Breast cancer is a main cause of cancer death, especially in women at increased risk for breast cancer. This risk is defined as a cumulative lifetime risk of more than 20%, and can be as high as 57% at the age of 70 in BRCA1-carriers.¹ Screening with only mammography is insufficient in these women and supplemental annual breast MRI is currently required.² In mammography screening it is regular practice to evaluate breast cancers detected in incident screening rounds and breast cancers detected in between screening round (interval cancers), and assess whether these cancers could have been detected earlier.^3,4 This is rare for MRI screening. The purpose of this study is to evaluate breast cancers detected in an intermediate and high risk screening program, and assess the visibility of these cancers on prior MRI examinations. To detect possible causes for non-recall, we investigated imaging, patient, and cancer characteristics. This retrospective study was approved by our local institutional board and the requirement for informed consent was waived. We collected all breast MRI screening examinations in the period from January 2003 - January 2014. To find all malignant lesions in this population, corresponding patient files were linked to the Netherlands Cancer Registry (NCR). For each patient with breast cancer detected on an MRI screen or interval cancer, we identified whether another MRI screen in 6 - 24 months before cancer detection was available (prior MRI). These MRI-scans were re-evaluated together with the MRI-scan in which cancer was detected (diagnostic MRI) in consensus by two radiologists with 8 and 12 years’ experience. The review was performed on an in-house developed breast MRI workstation, which provided T1-weighted images for all time points for both current and prior DCE-MRI, subtraction images, and their maximum intensity projection. Images were motion corrected using an algorithm described in Gubern-Mérida et al.⁵ No T2-weighted images or diffusion-weighted images were shown. On the diagnostic MRI morphological and enhancement characteristics of the known cancer were scored according to the Breast Imaging Reporting and Data (BI-RADS) MR-lexicon.⁶ In addition, background parenchymal enhancement (BPE) was scored as minimal (<25%), mild (25-50%), moderate (50-75%) or marked (>75%), and image quality (IQ) was scored as perfect, sufficient or bad. Thereafter, the prior MRI was analyzed. The visibility of the lesion, previously identified in the diagnostic MRI, was rated as visible (BI-RADS 4/5), minimally visible (BI-RADS 2/3), or invisible (BI-RADS 1) (Fig.1). In lesions classified as visible or minimally visible morphology and enhancement characteristics were scored. Pearson’s chi-square tests were used to test if imaging, patient, and cancer characteristics affect the visibility of the tumor on the prior MRI. Statistics were performed in SPSS. From January 2003 - January 2014, 10120 MRI-examinations were performed in 2797 women, including 807 BRCA-mutation carriers. In total, 153 cancers were found. For 69 screen-detected tumors a prior MRI was available (36 tumors in patients with a BRCA mutation). In retrospect, 20 (29%) tumors were visible on the prior MRI, 26 (38%) showed a minimal sign, and 23 (33%) were invisible. Furthermore, prior MRIs were also available for 12 interval cancers (6 tumors in patients with a BRCA mutation); 3 (25%) were visible, 4 (33%) showed a minimal sign, and 5 (33%) were not visible on the prior MRI. Tumors in BRCA patients, small tumors, tumors of high grade and hormone-negative tumors were more likely to be invisible on the prior MRI (p<0.001, p=0.039, p<0.001, p<0.001, respectively). The lack of detection of lesions scored as visible on the prior MRI was not related to BPE or IQ.A successful MRI screening program is based on the balance between the early detection and the false positive findings that result in unnecessary biopsies and anxiety. This might explain why not all visible lesions get recalled. However, in our study we show that almost one third of cancers were already visible on the prior MRI scan in retrospect and should have been recalled according to our consensus review. This fraction was similar for screen detected and interval cancers. A possible reason for the non-recall could be that the visible lesions were already present at an earlier time point and were regarded stable over time. Non-recall was not related to BPE or IQ. It was seen that 28% of breast cancers should have been recalled earlier based on consensus review. Only 35% was completely invisible in retrospect. This indicates that even highly specialized breast cancer screening programs can still be improved and that regular evaluation of screening practice is essential.