Mammary Tumors in Dogs and Cats: Symptoms, Causes, and Treatment
Mammary tumors are the most common type of tumor in unspayed female dogs and are also an important cause of cancer in cats. There are significant differences between the two species: approximately half of mammary tumors in dogs are benign, while the vast majority of tumors in cats are malignant and tend to behave aggressively. In this article, we discuss the symptoms, risk factors, diagnostic process, and treatment options for mammary tumors.
What Are Mammary Tumors?
Mammary tumors are masses that originate from mammary gland tissue. Dogs have five pairs of mammary glands, extending from the armpit to the groin in two “mammary chains.” Tumors may develop as a single mass in one gland or as multiple masses affecting several glands.
In dogs: Approximately half of mammary tumors are benign (adenomas, fibroadenomas, and mixed tumors), while the other half are malignant, most commonly various types of carcinomas.
In cats: The situation is more serious. The vast majority of mammary tumors are malignant, with a greater tendency for local invasion and distant metastasis compared with dogs.
The disease primarily occurs in females. Mammary tumors are extremely rare in males and, when they do occur, they generally have a more aggressive course.
Risk Factors
Hormonal status is one of the most important factors influencing the development of mammary tumors:
Timing of spaying directly affects risk: Dogs spayed before their first heat cycle have a very low risk, while the risk gradually increases with each subsequent heat cycle.
The risk is significantly higher in animals that remain unspayed or are spayed after their second year of life.
Age is an important factor: The risk begins to increase particularly after the age of seven and reaches its peak during middle to advanced age.
Mammary tumors may be more common in certain breeds, including Toy and Miniature Poodles, Spaniel breeds, and German Shepherd Dogs. However, a definitive hereditary association has not been established.
What Are the Symptoms?
The most common sign is a palpable mass in the mammary gland area. Other findings may include:
One or multiple masses along the mammary chain – Their size, shape, and consistency may vary, and they may be either fixed or movable.
Rapid growth of the mass
Redness, swelling, and increased skin temperature – These signs are particularly prominent in inflammatory mammary carcinoma.
Ulceration – This may occur in large, neglected tumors and carries a risk of infection.
Discharge from the nipple
Because cats may become uncomfortable when their abdominal area is touched, owners often notice mammary masses later than they do in dogs. As a result, mammary tumors in cats are frequently diagnosed at a more advanced stage.
How Are Mammary Tumors Diagnosed?
Fine-needle aspiration (FNA): A sample of cells can be collected from the mass. However, in mammary tumors, FNA may have limitations in reliably determining whether a mass is benign or malignant.
Surgical biopsy: Histopathological examination of the removed tissue is necessary to establish a definitive diagnosis and determine the tumor type.
Staging: Chest radiographs, blood tests, and evaluation of the regional lymph nodes are recommended before surgery.
Prognostic factors: Tumor size (less than 3 cm, 3–5 cm, or greater than 5 cm), lymph node involvement, histological type, and tumor grade play important roles in determining the treatment plan and expected outcome.
Treatment Options
Surgery
Surgery is the gold standard treatment for mammary tumors. Depending on the case, the tumor itself or the entire affected mammary gland may be removed. Complete removal of the entire mammary chain is rarely necessary in dogs.
Surgery is generally not recommended for inflammatory mammary carcinoma or widespread metastatic disease.
Spaying
Performing spaying at the same time as mammary tumor surgery remains a controversial issue. There is no consensus regarding its effect on recurrence or overall survival in all cases. However, for a specific estrogen-sensitive tumor type known as secretory carcinoma, spaying may be recommended to help reduce the risk of recurrence.
Chemotherapy
In dogs, unlike human breast cancer, there is no clear evidence that chemotherapy significantly prolongs survival, and it is generally not more effective than surgery alone.
In cats, chemotherapy may be considered on a case-by-case basis, particularly because feline mammary tumors tend to have a more aggressive course.
Hormonal Therapy
The effectiveness of hormonal therapy remains controversial because steroid receptors are frequently absent in malignant mammary tumors.
Prognosis: What Can Be Expected?
In dogs: The prognosis is generally very good when a tumor is detected and removed while it is smaller than 1 cm. As tumor size increases, lymph node involvement and a higher histological grade negatively affect prognosis.
In cats: When a tumor is smaller than 2 cm and treated surgically, cats may survive for several years. However, if the tumor exceeds 3 cm at the time of diagnosis, postoperative survival is generally less than one year. Therefore, early surgical intervention is particularly important in cats.
Inflammatory mammary carcinoma: This is a rare and extremely aggressive form of mammary carcinoma in both dogs and cats. There is currently no effective curative treatment, and surgery generally does not improve survival.
Recommendations for Pet Owners
The timing of spaying directly affects the risk of mammary tumors. Discuss the appropriate timing with your veterinarian early in your pet’s life.
Regularly examine your female pet’s mammary chain by gently feeling for any abnormalities. Cats may not tolerate abdominal handling well, so establish a gentle and consistent routine.
If you notice any mass, regardless of how small it is, seek veterinary evaluation rather than adopting a “wait and see” approach. Early intervention can be particularly important in cats.
Rapid growth of a mass, redness, swelling, or increased skin temperature requires prompt veterinary evaluation.
After surgery, discuss with your veterinary oncology specialist whether spaying or additional treatment is appropriate based on the tumor type and individual case.
Conclusion
Mammary tumors are generally manageable when detected early, particularly in dogs. In cats, however, their more aggressive behavior makes early diagnosis and prompt surgical intervention especially important.
If you notice any mammary mass in your female pet, it is recommended that you consult a veterinary oncology specialist without delay.
This content is intended for informational purposes only and does not replace an examination by a veterinarian. If you suspect that your pet has a health problem, please consult a veterinarian.
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