Anastrozole and Letrozole After Surgery for the Treatment of Stage I-III Breast Cancer

Inclusion Criteria:

* REGISTRATION-INCLUSION CRITERIA
* Disease characteristics:

  * Histological confirmation of invasive breast carcinoma
  * Stage I-III breast cancer
  * Estrogen receptor (ER) positive disease according to American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines as ER \>= 1% positive nuclear staining
* Completion of all planned cancer treatments prior to registration:

  * Surgical resection of breast and nodal surgery; (NOTE: Reconstructive surgery does not have to be completed)
  * Adjuvant radiation therapy, if needed
  * Neoadjuvant and/or adjuvant chemotherapy, if needed
* Post-menopausal defined as

  * Age \>= 60 and amenorrhea \> 12 consecutive months OR
  * Previous bilateral oophorectomy OR
  * Age \< 60 and amenorrhea \> 12 consecutive months and documented follicle stimulating hormone (FSH) level within post-menopausal range according to institutional standard

    * NOTE: Patients who did not meet these criteria at time of diagnosis and received pre-operative (neoadjuvant) or post-operative (adjuvant) chemotherapy will not be allowed to participate
* Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0, 1, or 2
* Hemoglobin \>= 8.0 g/dL (obtained =\< 14 days prior to registration)
* Absolute neutrophil count (ANC) \>= 1500/mm\^3 (obtained =\< 14 days prior to registration)
* Platelet count \>= 70,000/mm\^3 (obtained =\< 14 days prior to registration)
* Total bilirubin =\< 1.5 x upper limit of normal (ULN) (obtained =\< 14 days prior to registration)
* Alanine aminotransferase (ALT) and aspartate transaminase (AST) =\< 3 x ULN (obtained =\< 14 days prior to registration)
* Ability to swallow oral medication
* Provide written informed consent
* Willingness to provide mandatory blood specimens for correlative research
* Willing to return to enrolling institution for follow-up (during the Active Monitoring Phase of the study)
* RE-REGISTRATION-INCLUSION CRITERIA
* Confirmation that baseline blood sample was drawn and submitted
* Blood estrogen levels after cycle 1 anastrozole (ANA1) must meet the following criteria:

  * E1 \>= 1.3 pg/ml, AND
  * E2 \>= 0.5 pg/ml

Exclusion Criteria:

* REGISTRATION-EXCLUSION CRITERIA
* Pre-menopausal women receiving ovarian function suppression (goserelin, leuprolide, etc.)
* Stage IV (metastatic) breast cancer
* HER2 positive breast cancer as defined by

  * HER2 immunohistochemistry (IHC) \>= 3+
  * HER2/CEP17 \>= 2.0
  * HER2/CEP17 \< 2.0 and average HER2 copy number of \>= 6.0 signals/cell
* Prior endocrine therapy for this breast cancer. Exceptions:

  * Pre-operative aromatase therapy (anastrozole, letrozole, or exemestane) and last treatment was \>= 4 weeks prior to registration OR
  * Pre-operative tamoxifen therapy and last treatment was \>= 12 weeks prior to registration
* Currently receiving any of the following cancer-directed therapies:

  * Radiation therapy
  * Systemic therapy such as chemotherapy (standard or investigational)
* Bisphosphonate therapy started \< 4 weeks prior to registration

  * NOTE: If patient is currently on bisphosphonate therapy she must be on stable dose for \>= 4 weeks prior to registration. Patients not currently taking bisphosphonates will be allowed to start bisphosphonate therapy after completion of anastrozole (1 mg and 10 mg daily \[if given\]). Information regarding bisphosphonate therapy will be collected
* Current use of systemic or topical exogenous estrogen or progesterone (menopausal hormone replacement therapy \[HRT\])
* Prior ovarian function suppression (leuprolide, goserelin, etc.)
* Inability to provide informed consent
* History of contralateral ductal carcinoma in situ (DCIS) or invasive breast cancer

  * NOTE: Exception allowed if

    * Patient did not receive adjuvant endocrine therapy OR
    * Patient received adjuvant endocrine therapy but has been off treatment for at least 6 months prior to registration
* Concurrent active malignancy or history of malignancy =\< 3 years prior to registration

  * NOTE: Exceptions allowed for successfully treated cervical carcinoma in situ, lobular carcinoma in situ of the breast, papillary thyroid cancer, or non-melanoma skin cancer
* Prior prevention therapy with an aromatase inhibitor or a selective estrogen receptor modulator (SERM). Exception: Therapy with a SERM (tamoxifen or raloxifene) is allowed if patient has been off treatment for \>= 6 months prior to registration