This summary table contains detailed information about research studies. Summary tables offer an informative look at the science behind many breast cancer guidelines and recommendations. However, they should be viewed with some caution. In order to read and interpret research tables successfully, it is important to understand some key concepts. Learn how to read a research table.
Introduction: Ductal carcinoma in situ (DCIS) is a non-invasive breast cancer. In situ means "in place." In DCIS, abnormal cells are contained in the milk ducts, but have not invaded nearby breast tissue. Without treatment, DCIS can progress to invasive breast cancer over time. So, all cases of DCIS are treated (learn more).
Results from one randomized controlled trial showed tamoxifen can lower the risk of invasive and non-invasive breast cancer, both in the breast treated for DCIS and in the opposite breast, in women with DCIS who have lumpectomy plus radiation therapy . The benefit of tamoxifen appeared to be limited to women with estrogen receptor-positive DCIS .
A second randomized controlled trial found tamoxifen did not lower the risk of invasive cancer in the same breast, but did lower the risk of invasive cancer in the opposite breast. In contrast, findings from this trial showed tamoxifen lowered the risk of DCIS recurrence in the same breast, but did not lower the risk of DCIS in the opposite breast .
Learn more about treatment for DCIS.
Study selection criteria: Large randomized controlled trials and meta-analyses.
Table note: Relative risk above 1 indicates increased risk. Relative risk below 1 indicates decreased risk.
Study Population(number of participants)
Absolute Risk of Invasive Breast Cancer per year(number of cases per 1,000 women)
Relative Risk of Invasive Breast Cancer in Women Taking Tamoxifen Compared to Women Not Taking Tamoxifen, RR (95% CI)
Among Women Not Taking Tamoxifen
Among Women Taking Tamoxifen
Randomized controlled trials
National Surgical Adjuvant Breast and Bowel Project, Protocol B-24 [1-2]
1,799 women with DCIS treated with lumpectomy plus radiation therapy
UK Coordinating Committee on Cancer Research 
1,576 women with DCIS treated with lumpectomy either with or without radiation therapy
Staley et al. 
* Relative risk for invasive breast cancer in the breast treated for DCIS. Relative risk for invasive breast cancer in the opposite breast was similar, 0.68 (0.48-0.95). When results for 732 of participants were examined by hormone receptor status, relative risks showed benefit for hormone receptor-positive DCIS, but no benefit for hormone receptor-negative DCIS.
† Absolute risk not given. However, there was no difference between rates of invasive breast cancer for women not taking tamoxifen and women taking tamoxifen (9% vs. 9%).
‡ Results for all invasive breast cancers (most breast cancers occurred in the breast treated for DCIS).
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