Description: Publication date: Available online 3 July 2018 Source: Radiotherapy and Oncology Author(s): Patricia J.A.M. Brouwers, Erik van Werkhoven, Harry Bartelink, Alain Fourquet, Claire Lemanski, Judith van Loon, John H. Maduro, Nicola S. Russell, Luc J.E.E. Scheijmans, Dominic A.X. Schinagl, Antonia H. Westenberg, Philip Poortmans, Liesbeth J. Boersma Purpose In the Young Boost trial (YBT), breast cancer patients ≤50 years of age, treated with breast conserving therapy (BCT) were randomized between a 26 Gy boost dose and a 16 Gy boost dose, with local recurrence as primary and cosmetic outcome (CO) as secondary endpoint. Data of the YBT was used to investigate which factors are related with worse cosmetic outcome after BCT. Methods From 2004 to 2011, 2421 cT1-2N0-2a breast cancer patients were randomized. CO was scored subjectively by the patient and physician, and objectively using BCCT.core: at baseline, one and four years after treatment. Associations between potential risk factors for worse cosmetic outcome, based on the objective BCCT.core, were investigated using a proportional odds model. Results At four years, CO was significantly better in the standard boost group for all three scoring methods (satisfied CO ±65% vs 55%). A photon boost, high boost dose, poor cosmesis before radiation therapy, large boost volume and adjuvant chemotherapy significantly deteriorated CO. Conclusion Important risk factors for worse CO were the use of a photon boost instead of an electron boost, a high boost dose, cosmesis at baseline, adjuvant chemotherapy and boost volume. These results can be used to define strategies aimed at improving CO.