A protocol was developed which enables the routine clinical use of a 3D biological equivalent dose (BED) plan sum for retreated patients. This protocol is used for patients who are to be retreated, where one of the plans is in a standard dose fractionation and the other is in a hypofractionated regime. A deformable registration is applied to the original dose map to the subsequent CT scan. An in-house program has been developed to sum the registered dose between original and subsequent dose plans. After the sum is accomplished, the 3D BED sum is converted to dose through a 3D voxel by voxel solution of the linear quadratic BED equation. The use of BED based 3D dose summation for replanned patients gives an improved picture of the overall dose distribution for situations where the daily dose is different in both plans. This tool allows a more thorough evaluation of the plan sum and provides an alternative to the worst case BED scenario using point maximum statistics, giving a potential for better PTV coverage.