#1163 Background: Ductal carcinoma in situ (DCIS) accounts for about 20% of all female breast cancers and is now diagnosed 10 times more frequently than prior to the use of increased screening mammography. Although magnetic resonance imaging (MRI) has proven to be a useful imaging modality for invasive cancer, its role and impact in preoperative surgical planning for DCIS is unclear. Our aim is to determine if women diagnosed with pure DCIS on core-biopsy who have preoperative MRI are more likely to undergo mastectomy and sentinel lymph node biopsy (SLNB). Methods: Retrospective survey of women diagnosed with pure DCIS on stereotactic core biopsy between 2000-2007 at an academic tertiary referral center. All women underwent definitive surgical treatment for DCIS. Patient characteristics, surgical planning and surgical outcomes were compared between patients who underwent preoperative MRI and those without MRI. Continuous variables among the two groups were compared using the t-test. Differences in dichotomous variables between groups were compared using the chi-square test. Significance was determined if p<0.05. Results: Of 137 women diagnosed with DCIS, 39 underwent preoperative MRI. Mean age, DCIS size and grade, and presence of invasive cancer on surgical specimen were compared between the two groups. The only significant difference between the two groups was younger mean age in the group with preoperative MRI (51 vs 59 yrs, p=0.001). On univariate analysis, mastectomy and SLNB were more commonly employed in the MRI group. The mastectomy rate was 55% for women who had preoperative MRI and 17% for those who did not (p<0.0001). The SLNB rate was 46% for women had preoperative MRI and 23% for those who did not (p=0.009). Number of re-excisions, margin status and size were compared between the two groups. Preoperative MRI was not significantly associated with wider surgical margins or higher proportion of patients with negative margins. However, the mean number of re-excisions was lower in women who underwent preoperative MRI (0.7 vs. 1.2, p=0.003). The only independent factor associated with likelihood of preoperative MRI in the setting of DCIS was patient age. On multivariate analysis, use of SLNB was independently associated with mastectomy, DCIS size and DCIS grade (p=0.0001, p=0.011, p=0.024, respectively). In contrast, independent predictors of mastectomy were DCIS size and use of MRI (OR 5.1, 95% CI 2.0-13.2)). Conclusion: Women who underwent preoperative MRI for DCIS were younger and more likely to undergo mastectomy. However, women who had preoperative MRI required a lower number of re-excisions to obtain negative margins. Our study cannot address whether the strong association between preoperative MRI and mastectomy is a causal one. Nevertheless our data support a critical need for future studies to further define the benefit as well as consequences of the use of MRI for DCIS evaluation and treatment. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 1163.