BACKGROUND: Therapeutic Mammoplasty (TM) is increasingly becoming the standard of care, especially for patients with large tumor-to-breast volume ratios. The wider dissemination of oncoplastic skills warrants systems for the acquisition and assessment of safe skills. To date, TM simulations have not been developed for the acquisition or assessment of oncoplastic skills. This study aimed to design, develop, and validate a synthetic TM simulator for specialist surgical training and assessment. METHODS: A prospective, observational, and survey-based study. Breast surgeons collaborated with designers to construct a TM simulator. A modified Delphi approach was used to create a Competency Assessment Tool (CAT). Surgeons with varying operative experience performed simulated vertical scar TM. Procedures were videotaped (blinded, pseudo-anonymized), subsequently reviewed, and independently rated against CAT by three experts. Specimen radiographs and volumetric analysis were performed to assess specimen weight(g), volume(cm RESULTS: Thirty participants were recruited (10 consultants, 10 senior registrars (ST7-8), and 10 junior registrars (ST3-6)). Video-based rating scores (0-40) were significantly greater in consultants (median(IQR) = 34.0(30.5-38.0)) than in senior registrars (median(IQR) = 30.0(28.0-33.0)) and junior registrars (median(IQR) = 28.0(25.8-30.3)). The CAT scores varied significantly based on operator grade (p <
0.05). The inter-rater reliability showed fair agreement (κ = 0.379). Specimens resected by consultants had significantly greater weight and volume (p <
0.05). The consultants demonstrated the greatest uniformity in resection accuracy (p <
0.05). CONCLUSION: A novel oncoplastic simulator was developed to practice and assess TM skills. Video-based ratings and end-product assessments differentiated experts from novice surgeons, suggesting construct validity.