Breast deformation during the course of radiotherapy: The need for an additional outer margin

•Significant changes were observed in the breast surface during the course of breast cancer radiotherapy.•Because of deformations an additional outer margin is required which is important to consider when using IMRT or VMAT.•An additional margin of 8 mm outside the breast surface is required to take account of the changes in the breast surface.•The breast deformations were not fully predictable, although a moderate correlation was found with body mass index.