A Clinical Trial to Determine the Efficacy of Five Years of Letrozole Compared to Placebo in Patients Completing Five Years of Hormonal Therapy Consisting of an Aromatase Inhibitor (AI) or Tamoxifen Followed by an AI in Prolonging Disease-Free Survival in Postmenopausal Women with Hormone Receptor Positive Breast Cancer

INTERVENTION: Trade Name: Femara Product Name: Letrozole Pharmaceutical Form: Tablet Concentration unit: mg milligram(s) Concentration type: equal Concentration number: 2.5‐ Pharmaceutical form of the placebo: Tablet Route of administration of the placebo: Oral use CONDITION: Hormone Receptor Positive Breast Cancer PRIMARY OUTCOME: Main Objective: The primary aim is to determine whether or not prolonged adjuvant hormonal therapy with letrozole will improve disease‐free survival in postmenopausal women with ER‐positive and/or PgR‐positive tumors who have completed 5 years of hormonal therapy with 5 years of an aromatase inhibitor (AI) or 5 years of a combination of up to 3 years of tamoxifen followed by an AI. Primary end point(s): The primary endpoint for analysis is disease‐free survival (DFS). DFS events are local recurrence following mastectomy, local recurrence in the ipsilateral breast following lumpectomy (IBTR), regional recurrence, distant recurrence, second primary cancer (other than squamous and basal cell carcinoma of the skin, melanoma in situ, and carcinoma in situ of the colon and cervix), and death from any cause prior to recurrence or second primary cancer. Secondary Objective: Survival (S); Determine whether or not prolonged adjuvant hormonal therapy with letrozole will improve survival.; Breast cancer‐free interval:; Determine whether or not prolonged adjuvant hormonal therapy with letrozole will improve breast cancer‐free survival.; Distant recurrence; Determine whether or not prolonged hormonal therapy with letrozole will improve distant recurrence.; Osteoporotic‐related fractures (Colles', hip, and spine); Determine whether or not prolonged adjuvant hormonal therapy with letrozole will increase osteoporotic‐related fractures (Colles', hip, and spine).; Arterial thrombotic events; Determine whether or not prolonged adjuvant hormonal therapy with letrozole will increase arterial thrombotic events.; ; ; ; INCLUSION CRITERIA: 1‐ The patient must have consented to participate and must have signed and dated an appropriate IRB‐approved consent form . 2‐ Patients must be female. 3‐ Patients must have an ECOG performance status of 0 or 1 (0 = fully active, able to carry on all pre‐disease performance without restriction; 1 = restricted in physically strenuous activity but ambulatory). 4‐ Patients must be postmenopausal at the time of randomization. (Note: Premenopausal or perimenopausal women requiring therapy with luteinising hormone‐releasing hormone [LHRH] analogs to suppress ovarian function are not eligible.) For study purposes, postmenopausal is defined as: • age 56 or older with no spontaneous menses for at least 12 months prior to study entry, or • age 55 or younger with no spontaneous menses for at least 12 months prior to study entry (e.g., spontaneous or secondary to hysterectomy) AND with a documented estradiol level in the postmenopausal ran