Benign and Malignant Mammary Neoplasia

Mammary neoplasia in dogs and cats encompasses a heterogeneous group of benign and malignant tumors arising from the epithelial, myoepithelial, and mesenchymal components of the mammary gland. In the bitch, the mammary glands are hormonally responsive organs that undergo cyclical proliferation and differentiation under the influence of estrogen and progesterone during each estrous cycle. Mammary tumors are the most common neoplasms in intact female dogs, with an incidence of approximately 3.4% in the general population, and are the third most common neoplasm in cats. The disease is predominantly hormone-dependent, with a strong association with ovarian steroid hormones, particularly progesterone, which promotes tumorigenesis through the induction of growth factors and progesterone receptors. In cats, mammary tumors are more frequently malignant (85-90%) and are often highly aggressive, with a high metastatic potential to regional lymph nodes and lungs. The clinical presentation varies from solitary, well-circumscribed nodules to multiple, ulcerated, and fixed masses. Accurate classification, staging, and histopathological grading are essential for prognosis and therapeutic planning. The World Health Organization (WHO) classification system categorizes mammary tumors into benign (e.g., adenomas, fibroadenomas, benign mixed tumors) and malignant (e.g., carcinomas, sarcomas, carcinosarcomas) types, with further subtyping based on histological features and invasion. In the context of theriogenology, mammary neoplasia is a significant reproductive disease because of its strong hormonal etiology, impact on breeding decisions, and the need for surgical intervention (mastectomy) that may affect future lactation and breeding capability.

The etiology of mammary neoplasia is multifactorial, involving hormonal, genetic, environmental, and dietary factors. The primary hormonal drivers are estrogen and progesterone, which are produced by the ovaries during the estrous cycle. Progesterone, in particular, is a potent mitogen for mammary epithelium, and its continuous exposure through repeated estrous cycles increases the risk of tumor development. In dogs, the risk of mammary tumors is significantly reduced by ovariohysterectomy (OHE) before the first estrus (0.5% risk) compared to after the first estrus (8% risk) and after the second estrus (26% risk). This protective effect is attributed to the removal of the source of progesterone and estrogen. In cats, the risk is also reduced by early spaying, with a 91% reduction in risk if OHE is performed before 6 months of age. Genetic factors play a role, with certain breeds being predisposed, such as Poodles, English Springer Spaniels, Brittany Spaniels, and German Shepherds in dogs, and Siamese cats in felines. Specific genetic mutations, such as alterations in the p53 tumor suppressor gene, HER-2/neu oncogene amplification, and BRCA1/BRCA2 mutations, have been implicated in mammary carcinogenesis. Environmental factors, including exposure to exogenous hormones (e.g., progestin-based contraceptives), obesity, and high-fat diets, have been associated with increased risk. Additionally, chronic inflammation and oxidative stress may contribute to malignant transformation. In cats, feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) infections have been suggested as potential cofactors, although their direct role is not fully established. The molecular mechanisms involve dysregulation of cell proliferation, apoptosis, angiogenesis, and invasion, driven by steroid hormone receptor signaling pathways, growth factors (e.g., epidermal growth factor, insulin-like growth factor), and cyclooxygenase-2 (COX-2) expression.

Mammary neoplasia is the most common tumor in intact female dogs, with an incidence of 198 per 100,000 dog-years. The median age at diagnosis is 10-11 years, with a range of 4-15 years. The risk increases with age, and most tumors occur in dogs that have not been spayed or were spayed after 2.5 years of age. Breed predispositions include Poodles, English Springer Spaniels, Brittany Spaniels, Cocker Spaniels, German Shepherds, and Doberman Pinschers. In cats, mammary tumors are the third most common neoplasm, with an incidence of 25.4 per 100,000 cat-years. The median age is 10-12 years, and Siamese and other Oriental breeds are overrepresented. The vast majority of feline mammary tumors (85-90%) are malignant, whereas in dogs, approximately 50% are benign and 50% are malignant. The risk of malignancy is higher in dogs with multiple tumors and in those with tumors in the caudal glands (inguinal and abdominal). Parity and breeding status influence risk: nulliparous dogs and cats have a higher risk than parous animals, and the risk increases with the number of estrous cycles. The use of progestin-based contraceptives (e.g., megestrol acetate) has been associated with an increased risk of mammary tumors in both dogs and cats. The overall prevalence of mammary tumors in dogs is 3.4%, and in cats, it is 2.5%. The incidence of malignant tumors in dogs is approximately 50%, with a 5-year survival rate of 50% for malignant tumors. In cats, the 1-year survival rate for malignant tumors is less than 50%, and the median survival time is 12 months.

The pathophysiology of mammary neoplasia involves a complex interplay of hormonal stimulation, genetic mutations, and cellular proliferation. In the normal mammary gland, estrogen and progesterone regulate ductal and lobuloalveolar development. Estrogen promotes ductal growth, while progesterone stimulates lobuloalveolar proliferation. These hormones act through their respective nuclear receptors (ER and PR), which are expressed in mammary epithelial cells. In tumorigenesis, prolonged exposure to progesterone, especially during the luteal phase of the estrous cycle, leads to increased expression of growth hormone (GH) and insulin-like growth factor-1 (IGF-1) in the mammary gland, which stimulate cell proliferation. Progesterone also induces the expression of the Wnt signaling pathway and other mitogenic factors. Malignant transformation occurs when there is an accumulation of genetic mutations that dysregulate cell cycle control, apoptosis, and DNA repair. Key mutations include activation of oncogenes (e.g., HER-2/neu, c-myc, ras) and inactivation of tumor suppressor genes (e.g., p53, BRCA1/BRCA2). These mutations lead to uncontrolled cell growth, invasion of surrounding tissues, and metastasis. In dogs, approximately 50% of malignant tumors are hormone receptor-positive, and the expression of ER and PR is associated with a better prognosis. In cats, most tumors are hormone receptor-negative, which may explain their more aggressive behavior. The metastatic spread occurs primarily via the lymphatic system to regional lymph nodes (axillary and inguinal) and via the bloodstream to the lungs, liver, and other organs. The tumor microenvironment, including stromal fibroblasts, inflammatory cells, and angiogenesis, plays a crucial role in tumor progression. COX-2 is overexpressed in many canine and feline mammary tumors, contributing to inflammation, angiogenesis, and invasion.

Predisposing factors for mammary neoplasia include intrinsic and extrinsic factors. Intrinsic factors include age, breed, sex, and hormonal status. Age is a significant risk factor, with the incidence increasing dramatically after 6 years of age. Breed predispositions are well-documented, with purebred dogs and cats having a higher risk than mixed breeds. In dogs, breeds such as Poodles, English Springer Spaniels, Brittany Spaniels, and German Shepherds are at increased risk. In cats, Siamese and other Oriental breeds are predisposed. Sex is a factor, as mammary tumors are rare in males, accounting for less than 1% of cases. Hormonal status is critical: intact females have a significantly higher risk than spayed females, and the risk is inversely related to the age at OHE. OHE before the first estrus reduces the risk to 0.5%, after the first estrus to 8%, and after the second estrus to 26%. Nulliparity is a risk factor, as parous animals have a lower risk. Extrinsic factors include exogenous hormone administration, particularly progestins used for estrus suppression or contraception. Megestrol acetate and other progestins have been shown to increase the risk of mammary tumors in both dogs and cats. Obesity and high-fat diets have been associated with an increased risk, possibly due to increased estrogen production in adipose tissue. Environmental factors, such as exposure to carcinogens and radiation, may also contribute. Additionally, chronic inflammation and immune suppression may play a role in tumor development.

Clinical signs of mammary neoplasia vary depending on the tumor type, size, and stage. In dogs, the most common presentation is one or more palpable masses in the mammary glands, which may be firm, nodular, and well-circumscribed (benign) or irregular, fixed to underlying tissues, and ulcerated (malignant). The caudal glands (inguinal and abdominal) are more frequently affected than the cranial glands. The masses may be asymptomatic initially, but as they grow, they can cause discomfort, ulceration, bleeding, and secondary infection. In advanced cases, there may be systemic signs such as lethargy, anorexia, weight loss, and dyspnea if pulmonary metastasis is present. In cats, mammary tumors are often more aggressive, with rapid growth, ulceration, and invasion of the skin. The tumors may be multiple and involve multiple glands. Regional lymph node enlargement (axillary or inguinal) may be palpable. In both species, the presence of ulceration, fixation to the skin or underlying tissues, and rapid growth are suggestive of malignancy. In male dogs, mammary tumors are rare but may present as a single mass, often in the inguinal region. The clinical signs may also include pain, lameness if the tumor is large, and systemic signs of metastasis. It is important to note that some benign tumors, such as fibroadenomas, may be soft and cystic, while malignant tumors are typically firm and irregular.

Differential diagnoses for mammary neoplasia include: 1) Mammary hyperplasia (fibroadenomatous hyperplasia in cats, especially young intact females, which is a benign, hormone-induced proliferation of the mammary gland that can be distinguished by its rapid growth and association with progestin administration or estrous cycle; it is typically non-invasive and regresses after ovariohysterectomy or cessation of progestins). 2) Mammary adenitis (mastitis), which is an inflammatory condition of the mammary gland, usually due to bacterial infection, presenting with swelling, pain, erythema, and purulent discharge; it is more common in lactating animals and is distinguished by cytology showing neutrophils and bacteria, and by response to antibiotics). 3) Mammary gland cysts, which are fluid-filled cavities that may be palpable as soft, fluctuant masses; they are often benign and can be diagnosed by ultrasound or fine-needle aspiration showing fluid). 4) Skin tumors (e.g., mast cell tumors, lipomas, sebaceous adenomas) that may be located over the mammary glands; they can be differentiated by cytology and histopathology). 5) Subcutaneous abscesses or granulomas, which may be secondary to trauma or foreign bodies; they are typically painful and may have a draining tract). 6) Metastatic neoplasia from other sites (e.g., lymphoma, melanoma) that can present as mammary masses; histopathology is essential for differentiation). 7) In male dogs, prostatic disease (e.g., prostatic carcinoma) can occasionally metastasize to the inguinal region and mimic mammary tumors; imaging and biopsy are needed). 8) Ectopic mammary tissue or supernumerary nipples, which are congenital anomalies that may develop tumors). 9) In cats, feline mammary fibroadenomatous hyperplasia is a key differential, especially in young intact females or those receiving progestins; it is characterized by rapid, diffuse enlargement of multiple glands, and is distinguished by histopathology showing benign proliferation of ducts and stroma). 10) Inflammatory mammary carcinoma, a rare and aggressive form of mammary cancer that presents with diffuse, erythematous, painful swelling of the mammary glands, often with edema and peau d'orange appearance; it is highly malignant and has a poor prognosis).

The diagnostic algorithm for mammary neoplasia begins with a thorough history and physical examination, including palpation of all mammary glands and regional lymph nodes. The following steps are recommended: 1) Clinical staging: Assess the size, number, location, and characteristics of the masses (e.g., mobility, ulceration, fixation). Palpate the axillary and inguinal lymph nodes for enlargement. 2) Fine-needle aspiration (FNA) of the mass and any enlarged lymph nodes for cytology. Cytology can help differentiate between benign and malignant epithelial cells, but it has limitations in distinguishing between hyperplasia and well-differentiated tumors. 3) Complete blood count (CBC), serum biochemistry profile, and urinalysis to assess overall health and detect any systemic effects. 4) Thoracic radiographs (three views: right lateral, left lateral, and ventrodorsal) to evaluate for pulmonary metastasis. 5) Abdominal ultrasonography to assess the liver, spleen, and abdominal lymph nodes for metastasis, and to evaluate the ovaries and uterus for concurrent reproductive pathology. 6) If the mass is cystic, ultrasound-guided aspiration may be performed. 7) Definitive diagnosis requires histopathological examination of the excised mass (surgical biopsy or excisional biopsy). The tumor should be submitted for histopathology with evaluation of surgical margins, histological type, grade, and presence of lymphatic or vascular invasion. 8) Immunohistochemistry (IHC) for hormone receptors (ER, PR), HER-2/neu, and Ki-67 proliferation index may be performed to guide prognosis and treatment. 9) In cases of suspected inflammatory carcinoma, a skin biopsy may be needed. 10) Staging based on the WHO TNM system (Tumor, Node, Metastasis) should be performed to determine the prognosis and treatment plan.

Laboratory findings in mammary neoplasia are often non-specific but may reflect systemic effects of malignancy or concurrent disease. Complete blood count (CBC) may show anemia of chronic disease, leukocytosis with a left shift if there is secondary infection or inflammation, and thrombocytopenia in cases of disseminated intravascular coagulation (DIC). Serum biochemistry may reveal hypercalcemia, which is a paraneoplastic syndrome associated with some malignant tumors, particularly in dogs. Hypercalcemia can lead to polyuria, polydipsia, and renal dysfunction. Liver enzyme elevations (ALT, ALP) may indicate hepatic metastasis. Azotemia may be present if there is renal involvement. In cats, hyperglobulinemia may be seen. Urinalysis may show proteinuria or hematuria if there is urinary tract involvement. Hormonal assays may be performed to assess ovarian function, but they are not routinely used for diagnosis. Serum progesterone and estrogen levels may be measured to evaluate the hormonal milieu, especially in intact females. In cases of suspected hormone-responsive tumors, measurement of ER and PR on the tumor tissue is more informative. Vaginal cytology is not directly relevant to mammary neoplasia but may be performed to assess the estrous cycle stage if the animal is intact. Cytological evaluation of fine-needle aspirates from the mammary mass may show clusters of epithelial cells with variable atypia, and the presence of inflammatory cells if there is secondary infection. Histopathology is the gold standard for diagnosis and provides detailed information on tumor type, grade, and invasion.

Imaging plays a crucial role in the staging and management of mammary neoplasia. Thoracic radiography is essential to detect pulmonary metastasis, which is the most common site of distant spread. Three views (right lateral, left lateral, and ventrodorsal) are recommended to maximize sensitivity. Metastatic lesions typically appear as well-circumscribed, soft-tissue nodules in the lung fields, often in the caudal lobes. In advanced cases, a miliary pattern may be seen. Abdominal ultrasonography is used to evaluate the liver, spleen, and abdominal lymph nodes for metastasis. The liver may show hypoechoic or hyperechoic nodules, and the lymph nodes may be enlarged and hypoechoic. Ultrasonography of the mammary glands themselves can help characterize the masses (e.g., solid vs. cystic, well-circumscribed vs. invasive) and guide fine-needle aspiration. Mammary tumors typically appear as hypoechoic masses with irregular borders if malignant. Color Doppler ultrasound can assess vascularity, which may be increased in malignant tumors. Computed tomography (CT) and magnetic resonance imaging (MRI) are advanced imaging modalities that provide more detailed anatomical information and are particularly useful for surgical planning and detecting lymph node metastasis. CT is superior for evaluating the thorax and abdomen, while MRI provides excellent soft-tissue contrast for assessing tumor invasion into surrounding tissues. Lymphoscintigraphy or sentinel lymph node mapping may be used to identify the regional lymph node for biopsy. In cats, thoracic radiography is especially important due to the high metastatic rate. Imaging findings should be correlated with cytology and histopathology for accurate staging.

Cytology and histopathology are essential for the diagnosis and prognosis of mammary neoplasia. Fine-needle aspiration (FNA) cytology can be performed on palpable masses and enlarged lymph nodes. Cytological features of benign tumors include cohesive clusters of uniform epithelial cells with minimal atypia, while malignant tumors show cellular pleomorphism, anisocytosis, anisokaryosis, prominent nucleoli, and mitotic figures. However, cytology has limitations, as it cannot reliably distinguish between hyperplasia, adenoma, and well-differentiated carcinoma. Histopathology is the gold standard. The excised mass should be fixed in 10% neutral buffered formalin and submitted for evaluation. The pathologist will assess the tumor type, histological grade, surgical margins, and presence of lymphatic or vascular invasion. The WHO classification for canine mammary tumors includes: benign tumors (adenoma, fibroadenoma, benign mixed tumor, ductal papilloma) and malignant tumors (carcinoma in situ, simple carcinoma, complex carcinoma, solid carcinoma, anaplastic carcinoma, carcinosarcoma, sarcoma). Histological grading (e.g., the Elston-Ellis modification of the Scarff-Bloom-Richardson system) is based on tubule formation, nuclear pleomorphism, and mitotic count. In cats, most tumors are adenocarcinomas, which are graded similarly. Immunohistochemistry (IHC) can be performed to assess hormone receptor status (ER, PR), HER-2/neu expression, and Ki-67 proliferation index. Hormone receptor-positive tumors are more likely to respond to hormonal therapy and have a better prognosis. HER-2/neu overexpression is associated with aggressive behavior. Ki-67 index correlates with tumor grade and prognosis. Special stains, such as cytokeratin and vimentin, can help differentiate epithelial from mesenchymal tumors.

The treatment of mammary neoplasia depends on the tumor type, stage, and the patient's overall health. The primary treatment is surgical excision. For benign tumors, a simple lumpectomy or mastectomy may be curative. For malignant tumors, a more aggressive surgical approach is recommended, including regional mastectomy (removal of the affected gland and adjacent glands) or radical mastectomy (removal of all mammary glands on the affected side). The choice of surgery depends on the size and location of the tumor, and the presence of multiple tumors. In dogs, a unilateral or bilateral radical mastectomy is often performed for malignant tumors, as it reduces the risk of local recurrence. In cats, a bilateral radical mastectomy is commonly recommended due to the high rate of malignancy and the tendency for multiple gland involvement. Ovariohysterectomy (OHE) is recommended as part of the treatment for intact females, as it removes the source of estrogen and progesterone, which may slow the growth of hormone-responsive tumors. However, OHE is not a substitute for surgical excision of the tumor. Adjuvant therapy may be recommended for malignant tumors, especially those with poor prognostic factors (e.g., high grade, lymph node metastasis, vascular invasion). Chemotherapy with doxorubicin, carboplatin, or mitoxantrone may be used, but the response rates are variable. In cats, doxorubicin-based protocols have shown some benefit. Radiation therapy may be used for local control in cases of incompletely excised tumors or for palliative treatment of inoperable masses. Hormonal therapy with anti-estrogens (e.g., tamoxifen) or aromatase inhibitors has been explored, but their efficacy is limited and they may have side effects. COX-2 inhibitors (e.g., piroxicam) have been used in dogs with malignant tumors, as they may have anti-tumor effects. Supportive care includes pain management, antibiotics for secondary infections, and nutritional support. The prognosis is guarded for malignant tumors, especially in cats, and the owner should be counseled accordingly.

The prognosis for mammary neoplasia varies widely depending on the tumor type, stage, and treatment. In dogs, benign tumors have an excellent prognosis after surgical excision, with a low rate of recurrence. Malignant tumors have a more guarded prognosis, with a 5-year survival rate of approximately 50%. Factors associated with a worse prognosis include tumor size >3 cm, lymph node metastasis, distant metastasis, high histological grade, vascular invasion, and negative hormone receptor status. In cats, the prognosis is generally poor, with a median survival time of 12 months for malignant tumors. The 1-year survival rate is less than 50%. Factors associated with a better prognosis in cats include tumor size <2 cm, well-differentiated histology, and absence of lymph node metastasis. Early detection and surgical excision with clean margins offer the best chance for long-term survival. The role of OHE in improving prognosis is controversial, but it is generally recommended for intact females. Adjuvant chemotherapy may prolong survival in some cases, but the overall response rates are low. The prognosis for inflammatory mammary carcinoma is grave, with a median survival time of 1-2 months. In male dogs, mammary tumors are rare but tend to be malignant and have a poor prognosis. Overall, the prognosis is better for dogs than for cats, and for benign tumors than for malignant tumors.

Follow-up care for patients with mammary neoplasia is essential to monitor for recurrence and metastasis. After surgical excision, the patient should be re-examined every 1-3 months for the first year, then every 6 months thereafter. Physical examination should include palpation of the surgical site and regional lymph nodes. Thoracic radiographs should be repeated every 3-6 months for the first 2 years, then annually, to detect pulmonary metastasis. Abdominal ultrasonography may be performed if there is suspicion of abdominal metastasis. For patients receiving chemotherapy, regular CBC and biochemistry panels are needed to monitor for toxicity. For patients with hormone receptor-positive tumors, serial hormone receptor testing is not typically repeated, but the patient's reproductive status should be managed. If the patient is intact, OHE is recommended. For breeding animals, the owner should be advised that the risk of recurrence and metastasis may be increased if the animal is bred, and that the tumor may affect lactation. In cases of benign tumors, the prognosis is excellent, but the patient should still be monitored for the development of new tumors. In cases of malignant tumors, the owner should be informed of the signs of metastasis, such as coughing, dyspnea, lethargy, and weight loss, and should seek immediate veterinary attention if these occur. The follow-up schedule should be tailored to the individual patient's risk factors and tumor characteristics.

Clinical pearls: 1) Ovariohysterectomy before the first estrus is the most effective preventive measure for mammary neoplasia in dogs and cats. 2) In dogs, the risk of malignancy is higher in tumors that are >3 cm in diameter, fixed to underlying tissues, or ulcerated. 3) In cats, any mammary mass should be considered malignant until proven otherwise, and a bilateral radical mastectomy is often recommended. 4) Fine-needle aspiration cytology is useful for differentiating between benign and malignant tumors, but histopathology is required for definitive diagnosis and grading. 5) Thoracic radiographs are essential for staging, as pulmonary metastasis is common in malignant tumors. 6) Hormone receptor status (ER/PR) can guide prognosis and treatment; receptor-positive tumors have a better prognosis and may respond to hormonal therapy. 7) COX-2 inhibitors may have a role in the treatment of malignant tumors, especially in dogs. 8) Inflammatory mammary carcinoma is a highly aggressive form that presents with diffuse, erythematous, painful swelling of the mammary glands; it has a very poor prognosis. Pitfalls: 1) Failure to perform a thorough staging workup (including thoracic radiographs and abdominal ultrasound) before surgery can lead to incomplete treatment and poor outcomes. 2) Incomplete surgical excision (positive margins) is a common cause of local recurrence; therefore, wide surgical margins are essential. 3) Relying solely on cytology to determine the benign or malignant nature of a tumor can be misleading; histopathology is mandatory. 4) Delaying surgery in intact females may allow the tumor to progress and metastasize. 5) In cats, conservative surgery (lumpectomy) is associated with a high rate of recurrence; a more aggressive approach is recommended. 6) Not recommending OHE in intact females with mammary tumors may miss the opportunity to reduce hormonal stimulation and prevent future tumors. 7) Overlooking the possibility of metastasis to regional lymph nodes; lymph node biopsy or aspiration should be performed if enlarged. 8) Failing to consider the possibility of inflammatory mammary carcinoma, which requires a different treatment approach (medical management rather than surgery).

Current drug protocols for mammary neoplasia are primarily focused on adjuvant chemotherapy and supportive care. In dogs, the most commonly used chemotherapeutic agents are doxorubicin and carboplatin. Doxorubicin is administered at a dose of 30 mg/m² IV every 3 weeks for 4-6 cycles. Carboplatin is given at a dose of 300 mg/m² IV every 3 weeks for 4 cycles. These protocols are used for malignant tumors with high-grade features or metastasis. In cats, doxorubicin is used at a dose of 25 mg/m² IV every 3 weeks for 4-6 cycles, but it is important to monitor for nephrotoxicity and cardiotoxicity. Mitoxantrone (6.5 mg/m² IV every 3 weeks) is an alternative. COX-2 inhibitors, such as piroxicam (0.3 mg/kg PO every 24 hours) or firocoxib (5 mg/kg PO every 24 hours), may be used in dogs with malignant tumors, as they have been shown to have anti-tumor effects and may improve survival. In cats, meloxicam (0.1 mg/kg PO every 24 hours) may be used, but caution is needed due to renal toxicity. Hormonal therapy with tamoxifen (1 mg/kg PO every 24 hours) has been used in dogs, but its efficacy is limited and it can cause side effects such as pyometra and vulvar swelling. Aromatase inhibitors (e.g., anastrozole) are not commonly used. Supportive care includes pain management with NSAIDs or opioids, antibiotics for secondary infections (e.g., amoxicillin-clavulanate 12.5-25 mg/kg PO every 12 hours), and nutritional support. For inflammatory mammary carcinoma, a combination of piroxicam and toceranib phosphate (Palladia) has been used in dogs, but the prognosis remains poor. It is important to note that chemotherapy protocols should be tailored to the individual patient, and consultation with a veterinary oncologist is recommended.

Evidence-based literature on mammary neoplasia in dogs and cats is extensive. Key studies include: 1) The landmark study by Schneider et al. (1969) demonstrated that ovariohysterectomy before the first estrus reduces the risk of mammary tumors to 0.5%, after the first estrus to 8%, and after the second estrus to 26%. 2) A study by Misdorp et al. (1999) established the WHO histological classification of mammary tumors in dogs and cats, which is still widely used. 3) A meta-analysis by Sorenmo et al. (2009) confirmed that tumor size, lymph node status, and histological grade are significant prognostic factors in dogs. 4) A study by MacEwen et al. (1982) showed that adjuvant chemotherapy with doxorubicin and cyclophosphamide did not significantly improve survival in dogs with malignant mammary tumors, but more recent studies have shown a benefit in certain subsets. 5) A study by Novosad et al. (2003) evaluated the use of COX-2 inhibitors in dogs with mammary tumors and found that piroxicam had anti-tumor activity. 6) In cats, a study by Hayes et al. (1981) demonstrated that ovariohysterectomy before 6 months of age reduces the risk of mammary tumors by 91%. 7) A study by Castagnaro et al. (1998) evaluated the prognostic value of hormone receptor status in feline mammary tumors and found that ER-negative tumors have a worse prognosis. 8) A study by Zappulli et al. (2015) provided a comprehensive review of feline mammary tumors, highlighting the aggressive nature and the importance of early surgical intervention. 9) The BSAVA Manual of Canine and Feline Reproduction and Neonatology (2011) provides guidelines for the management of mammary tumors in breeding animals. 10) The American College of Veterinary Surgeons (ACVS) has published consensus statements on the surgical management of mammary tumors. Overall, the evidence supports early spaying, aggressive surgical excision, and the use of adjuvant chemotherapy for high-risk tumors, but the prognosis for malignant tumors, especially in cats, remains guarded.