Photon dose at the maze entrance door: The comparison of flattening filter and flattening filter free working modes.

In recent years, field flattening free accelerators have been introduced in therapy practice. One of the objective of these measurements was to establish if the maze door, designed for accelerators operating with flattening filter can provide adequate shielding in field flattening free mode of operation. Linac installed in this standard one band maze vault is equipped to operate at 6 MV with field flattening filter and in field flattening free mode of operation. Series of measurements of the photon dose at the maze door (with different jaws openings and gantry positions) were performed in both operation modes with and without water canister to mimic standard therapy conditions. In this paper results of photon dose measurements, performed at the maze door of the therapy linear accelerator vault are presented in order to compare photon dose in flattening filter and flattening filter free operation modes. It was obtained that in field flattening free mode of operation, the dose at the maze door is always lower than the dose measured in standard mode of operation with the field flattening filter. In the case when FFF therapy practice should start in some existing therapy vault, no additional shielding measures need to be added at the existing maze door.