Current recommendations for mammary gland tumors in dogs - DVM
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Current recommendations for mammary gland tumors in dogs


Lumpectomy vs. mastectomy

In general, the goal of surgery is to completely excise the tumor with the simplest surgery possible. It is important to take into account the size and location of the tumor, whether the tumor is movable and whether there are multiple tumors or palpable lymph nodes. For larger tumors requiring a more aggressive surgery such as a regional mastectomy, the pattern of lymphatic drainage may also be important.

A recent prospective study found that 58 percent of dogs with a malignant tumor removed via a regional mastectomy developed a second tumor in an ipsilateral gland.3 The researchers' conclusion was that all malignant tumors should be removed with a radical mastectomy. This recommendation may prevent the development of future tumors but does not change the rate of local recurrence provided a wide excision is achieved. In addition, it should be noted that the dogs in this study were all intact females at the time of entry and were not spayed during the course of the study, which may have influenced the outcome.

Lumpectomies are generally reserved for those tumors that are small (< 0.5 cm), movable and benign. If a malignant tumor is removed via a lumpectomy and the margins are inadequate (< 1 to 2 cm), a second surgery to widen the margins is recommended. For tumors that are larger (1 cm) and localized within a single gland, a mammectomy should be considered. A regional or radical mastectomy is generally reserved for those dogs with larger tumors or that have multiple affected glands. If the tumor is fixed to the underlying tissue or skin, it is also important to remove skin and abdominal wall muscle or fascia to obtain an adequate excision.

If either the axillary or inguinal lymph nodes are enlarged, they should be removed and/or biopsied at the time of surgery. When the axillary lymph node is not enlarged, it can be difficult to find by surgical dissection, so removal of the node in these cases is not recommended. The inguinal lymph node is closely associated with the fifth mammary gland and is removed whenever the fifth gland is excised regardless of whether it is enlarged or not.

To spay or not to spay

There is still the debate as to whether or not an OHE at the time of tumor removal in intact dogs improves survival time. Early studies did not support the recommendation of an OHE at the time of diagnosis. However, two recent studies were able to demonstrate a survival benefit for those dogs that were spayed at the time of diagnosis.1,4 It is logical to consider that an OHE would improve survival time since it removes the major source of estrogen and progesterone production. In a sense, it could be compared to using an anti-estrogenic drug such as tamoxifen. It would be expected that dogs that had estrogen or progesterone receptor positive tumors would benefit most from an OHE as this would eliminate a source of hormonal stimulation for any remaining tumor cells.

To give or not to give adjuvant therapy

Given that 25 percent of dogs with mammary gland tumors cannot be successfully treated with surgery alone, it is desirable to consider some type of adjuvant chemotherapy for these patients. However, there is not a standard approach in treating canine mammary gland tumors with chemotherapy. In people, the most effective drugs are the anthracyclines (i.e., doxorubicin) and the taxanes (i.e., paclitaxel), either alone or in combination with other drugs. Doxorubicin is frequently combined with cyclophosphamide as a first-line adjuvant therapy for high-risk human patients.

In veterinary medicine, we have taken a similar approach to adjuvant therapy, though the data to support the use of chemotherapy are scarce and there are conflicting reports in regards to efficacy. Drugs that have been evaluated include cyclophosphamide, 5-fluorouracil, doxorubicin, docetaxel and gemcitabine.5,6 Given the small number of patients in these studies, it is difficult to ascertain the efficacy of chemotherapy. Anti-estrogenic drugs such as tamoxifen have also been evaluated but have not yet proved to be of benefit and carry a high risk of side effects including stump pyometra.

Despite the limitations, oncologists still recommend chemotherapy for high-risk patients and for those with metastatic or nonresectable disease. The protocol is often determined by an oncologist's clinical experience as well as the specifics of the case. It is hoped that more effective targeted therapies will be developed as we continue to explore the molecular basis of these tumors.

Dr. Cronin earned her DVM degree from Cornell University in 1990. She completed an internship at the Animal Medical Center in New York and a medical oncology residency at North Carolina State University. She is a diplomate of the American College of Veterinary Internal Medicine in the specialty of oncology. After completing her residency, she was lecturer at the University of Pennsylvania Veterinary Teaching Hospital and a medical oncologist at Angell Memorial Animal Hospital in Boston. In 2001, she co-founded the New England Veterinary Oncology Group in Waltham, Mass.


Source: DVM360 MAGAZINE,
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