Mammary Gland Tumors in Dogs
Key takeaways
Mammary gland tumors (MGTs) result from the uncontrolled growth of mammary gland (breast) cells, which group together to form lumps in the mammary glands.
- MGTs vary in size, appearance, location, and density
- MGT is not a specific diagnosis, but describes all tumors found in the mammary glands
- Any dog can develop MGTs but older, intact females are predisposed
- The exact cause of MGTs is unclear but they are linked to the presence of sex hormones
- Sexual alteration (spay/neuter) at a young age has a protective effect
- A specific diagnosis is made with biopsy results; diagnostic imaging assesses the internal organs for tumor spread
- Masses may be benign and require no treatment
- Cancerous masses may be treated surgically with or without chemotherapy
- Survival times vary according to the severity of metastasis and the aggressiveness of cancer treatment
- Prompt veterinary treatment should be sought on discovery of MGTs
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A closer look: Mammary Gland Tumors in Dogs
Prognosis varies due to type of MGT. Benign fibroadenomas do not usually require treatment and carry a good prognosis. Inflammatory metastatic mammary gland carcinoma is a rare, but rapidly fatal condition.
MGTs vary in how they feel. Some lumps are solitary and firm whereas others feel like gravel under the skin. 70% of MGTs occur in the rear two glands. While MGTs are rarely an emergency, prompt veterinary treatment is recommended if a tumor or mass is suspected.
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Risk factors
MGTs are a common presentation in dogs, but the incidence varies depending on sex, age, neutering status, and breed. MGTs are rare in male dogs, whereas unspayed female dogs over 8 years old commonly develop tumors.
Possible causes
The cause of MGTs is unknown, however, the presence of sex hormones in female dogs is a predisposing factor.
Main symptoms
Symptoms of MGTs are localized to the nipple area and surrounding mammary tissue.
Testing and diagnosis
Investigation of MGTs involves a definitive diagnosis of the lump. A biopsy is taken of the mass and the local lymph node(s). Chest X-rays and abdominal ultrasound assess for metastasis. This information allows for staging of the MGT.
Steps to Recovery
Treatment normally involves surgical excision. Local excision removes the lump and the associated mammary gland. Where multiple lumps are present, a mammary strip removes all the mammary tissue on one side of the patient. This procedure is sometimes repeated on the other side once healing from the first surgery is complete.
In cases where the lump has spread internally, or was not completely excised, follow-up chemotherapy is recommended.
Sexual alteration (spay/neuter) is recommended to prevent continued growth of any cancerous tissue left behind or recurrence.
Duration and outcome vary significantly depending on the tumor type. A simple fibroadenoma usually requires no further treatment. A single adenocarcinoma that has not spread is sometimes cured with mastectomy alone. This scenario has an excellent prognosis, although new tumors are common. Multiple tumors that have already metastasized to other organs carry a less favorable prognosis and require extensive surgical, radiation, and/or pharmaceutical treatment, which can last for weeks or months. More complex MGTs are often life-shortening conditions.
Prevention
The incidence of MGTs is significantly reduced by early spaying. The protective effect is most significant prior to the first heat cycle (estrus), and the effect reduces with each subsequent season.
- Prior to first season = 95.5% reduction in risk
- Prior to second season = 92% reduction in risk
- Prior to third season = 74% reduction in risk
- Protective effect is lost after the fourth season
Are Mammary Gland Tumors in Dogs common?
MGTs are common. They account for 70% of tumors in unspayed female dogs over 8 years old. In the general dog population, MGTs account for 25% of tumors.
Typical Treatment
- Simple mastectomy
- Mammary strip - removal of all the mammary tissue from one side of the body
- Removal of the local lymph nodes
- Chemotherapy
- Radiotherapy
- Spaying/neutering
