Canine mammary tumors, the equivalent of breast cancer in dogs, are one of the most common neoplasms diagnosed in female dogs. These growths originate in the mammary gland tissue, which in dogs consists of four to five pairs of glands running along their underside. Approximately half of all mammary tumors are malignant, meaning they have the potential to spread to other parts of the body.
Identifying Potential Signs
The most common sign of a mammary tumor is a palpable lump or nodule under the skin of a dog’s abdomen. These masses feel firm and can range from small, pea-sized bumps to much larger growths. They can appear as single nodules or in multiples. In some cases, the skin over the mass may become ulcerated, leading to bleeding or a non-healing sore.
Owners might also notice other changes in the mammary area. The surrounding tissue can become swollen, warm, or painful. Some dogs may have discharge from a nipple. As the disease progresses, systemic signs like lethargy, decreased appetite, and weight loss can appear, though many dogs seem healthy when a tumor is first found.
Monthly at-home checks can help with early detection. Gently palpate the entire length of your dog’s two mammary chains, from the chest to the groin, feeling for unusual firmness or lumps. The glands should feel soft and pliable. Note any changes in the size, shape, and texture of the nipples and surrounding skin.
Causes and Prevention
The development of canine mammary tumors is strongly linked to hormonal exposure, particularly estrogen from heat cycles. Female dogs who are not spayed, or are spayed later in life, are at a significantly higher risk because repeated hormonal stimulation can promote the growth of abnormal cells over time. The timing of spaying (ovariohysterectomy) has a major impact on risk reduction. Spaying a dog before her first heat cycle is the most effective preventive measure, as the risk progressively increases with each subsequent cycle.
While hormones are the primary factor, other elements contribute to the risk. Certain breeds, such as Poodles, Spaniels, and German Shepherds, appear to have a genetic predisposition. Age is also a factor, with most tumors appearing in middle-aged to older dogs. Obesity, especially when a dog is young, may also play a role in increasing risk.
The Diagnostic Process
When a lump is found, a veterinarian will begin with a physical examination. This involves palpating the mass and associated lymph nodes to assess its size and characteristics, as well as checking all mammary glands for additional tumors.
To determine the nature of the lump, a tissue sample is required. A fine-needle aspirate (FNA) is a common first step to collect cells for evaluation, but it may not definitively distinguish between benign and malignant tumors. For a conclusive diagnosis, a surgical biopsy is the standard, where part or all of the mass is removed and sent to a pathologist for histopathology.
If a malignant tumor is confirmed, the next phase is “staging” to determine if the cancer has spread (metastasized). This process involves chest X-rays to look for evidence of spread to the lungs, a common site for metastasis. Blood work and a urinalysis are also performed to assess the dog’s overall health and organ function, while an abdominal ultrasound may be recommended to check for spread to internal organs.
Treatment Pathways
Surgery is the primary treatment for canine mammary tumors, with the goal of completely removing the tumorous tissue. The extent of the surgery depends on the size, location, and number of tumors present.
Surgical procedures range from a lumpectomy (removing only the mass) to a simple mastectomy (removing the tumor and its gland). For multiple or larger tumors, a radical chain mastectomy, which removes an entire side of the mammary chain and lymph nodes, may be recommended. If the dog is not spayed, performing an ovariohysterectomy at the same time is often advised.
Chemotherapy is reserved for specific circumstances, such as when the cancer is aggressive, has spread, or was not completely removed with surgery. For a rare type called inflammatory mammary carcinoma, which causes significant swelling, surgery is not the preferred approach. Instead, radiation and medication are used.
Prognosis and Recovery
The long-term outlook for a dog with a mammary tumor depends on several factors. The tumor’s size is a significant indicator; dogs with tumors smaller than three centimeters in diameter have a better prognosis. The histopathologic grade, which describes how aggressive the cancer cells appear, and whether the cancer has metastasized to the lymph nodes or lungs are also important determinants.
With early detection and complete surgical removal of a small, non-metastasized tumor, the prognosis can be quite good. For dogs with larger or more aggressive cancers that have spread, the prognosis is more guarded, with survival times varying based on the specifics of the case.
Post-surgical recovery involves careful home management. Owners must care for the incision site, keeping it clean and preventing the dog from licking the stitches, often with an Elizabethan collar. Pain medication will be prescribed for comfort, and activity should be restricted during the healing period.