In the dose calculation at an arbitrary point in a water-equivalent homogeneous medium irradiated by an X-ray megavoltage wedged beam (w), several treatment planning systems (TPS) based on the Bently-Milan data processing procedure use algorithms which do not distinguish between wedged OF(s)w and open OF(s)o output factor or between field size dependent WF(s) and not WF wedge factor. This is equivalent to neglecting the difference between open (o) and wedged (w) fields both in relation to back-scattered radiation from secondary collimators to the beam monitor chamber and to scattered radiation generated by the water-phantom at the depth of measurement. The output factor can be expressed by: OF(s) = Sc(s)* Sp(s) where Sc(s) is the collimator scatter factor and Sp(s) is the phantom scatter factor. The approximation between wedged OF(s)w and open OF(s)o is thus equivalent to ignoring the fact that the insertion of the wedge in an open field modifies the back-scattered radiation from secondary collimators to the beam monitor chamber and the scattered radiation in the phantom. The lack of wedge factor's field size dependence WF(s) or the equivalent lack of wedged field output factor OF(s)w in the beam data installed in TPS is in appreciable source of systematic error in the calculation of the absorbed dose. This systematic uncertainty can be evaluated by measuring the wedge factor's field size dependence. In our study of WF(s) for 25-MV X-rays generated by our Linac "Saturne-43" (by GE), the relative wedge factor variation with respect to the reference value WF(so) ranges from (-.7%) for the smallest wedged field size (5 x 5) cm2 to (+2.6%) for the largest wedged field size (20 x 20) cm2. It is clear that the same relative variation reproduces itself identically as a systematic uncertainty in the calculation of the absorbed dose.