Purpose: To determine the risk of ipsilateral breast events in patients with ductal carcinoma in situ (DCIS) treated with local excision without irradiation.

Patients and Methods: Patients with either low- or intermediate-grade DCIS measuring 2.5 cm or smaller, or high-grade DCIS measuring 1 cm or smaller who had microscopic margin widths of 3 mm or wider and no residual calcifications on postoperative mammograms were eligible for a prospective trial conducted from 1997 to 2002 by the Eastern Cooperative Oncology Group and North Central Cancer Treatment Group. Patients entered in 2000 and later could take tamoxifen if they wished. Median age at last surgery for the entire population was 60 years (range, 28 to 88 years), and median tumor sizes in the two strata were 6 mm and 5 mm, respectively.

Results: With a median follow-up of 6.2 years, the 5-year rate of ipsilateral breast events in the 565 eligible patients in the low/intermediate grade stratum was 6.1% (95% CI, 4.1% to 8.2%). With a median follow-up of 6.7 years, this incidence for the 105 eligible patients in the high-grade stratum was 15.3% (95% CI, 8.2% to 22.5%).

Conclusion: Rigorously evaluated and selected patients with low- to intermediate-grade DCIS with margins 3 mm or wider had an acceptably low rate of ipsilateral breast events at 5 years after excision without irradiation. Patients with high-grade lesions had a much higher rate, suggesting that excision alone is inadequate treatment. Further follow-up is necessary to document long-term results.