The variation of wedge factors with field size on a linear accelerator with wedge tray beneath secondary collimators

We refer to the recent technical note by Thomas (1990) in which the variation in wedge factor with field size was discussed. The linear accelerator used to produce the results in that article was a Dynaray LA 20 (Asea-Brown Boveri), in which the motorized wedge lies between the beam monitor and the movable, secondary collimators (Fig. 1a). We suggest that this wedge factor variation with field size may not occur with all designs of linear accelerator.