Evaluation of the scatter field for high-energy photon beam attenuators.

Based upon sector integration, a method has been developed to evaluate the scatter from attenuating beam modulators at any point in the field for arbitrarily shaped fields and attenuators with variable thickness. The method requires preliminary measurements of narrow and broad beam transmission fractions as a function of filter thickness and field size. The ratio Sp of the contribution from photons scattered by the attenuator to the non-attenuated primary contribution was derived from these measurements. Sp was determined for x-ray beam energies between 4 and 23 MV with brass and lead attenuators. This quantity was found to be practically independent of beam energy for a given field size and material. The variation of Sp as a function of slab thickness for attenuators covering the entire beam showed a maximum for a thickness of approximately one mean free path. This maximum represents about 6.0% of the transmitted primary dose for an extreme case of a very heavily (1.6 cm thick lead slab) attenuated 15 cm x 15 cm field. The 'scatter field', corresponding to the scatter contribution from the attenuator across the field, was calculated for different partial attenuators and wedges. The results show that this component has a limited influence on calculation of dose distribution, but should be taken into account in absolute dosimetry analysis for large fields and thick wedge filters.