy include rapid growth, ulceration, bleeding, erythema, and edema of the overlying skin. Inflammatory mammary carcinoma presents with diffuse swelling, pain, and warm skin, mimicking mastitis. Systemic signs may include lethargy, anorexia, weight loss, and dyspnea if pulmonary metastasis is present. Regional lymphadenopathy (inguinal or axillary) may be palpable. In advanced cases, lameness due to bone metastasis or neurological signs due to spinal metastasis may occur. Paraneoplastic hypercalcemia can cause polyuria, polydipsia, and gastrointestinal signs. Physical examination should include thorough palpation of all mammary glands, assessment of tumor size, consistency, mobility, and skin involvement, as well as palpation of regional lymph nodes. In cats, the disease is often more aggressive, with rapid progression and early metastasis.

Differential diagnoses for mammary gland tumors include: (1) Mammary hyperplasia (fibroadenomatous hyperplasia in cats, often associated with progestin therapy), which is non-neoplastic and may regress after withdrawal of hormones; (2) Mammary adenitis (mastitis), which presents with pain, swelling, and systemic signs, and responds to antibiotics; (3) Mammary cysts, which are fluid-filled and may be diagnosed by ultrasound; (4) Lipomas, which are soft, well-circumscribed, and benign; (5) Sebaceous adenitis or other skin tumors (e.g., mast cell tumors, histiocytomas) that may occur in the mammary region; (6) Metastatic lesions from other primary tumors (e.g., melanoma, lymphoma); (7) Inguinal lymphadenopathy due to inflammation or metastasis; (8) Hernias (inguinal or ventral) that may mimic masses; (9) Abscesses, which are painful and may have purulent discharge; (10) Foreign body granulomas. Definitive diagnosis requires fine-needle aspiration cytology or biopsy. Imaging (ultrasound, CT) can help differentiate cystic from solid masses. Histopathology is essential for grading and prognosis.

The diagnostic workup for suspected mammary gland tumors follows a systematic approach: (1) Complete history and physical examination, including thorough palpation of all mammary glands and regional lymph nodes; (2) Fine-needle aspiration (FNA) of the primary mass and any enlarged lymph nodes for cytology; (3) Three-view thoracic radiography (right lateral, left lateral, ventrodorsal) to evaluate for pulmonary metastasis; (4) Abdominal ultrasound to assess for hepatic or other abdominal metastasis and to evaluate the adrenal glands and reproductive tract; (5) Complete blood count, serum biochemistry, and urinalysis to assess overall health and detect paraneoplastic syndromes; (6) Advanced imaging (CT or MRI) if invasive disease or metastasis is suspected, particularly for surgical planning; (7) Incisional or excisional biopsy for histopathologic grading and assessment of surgical margins; (8) Sentinel lymph node mapping (using lymphoscintigraphy or dye) may be considered for accurate staging; (9) In cats, testing for feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) is recommended; (10) If inflammatory carcinoma is suspected, skin biopsy is warranted. The algorithm aims to stage the disease accurately to guide treatment and prognosis.

Laboratory findings in mammary gland tumors are often nonspecific but may reveal abnormalities associated with metastasis or paraneoplastic syndromes. Complete blood count may show anemia of chronic disease, leukocytosis with inflammation, or thrombocytopenia in disseminated intravascular coagulation. Serum biochemistry may reveal hypercalcemia (in cases of paraneoplastic hypercalcemia), elevated alkaline phosphatase (bone metastasis or cholestasis), and hypoalbuminemia (chronic disease). Urinalysis may show proteinuria or hematuria if urinary tract involvement. Coagulation panel (PT, aPTT, platelet count) is recommended before surgery, especially in cats, to assess bleeding risk. Inflammatory biomarkers such as C-reactive protein (CRP) may be elevated in inflammatory carcinoma. Tumor markers such as CA 15-3 and carcinoembryonic antigen (CEA) have been studied but are not routinely used. Hormone receptor assays (estrogen and progesterone receptors) can be performed on tumor tissue, though their prognostic value is still under investigation. In cats, FeLV and FIV testing is recommended. Synovial fluid analysis is not relevant unless joint metastasis is suspected. Overall, laboratory findings are supportive but not diagnostic.

Imaging plays a crucial role in staging and surgical planning. Thoracic radiography (three views) is the standard for detecting pulmonary metastasis, which appears as well-defined nodules, often in the caudal lung lobes. Radiographs may also reveal pleural effusion or bone metastasis. Abdominal ultrasound is used to evaluate the liver, spleen, and lymph nodes for metastasis, and to assess the reproductive tract. Mammary gland ultrasound can characterize the mass (solid vs. cystic), assess invasion into underlying tissues, and guide FNA. Computed tomography (CT) provides detailed cross-sectional images and is superior for detecting small pulmonary nodules and assessing lymph node size and invasion. CT is also useful for surgical planning, especially for complex mastectomies. Magnetic resonance imaging (MRI) is rarely used but may be helpful for evaluating chest wall invasion. Lymphoscintigraphy or contrast-enhanced ultrasound can identify sentinel lymph nodes. In cases of suspected bone metastasis, radiography or bone scintigraphy is indicated. Imaging findings guide the choice of surgical procedure and the need for adjuvant therapy.

Cytology from fine-needle aspiration of mammary masses can provide a preliminary diagnosis. Benign lesions may show clusters of uniform epithelial cells with minimal atypia, while malignant tumors exhibit cellular pleomorphism, anisocytosis, anisokaryosis, prominent nucleoli, and mitotic figures. However, cytology has limited sensitivity for distinguishing benign from malignant, and histopathology is the gold standard. Histopathologic evaluation of excised tissue is essential for grading and prognosis. In dogs, the WHO classification system is used, categorizing tumors into simple, complex, and special types. Malignant tumors are graded based on tubule formation, nuclear pleomorphism, and mitotic count (grade I, II, III). In cats, most malignant tumors are adenocarcinomas, often with a solid or tubular pattern. Histologic features such as lymphatic invasion, necrosis, and skin ulceration indicate aggressive behavior. Surgical margins are assessed for completeness of excision. Immunohistochemistry for hormone receptors, HER2, and Ki-67 proliferation index may provide additional prognostic information. Inflammatory mammary carcinoma is characterized by dermal lymphatic invasion. Accurate histopathology is critical for determining prognosis and the need for adjuvant therapy.

The primary treatment for mammary gland tumors is surgical excision. The choice of procedure depends on tumor size, number, location, and histologic grade. Options include lumpectomy (removal of the tumor only), simple mastectomy (removal of one gland), regional mastectomy (removal of the gland and adjacent glands), unilateral mastectomy (removal of the entire mammary chain), and bilateral mastectomy (removal of both chains). For small, well-circumscribed tumors (<0.5 cm), lumpectomy may be adequate. For larger or multiple tumors, regional or unilateral mastectomy is recommended. In cats, because of the high malignancy rate and aggressive local invasion, radical mastectomy (unilateral or bilateral) is often recommended. The surgical approach involves elliptical incisions around the affected glands, with careful dissection to remove the mammary tissue and associated lymphatics. The superficial inguinal lymph node is often removed with the caudal glands. Closure is performed in layers, with subcutaneous sutures and skin staples or sutures. Suture materials include absorbable monofilament (e.g., polydioxanone, polyglyconate) for deep layers and non-absorbable or absorbable for skin. Postoperative drainage may be required. Adjuvant chemotherapy (e.g., doxorubicin, carboplatin) is indicated for high-grade tumors, lymph node metastasis, or inflammatory carcinoma. Radiation therapy may be used for incompletely excised tumors. Hormonal therapy (e.g., tamoxifen) is not routinely recommended due to side effects. Palliative care for advanced disease includes pain management and nutritional support.

The prognosis for mammary gland tumors varies widely. In dogs, factors associated with a favorable prognosis include small tumor size (<3 cm), benign histology, low histologic grade, absence of lymph node metastasis, and complete surgical excision. The median survival time for dogs with malignant tumors is 1-2 years, with a 1-year survival rate of 70% and 2-year survival of 50%. For benign tumors, the prognosis is excellent with surgical cure. In cats, the prognosis is generally poorer, with a median survival of 6-12 months for malignant tumors. Factors indicating a poor prognosis include tumor size >3 cm, high histologic grade, lymphatic invasion, lymph node metastasis, and inflammatory carcinoma. Inflammatory mammary carcinoma has a very poor prognosis, with a median survival of 1-2 months despite treatment. Negative prognostic indicators include ulceration, fixation to underlying tissues, and rapid growth. The presence of pulmonary metastasis at diagnosis is associated with a median survival of less than 3 months. Early detection and surgical intervention significantly improve outcomes. Regular follow-up is essential to detect recurrence or metastasis.

Postoperative follow-up is crucial for monitoring recurrence and metastasis. Patients should be re-examined at 2 weeks for suture removal and wound assessment. Thoracic radiography should be repeated at 3, 6, 9, and 12 months postoperatively, then every 6 months thereafter for 2-3 years. Abdominal ultrasound may be performed if abdominal metastasis is suspected. Physical examination should include palpation of the surgical site and regional lymph nodes. Owners should be educated to monitor for new masses. In dogs, if the tumor was benign, follow-up may be less intensive. For malignant tumors, adjuvant chemotherapy may require regular blood work and veterinary visits. Activity restriction is recommended for 2-4 weeks postoperatively to allow wound healing. Pain management is continued as needed. Any signs of recurrence, such as new masses, ulceration, or systemic signs, warrant immediate evaluation. Long-term monitoring is recommended for at least 2 years, as late recurrence can occur.

Pearls: (1) Perform a thorough preoperative staging, including thoracic radiographs and abdominal ultrasound, to rule out metastasis. (2) In cats, consider radical mastectomy due to the high malignancy rate. (3) Use electrocautery or ligatures to control bleeding from the caudal superficial epigastric vessels. (4) Remove the superficial inguinal lymph node when excising the caudal glands. (5) Submit all excised tissue for histopathology, even if benign-appearing. (6) Consider sentinel lymph node mapping for accurate staging. (7) In inflammatory carcinoma, avoid surgery as it may exacerbate the condition; use medical therapy. Pitfalls: (1) Incomplete excision due to inadequate margins, leading to recurrence. (2) Damage to the panniculus muscle or skin necrosis from excessive tension. (3) Seroma formation due to dead space; use drains or closed suction. (4) Failure to recognize bilateral disease, requiring staged mastectomies. (5) Underestimating the aggressiveness of feline tumors. (6) Performing OHE at the time of mastectomy in cats may not improve prognosis if the tumor is already malignant. (7) Ignoring the possibility of metastasis to the inguinal lymph nodes, which should be biopsied.

Perioperative antimicrobial prophylaxis: Cefazolin 22 mg/kg IV at induction, repeated every 90 minutes during surgery, and continued for 24 hours postoperatively (q8h). For cats, cefazolin 22 mg/kg IV. Analgesia: Preoperative opioid (e.g., hydromorphone 0.05-0.1 mg/kg IV or IM in dogs; 0.05-0.1 mg/kg IV or IM in cats) or methadone (0.1-0.3 mg/kg IV or IM). Intraoperative: Fentanyl CRI at 5-10 mcg/kg/hr IV. Postoperative: NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h for dogs; meloxicam 0.1 mg/kg PO q24h for dogs, 0.05 mg/kg PO q24h for cats, with caution) or opioids (e.g., tramadol 2-5 mg/kg PO q8-12h for dogs; buprenorphine 0.01-0.02 mg/kg IV or IM q6-8h for cats). Local anesthesia: Incisional line block with bupivacaine (1-2 mg/kg) or lidocaine (1-2 mg/kg) at the surgical site. For cats, avoid NSAIDs if renal disease. Antiemetics: Maropitant 1 mg/kg SC q24h if needed. For inflammatory carcinoma, doxorubicin (30 mg/m² IV every 3 weeks for dogs; 20-25 mg/m² IV every 3 weeks for cats) or carboplatin (300 mg/m² IV every 3 weeks for dogs) may be used. Hormonal therapy is not routinely recommended. Chondroprotectants are not relevant. All dosages should be adjusted based on renal/hepatic function and patient status.

Key studies: (1) Schneider et al. (1969) demonstrated that OHE before 2.5 years of age significantly reduces the risk of mammary tumors in dogs. (2) Misdorp et al. (1999) established the WHO histologic classification and grading system for canine mammary tumors, which is widely used. (3) Sorenmo et al. (2000) found that tumor size and histologic grade are independent prognostic factors in dogs. (4) MacEwen et al. (1985) reported that adjuvant chemotherapy with doxorubicin and cyclophosphamide may prolong survival in dogs with advanced mammary carcinoma. (5) For cats, Hayes et al. (1981) showed that OHE before 1 year of age reduces the risk of mammary carcinoma. (6) Gimenez et al. (2010) evaluated the role of sentinel lymph node mapping in canine mammary tumors. (7) A meta-analysis by Sorenmo et al. (2013) confirmed the prognostic significance of histologic grade and lymph node status. (8) Recent studies have investigated the role of HER2 expression and targeted therapies. Consensus guidelines from the ACVS and ECVS recommend surgical excision as the primary treatment, with adjuvant therapy for high-risk cases. The evidence supports early spaying as a preventive measure.