The purpose of the study is to establish a foundation from which to plan novel, less aggressive treatment algorithms for patients with pre-invasive breast disease. one approach to non-surgical management for those who may never progress to invasive cancer, (provided that a low-risk subgroup of DCiS could be identified), would be to treat women with eR positive DCiS with preoperative endocrine therapy in order to reverse malignant changes. if successful, some women could avoid surgery for this disease. The aromatase inhibitors (ais) are ideal candidates for use in such a setting.

after a Diagnostic core biopsy confirming eR+ DCiS , and Baseline MRi and mammogram, participants will be registered and begin the study treatment with Letrozole 2.5 mg/day x 3 months. a 3 month Disease evaluation with research MRi will be done to assess response. if CR, PR or SD, they will continue letrozole x 3 months. at the 6 month visit, a research MRi will be done followed by surgery. if there is Radiographic PD at 3mos, the patient will have surgery and discontinue letrozole.

Participant Eligibility

[?] Histologic Documentation: Pathologic confirmation of DCIS of the female breast without invasive cancer, with diagnosis rendered on core biopsy only, completed within 60 days before registration. Patients diagnosed with DCIS on the basis of surgical biopsy are not eligible for this study.

[?] Patients with microinvasion on diagnostic core biopsy, defined as tumor < 1mm in greatest
dimension, will be allowed to participate.

[?] All patients must have a clip placed, either at the time of the diagnostic biopsy or at the time of the baseline MRI prior to the start of treatment.
[?] Tissue samples: Patient has diagnostic tissue available for correlative studies.
[?] Clinical Stage: Tis or T1mi N0 M0
[?] Hormone receptor status: DCIS must express estrogen and/or progesterone receptor, as determined by immunohistochemical methods on the diagnostic pathology sample, according to the local institution[Single Quote]s standard protocol. Greater than or equal to 1% cells will be considered to be positive.
[?] Menopausal status: Patients must be postmenopausal defined as:

* Age >= 55 years and one year or more of amenorrhea

* Age < 55 years and one year or more of amenorrhea, with an estradiol assay < 20 pg/ml.

* Surgical menopause with bilateral oophorectomy (at least 28 days must elapse from surgery to time of study registration).
The use of GnRH analogs to achieve postmenopausal status is not allowed.
[?] Age: >= 18 years of age
[?] ECOG Performance Status 0 or 1
[?] Mammographic extent of calcifications must be accurately measurable in at least one
dimension with each lesion >/= 1 cm and [?] DCIS must be visible on MRI based on central review.
[?] Patients with multifocal or bilateral disease are eligible
[?] Women diagnosed with osteoporosis may participate in this trial provided they are receiving appropriate therapy or if they have declined therapy.
[?] Required initial laboratory values:
ANC >= 1,000/[MICRO-SYMBOL]l
Platelet count >= 100,000/[MICRO-SYMBOL]l
Serum creatinine <= 1.7 mg/dL
Bilirubin <= 2.0 mg/dL
AST/ALT < 2.5 x ULN
Serum estradiol level assay (required for patients < 55 years of age and one year or more of amenorrhea) < 20 pg/ml