The main objective of this protocol is to evaluate the value of MRI+biopsy in the staging of mammography-detected DCIS in an unselected population of women. It is a multicentric, randomised study including patients from France presenting a unique, limited (< 30 mm) DCIS detected by mammography or breast ultrasound. The two compared arms are: preoperative breast MRI+biopsy versus no additional diagnostic examination. High resolution dynamic 3D images will be obtained in order to detect very small suspicious lesions, typical of DCIS. The main endpoint is the re-excision rate for involved margins. In order to show a reduction of 50% of the rate of second surgery (from 25% to 12.5% in the MRI arm), it is necessary to include 360 patients (bilateral test, alpha=5%, power=80%).