Evaluation of hormone receptor expression for use in predicting survival of female dogs with malignant mammary gland tumors

Objective-To evaluate the prognostic potential of expression of hormone receptors in malignant mammary gland tumors of dogs. Design-Cohort study. Animals-89 female dogs with malignant mammary gland tumors and 24 female dogs with benign mammary gland tumors. Procedures-Female dogs with malignant (n = 89 dogs) and benign (24) mammary gland tumors were evaluated to determine the prognostic value of the expression of estrogen receptor (ER)alpha or the progesterone receptor (PR), as determined by use of immunohistochemical methods. Results-In this study, 68 (60.2%) and 88 (779%) of the 113 dogs with mammary gland tumors had expression of ER alpha and PR, respectively. Expression of ER alpha and PR was detected proportionately more frequently in benign tumors (23/24 (95.8%] and 24/24 [100%], respectively) than in malignant tumors (45/89 [50.6%] and 64/89 [71.9%]). Percentage of tumors with positive results for ER alpha and PB was significantly higher in tumors < 5 cm in diameter; as clinical stage I, II or III; and without metastasis to lymph nodes or distant metastasis. However, only PR expression in tumor cells was significantly associated with 1-year survival after surgical removal of the tumor. Moreover, dogs with malignant tumors expressing ER alpha and PR had a significantly higher survival rate, compared with the rate for dogs with malignant tumors expressing ER alpha but not PR. Conclusions and Clinical Relevance-These findings strongly suggested that expression of PR could be used as a prognostic factor for survival, especially in female dogs with malignant mammary gland tumors with ERa expression. (J Am Vet Med Assoc 2009;235:391-396)