Background: Preoperative objective predictions of cosmetic result after breast conserving surgery (BCS) has the potential to aid in surgical treatment decision making. Our aim was to investigate the predictive value of tumor volume in relation to breast volume (TV/BV ratio) for cosmetic result. Methods: Sixty-nine invasive breast cancer women with preoperative MRI and treated by BCS and radiotherapy in 2007-2012 were prospectively included. Simple excision or basic oncoplastic techniques were used, but no volume displacement. TV/BV ratio was measured in the MRI while 3D-projected in virtual reality environment (I-Space). Cosmetic result was assessed by patient questionnaire, panel evaluation, and breast retraction assessment (BRA). Quality-of-life was assessed by EORTC QLQ-C30 and BR23. Results: Intraobserver and interobserver correlation coefficients for tumor and breast volume were all >0.95. Increasing TV/BV ratio correlated with decreasing cosmetic result as determined by patient, panel, and BRA. TV/BV ratio was a significant independent predictor for the panel evaluation (P = 0.028), as was tumor location (P < 0.05), and together they constituted a good prediction model (AUC 0.83). Conclusions: TV/BV ratio was a precise and independent predictor for cosmetic result determined by a panel and can be used as preoperative prediction tool to enable more informed surgical treatment decision making. (C) 2014 Wiley Periodicals, Inc.