PURPOSE: To analyse the influence of tumor volume/breast volume ratio (TV/BV ratio) measured on magnetic resonance imaging (MRI) and other factors on surgeons’ choice between large volume displacement oncoplastic surgery (LVOS) versus mastectomy (M) in patients with large sized tumors. Secondly, we investigate the predictive value of TV/BV ratio and other possible predictors for cosmetic results. MATERIALS AND METHODS: We retrospectively reviewed 80 MRI examinations performed on 77 breast cancer patients who underwent M (58 breasts, 72.5%) or LVOS (22 breasts, 27.5%) at our institution between January 2016 and December 2017. The TV and BV measurements were performed by a semiautomated analysis and the TV/BV ratio was calculated by dividing TV by BV in cm3 and multiplying it by 1,000. Cosmetic result was determined by an expert panel assessment of postoperative photographs. RESULTS: Median TV/BV was significantly higher in the M group (44,8 IQR 17,3-93,6) than in the LVOS group (17,5 IQR 11,7-57,5) (P=0.002). Multifocal/multicentric disease (P=0.005), lower degree of breast ptosis (P<0.0001) and unfavourable tumor location (P=0.024) are significantly more frequent in the M group. After multivariable linear regression, the independent predictors for cosmetic result were: unfavourable tumor location (OR 6.637 95% CI 1.564– 28.172 P=0.010) and a higher TV/BV ratio (OR 4.907 95% CI 1.461–16.478 P=0.010). CONCLUSION: Preoperative evaluation of TV/BV ratio, tumor location and tumor multifocality/multicentricity could improve treatment decision making (LVOS versus M) in breast cancer patient eligible for both options. Increasing TV/BV ratio and unfavourable tumor location are adversely affecting cosmetic result.