SHORTened Duration of TAMoxifen

Inclusion Criteria:

• 1. Patient at high risk for invasive breast cancer (either A or B)

A. Post-menopausal women diagnosed with ductal carcinoma in situ (DCIS), Lobular carcinoma in situ (LCIS), atypical ductal hyperplasia (ADH), and atypical lobular hyperplasia (ALH) who meet the following criteria:

• DCIS/LCIS

  • Lesion size ≤3cm
  • Low or intermediate grade
  • Lesion completely excised by surgical excision
  • Margins >2cm for DCIS
  • Estrogen receptor (ER) positive and/or Progesterone receptor positive (defined as ≥10%)
• ADH/ALH o Lesion completely excised by surgical excision

B. Patients at increased risk of breast cancer based on Gail model criteria (5- year risk ≥1.66%) without prior history of DCIS, LCIS, atypical hyperplasia, or invasive breast cancer.

2. No history of pathogenic germline mutations that increase the lifetime risk of breast cancer. If genetic testing was not done, then patients must have no more than two first-degree relatives diagnosed with breast cancer.

3. Should have received adjuvant radiation for DCIS, if recommended by the radiation oncologist.

4. No prior history of invasive carcinoma, DCIS, LCIS, ADH, and/or ALH in bilateral breasts.

5. ECOG performance status ≤ 2 6. No use of hormone replacement therapy for at least 5 years.

Exclusion Criteria:

1. Evidence of invasive or microinvasive breast cancer.
2. Current use of dicumarols and CYP2D6 inhibitors, such as selective serotonin reuptake inhibitors.
3. Patients who have had a bilateral mastectomy for risk reduction.
4. Prior history of any malignancy in the last 5 years (except squamous cell carcinoma/basal cell carcinoma of skin)
5. Patients who are known to be pregnant
6. Patients who cannot voluntarily provide informed consent. Evidence of dementia, altered mental status, or any psychiatric condition that would prohibit understanding or rendering of informed consent.
7. Life expectancy of less than 1 year per treating physician.

8. Prior treatment with tamoxifen or an aromatase inhibitor.

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