This probably is one of the most important prognostic factors for a dog with a mammary mass. Dogs with mammary tumors less
than 3 cm in diameter have a significantly better prognosis than dogs with larger tumors. In another study, dogs with tumors
larger than 5 cm in diameter were more likely to develop metastases than dogs with smaller tumors and were seven times more
likely to die in the first two years after surgery.
Metastases to regional lymph nodes have been associated with an increased risk for tumor recurrence and for decreased overall
survival. The presence of distant metastases was found to be prognostically important in other studies. Dogs with no metastases
were more than three times as likely to survive one year from diagnosis.
Older dogs have a worse prognosis in some studies. It is unclear if this is due to tumor-related factors or competing risks.
Diet and body weight
In one study, the effect of diet in the year prior to diagnosis on survival after surgery showed that dietary fat and dietary
protein together influenced outcome. When dogs were categorized by the percent of total calories they derived from fat and
protein, the median survival time for dogs fed a low-fat diet (<39 percent) with protein greater than 27 percent and less
than 23 percent was three years, 1.2 years, and six months, respectively. For dogs fed a high-fat diet (>39 percent), there
was no difference in survival for the different intake levels of dietary protein. In addition, body conformation one year
prior to diagnosis affected survival.
Degree of invasion and ulceration
Dogs with tumors that ulcerate overlying skin have a worse prognosis (shorter overall survival times) than those with tumors
without ulceration. Rapid and invasive growth correlates with a worse prognosis, which may be recognized as fixation of the
tumor to the underlying skin. Vascular or lymphatic invasion is a poor prognostic factor; dogs with histopathologic evidence
of invasion have a shorter median survival.
Several histopathologic grading schemes are of prognostic significance. Important factors include histopathologic classification,
degree of nuclear differentiation and the presence of lymphoid accumulation. In general, the more highly differentiated the
tumor, the better the prognosis. Poorly differentiated tumors are much more likely to recur than well-differentiated tumors.
The chance of recurrence for poorly differentiated canine mammary tumors is 90 percent; for moderately differentiated tumors,
68 percent; and for well-differentiated tumors, 24 percent. Dogs that have mammary cancer but no evidence of lymphoid cellular
reactivity at the time of initial mastectomy have a three-fold increased risk of developing recurrence within two years, compared
to those with such reactivity.
Dogs with inflammatory carcinomas also have a poor prognosis. When reviewing a histopathology report, the veterinarian should
look for information regarding completeness of the surgical excision, invasion into lymphatics or blood vessels and differentiation
of the tumor.
Dogs with tumors that are estrogen-receptor and/or progesterone-receptor positive have a better prognosis than dogs with tumors
that do not have receptors (i.e., longer disease-free and overall survival times). Receptor-positive tumors are likely to
be benign. While presence of estrogen and progesterone receptors was predictive for disease-free survival after surgery in
one study, on multivariate analysis the effects of tumor size and histopathologic grade overpowered that effect.
Dogs with tumors that showed a high proportion of Ki-67 staining (which is an immunohistochemical marker for cellular proliferation)
were more likely to develop metastases in studies. Additionally, Ki-67 staining was inversely related to survival time.