Effects of scatter generated by beam-modifying absorbers in megavoltage photon beams

Transmission through a beam-modifying absorber consists of attenuated primary beam and scattered radiation generated by the absorber. The primary component of the transmitted beam is characterized by the narrow beam attenuation coefficient which depends upon the energy of the beam and type of the absorber. In addition to beam energy and absorber material, the scatter component also depends on field size, thickness and shape of the absorber, location of the absorber with respect to the source, and the point of calculation. Based upon Compton first-scatter, a method has been developed to calculate effective broad beam transmission through any arbitrarily shaped absorber with variable thickness for any points on and off the central axis. The method requires predetermined narrow beam attenuation coefficients as a function of thickness. Transmission calculations for various absorbers such as wedges and attenuators were performed for cobalt-60 and 6-MV beams and were compared with measured data. For a cobalt-60 beam, the measured transmission fraction through a 1.33-cm-thick absorber (alloy, consisting of 55% bismuth and 45% lead) for a field size of 24 x 24 cm2 is 17% higher than the calculated value using a narrow beam attenuation coefficient. Also, for the same absorber, measured central axis transmission is as much as 3.6% higher compared to off-axis locations. The measured transmission fraction through a 1.33-cm absorber was found to differ by as much as 13% and 14% for Cobalt-60 and 6 MV, respectively, as the chamber-to-source distance was varied from 70 to 110 cm. The agreement between calculated and measured values is within 0.5% for both energies whereas conventional narrow beam calculations would have yielded errors of 18% and 19%, respectively. Similar agreement was obtained when comparing calculated and measured wedge factors as a function of field size, with the maximum deviation being 0.7%. Measured scattered doses, due to an attenuator covering part of a beam, show a maximum for a thickness of approximately one mean-free path. This is also predicted by calculations with an agreement of 0.3%.