Ductal carcinoma in situ (DCIS) is a very early form of breast cancer. By definition, it has not spread outside the breast duct, and it can often be cured with surgical removal of the DCIS-containing breast tissue. DCIS is generally treated with lumpectomy (removal of the tumor spot), followed by breast radiation. Radiation has been shown to decrease the risk that the DCIS will return to the area of the breast that contained it. Breast radiation after lumpectomy is sometimes omitted in special circumstances, such as with an older woman who has a very small area of DCIS. Alternatively, some women may elect to have a mastectomy (surgical removal of the whole breast). A mastectomy may be recommended if the DCIS area is very large, if there are multiple areas of DCIS in the breast, or if for some reason a patient isn't a candidate for radiation. Radiation is not needed after a mastectomy for DCIS because there is insufficient breast tissue left behind for the DCIS to recur in.

Women with DCIS do not require chemotherapy because the risk that the cancer will recur in areas outside the breast is very, very low. However, women with DCIS can be at risk for developing a new DCIS area in the remaining breast tissue (if a lumpectomy was performed) or in the other breast. There is also a small chance that an invasive breast cancer (one that invades from the breast duct into the breast tissue) could develop.

Tamoxifen has been shown to decrease the risk of developing an invasive breast cancer among women with DCIS. In a large clinical trial, about 1,800 women were randomly assigned to have their DCIS treated either with lumpectomy followed by radiation and a daily placebo tablet, or with lumpectomy followed by radiation and daily tamoxifen. The women were followed to determine how many developed invasive breast cancer. The study found that treatment with tamoxifen reduced the risk for breast cancer events from 13.4 percent among women assigned to the placebo group to 8.1 percent among women assigned to tamoxifen during the five years after DCIS diagnosis. This means that about 87 percent of women who undergo a lumpectomy and radiation for DCIS, but are not treated with tamoxifen, will not have new DCIS or invasive breast cancer in the five years after DCIS diagnosis. Ninety-two percent of women who undergo lumpectomy and radiation and who take tamoxifen will have no new DCIS or invasive breast cancer in the five years after DCIS diagnosis. Based on the results of this trial, tamoxifen is routinely offered to women with DCIS after lumpectomy and radiation. The data for tamoxifen use are not as straightforward for women who have had a mastectomy to treat DCIS. Since there is no breast tissue at risk on the mastectomy side, the tamoxifen benefit would only involve the other breast.

You should discuss the specifics of your DCIS with your oncologist. For instance, if you have had a mastectomy and your DCIS was low-grade and very small, the absolute benefit of tamoxifen may be quite small. Alternatively, there may be issues, such as a strong family history of breast and ovarian cancer, that put you at risk for developing an invasive breast cancer in the future. There could also be some higher-risk features of your DCIS that would strengthen the case for adjuvant or preventive use of tamoxifen. If you and your doctor decide to proceed with tamoxifen treatment, you can be reassured that the drug is generally very well tolerated. Some fairly common side effects include hot flashes, vaginal dryness, and a mild vaginal discharge. Rarely, tamoxifen may be associated with an increased risk of blood clots. Even more rarely, but potentially seriously, women who still have a uterus and take tamoxifen have a small increased risk for developing endometrial cancer.

Finally, regarding your specific question about thyroid function and tamoxifen, the use of this drug may change the blood levels of thyroid hormone. However, the "free," or functional, thyroid hormone levels remain in the normal range, and women taking tamoxifen continue to have normal thyroid function.

Learn more in the Everyday Health Breast Cancer Center.