Compensators for intensity-modulated beams.

This study describes the importance of attenuator scatter in the construction of compensators. The attenuator used in this study was Lipowitz metal, commonly referred to as cerrobend. Linear attenuation coefficients of cerrobend were measured in air for different thickness of cerrobend sheets and different field sizes for a 6-MV photon beam. The magnitude of the dose contribution from photons scattered by the attenuator was measured. The variations of beam hardening and the scatter to primary ratio as a function of the thickness of cerrobend and varying field size were investigated. The compensators in this study were produced using a simple exponential attenuation model and the measured linear attenuation coefficients. It was found that the beam hardening effect was significant, and can lead to an error of 6.2% in the transmission, for 6 cm of cerrobend in the beam. The maximum scatter contribution to the measured fluence was 19.8% of the transmitted primary dose for a 20 x 20-cm2 field size, and 6 cm of cerrobend in the beam. For a simple wedge-step compensator; there was a maximum deviation of 6% between the measured and our predicted fluence profile. For simple compensators, this deviation can be attributed to scatter.