Clinical Significance

The role of ER and PR receptor testing as markers of prognosis and predictors of response to antiestrogen therapy is established as a standard of care for patients with breast cancer.88'89 The majority of breast cancers in the United States are evaluated for ER and PR status using IHC. Results of ER and PR IHC are used as the basis for selection of patients as candidates for hormonal therapy. The response to standard antiestrogen therapies such as Tamoxifen in first-line endocrine treatment of metastatic breast cancer averages 30% for either ER- or PR-positive tumors.92 For patients with both ER- and PR-positive breast cancers' response rates have ranged from 50% to 70%.92-94 For patients with ER- and PR-negative tumors' there is only a 5% to 10% chance that the patient will respond to hormonal therapy.92-94 The introduction of specific estrogen response modulators95 and aromatase inhibitors, such as anastrozole, letrozole, and extremestane,96'97 have added new strategies of hormonal therapy for breast cancer patients.

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