516 Background: Although randomized trials have demonstrated the efficacy of radiation (RT) in reducing the risk of breast cancer after lumpectomy for Ductal Carcinoma in Situ (DCIS), the use of RT in the population remains relatively low. This study was designed to determine the effectiveness of RT after lumpectomy for DCIS in a population-based fashion using SEER data. Methods: We identified women with DCIS treated with lumpectomy in SEER registry data from 1987 through 1996. We evaluated the effect of RT on development of subsequent invasive breast cancer: the overall rate, the rate of ipsilateral invasive cancer, and the rate of contralateral invasive cancer. Using a proportional hazards model, we evaluated the effect of RT on development of invasive breast cancer over time adjusting for potential confounders. Results: A total of 10,034 women underwent lumpectomy for DCIS in this time period; 5063 underwent lumpectomy only and 4971 underwent RT after lumpectomy. Invasive breast cancer developed in 854...