Improving the dose distribution for wedged fields

Full text: The aim of this research was to construct a wedge for a radiotherapy beam which reproduced the dose distribution planned by a treatment planning computer in the non-wedged axis. Dose profile curves in the non-wedged axis for a wedged field irradiation show a decrease in dose (sagging) with off-axis distance. This sagging is mainly due to the softening of the energy spectrum off-axis (Chui C-S and LoSasso T, Med. Phys. 21:1685-90, 1994). Other contributing factors include the increase in scattered photon fluence and the increase in the pathlength of off-axis photons due to the angulation of the beam off axis. Because of this sagging, there is an underdosing of up to 7% in the non-wedged direction. This discrepancy increases with off-axis distance. Ideally, the non-wedged profile shape should match that of an open field profile, as the planning computer assumes this shape in the dose calculation. Theory based on the dose distribution beneath the old wedge was used to calculate the required thickness of the new wedge at each off-axis point. Two assumptions were made about the particle transport: (i) that dose = collision kerma (which is a valid assumption at the measurement point) and (ii) that the photon spectrum at a point beneath the wedge is unchanged if the wedge thickness at that point is changed slightly. This second assumption is valid if the change in wedge thickness is small, and hence assumes that the change in wedge thickness will only affect the photon number beneath the wedge. Using the wedge thicknesses derived from the theoretical calculations, a new 60deg wedge was milled, and the dose distribution beneath this wedge was measured, and compared to open field profiles in the non-wedged axis direction. Dose profile results using the new wedge in the non-wedged axis direction differed from the open field profile by less than 1%, an improvement of over 6% on the old wedge. Making new wedges as described above will give a dose distribution which does not exhibit sagging in the non-wedged direction, and is in better agreement with the dose distribution calculated by the treatment planning computer