The number of breast cancer cases is increasing annually throughout the world and it has more than doubled from ~800,000 in 1990 to 1.68 million in 2016(1). The 10-year survival rate for stage I primary breast cancer is >95% with good prognosis; however, that for metastatic/recurrent breast cancer is only ~5% (2). Proper definitive treatment is important to prevent metastasis and recurrence. Surgical treatment is known to have an important role as the initial treatment. The standard surgery for breast cancer is generally mastectomy or partial mastectomy; i.e., breast-conserving surgery (BCS), but it is important to obtain a negative surgical margin, particularly in BCS. A meta-analysis by the Early Breast Cancer Trialists' Collaborative Group reported that positive surgical margins in BCS increased breast cancer-related mortality (3). Accurate diagnosis of the spread of breast cancer is necessary to obtain negative surgical margins. The gold standard imaging examination for investigating the spread of breast cancer is ultrasound (US) (4). However, for diagnoses made by US, the rate of positive surgical margins is 24-27% (5,6), which is clearly insufficient and requires to be improved. As an alternative technique, magnetic resonance imaging (MRI) with the patient in the prone position has recently been used for this purpose. The prone position has the advantages of minimal deterioration of image quality due to respiratory movements and of improved ability to examine intraductal components of the mammary glands without distortion, as they are stretched when the breast is ‘hanging’ in this position (7,8). At present, US and MRI in the prone position are in widespread use as standard methods despite a lack of significant results from randomized controlled trials (RCTs) (6). An RCT conducted in 2001 compared the reoperation rate for BCS between a group with MRI in the prone position (n=816) and a triple assessment group of mammography (MG), US and core needle biopsy (non-MRI group, n=807). There was no significant reduction in the reoperation rate in the MRI group (153/816, 19%) compared with the non-MRI group (156/807, 19%; odds ratio: 0.96, 95% CI: 0.75-1.24, P=0.77) (9). A disadvantage of MRI in the prone position is that it differs from the operating position (10,11). Even if spread of the lesion can be correctly diagnosed, it is considered that a certain shift occurs due to the change in posture between imaging and surgery. The present study was performed based on the notion that supine MRI has an advantage compared with prone MRI because the images are acquired in the same position as the surgical position. In addition, MRI image rendering technology has improved to the point that previous problems associated with supine MRI have been overcome. The aim of the present study was therefore to examine the rate of positive surgical margins for supine MRI.

The rate of positive surgical margins for MRI in the supine position in the present study was 18.8%, which is similar to 19.7-20.0%, the rates reported for MRI in the prone position (6,12-14). US may also be used for preoperative imaging and has the advantages of being simple and inexpensive. It may be performed in the operating position; however, it has the disadvantage that only part of the breast may be visualized at a time. For this reason, MRI imaging in the prone position has been employed for preoperative assessment (15). The present results indicated comparable rates of positive margins for supine- and prone-position MRI. As the present data were obtained between 2012 and 2013, it is possible that the current positive margin rate is further reduced when examined by MRI using the most advanced technology. In the present subgroup analysis of tumors with a diameter of 2 cm or less, the positive rate was almost comparable between supine and prone MRI, similar to the results of previous studies (16,17). However, only the tumor size cannot be deduced from the positive rate. The present study indicated an increased rate of positive margins for prone MRI in the case of tumors >2 cm, but this was not the case for supine MRI in the current study. It may be difficult to determine the extent of resection in larger tumors imaged with prone-position MRI, as the relationship of the resection line to the complex tumor differs considerably between the prone imaging and supine surgical positions. By contrast, spatial variation is minimized with supine MRI due to its similarity with the supine surgical position. A total of three RCTs (6,9,12) of perioperative procedures using supine MRI reported percentages of positive margins, although the positive margin rates were different for ductal carcinoma in situ and invasive ductal carcinoma. The combined positive margin rate of 19.7-20.0% is comparable to the 18.8% reported in the present study.

Higher positive margin rates have been reported with both supine- and prone-position MRI for the HER2-positive type compared with non-HER2 type breast cancer (18). In the latter study, there are reports of a higher rate of positive margins in HER-positive breast cancer, while this does not appear to be the case for ER-positive patients. A previous RCT (6) has reported that positive margins are more likely to occur in cases of in situ HER2-positive breast cancer because ductal spread is poorly detected even by MRI. Expected improvements in MRI image-rendering technology should increase iability to detect ductal carcinoma in situ (16,19). Prospective cohort studies will then be required to evaluate the efficacy of the new technology. Previous studies have reported a difference in tumor size between positive and negative surgical margin rates (12-14), but in the present study, no such difference was found.

There are certain limitations to the present study. As its retrospective design may have introduced bias, the results should be interpreted with caution. In the present study, there was no prone position group included, so there was no direct comparison between prone- and supine-position patients. Furthermore, as it appears that there were differences in procedures among the different hospitals included, this may have introduced heterogeneity. However, it may be assumed that the data are meaningful because of the relatively large size of the study cohort. The correlation between phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit α gene mutation and positive resection margins or between PI3K/AKT/mTOR pathway-related mutations and positive resection margins is important in the analysis of clinicopathological factors, but lack of preparation, including informed consent, made it difficult to examine this in the present study.

In conclusion, the rate of positive surgical margins in patients who underwent preoperative imaging with supine MRI prior to BCS was 18.8%, which is similar to the 20% reported previously (6). Supine MRI may provide useful information for determining the extent of resection.