Broad beam attenuation of cobalt-60 gamma rays and 6-, 18-, and 25-MV x rays by lead
The measurement of photon attenuation by absorbing materials in radiation beams is usually performed using narrow beam geometry. However, radiotherapy treatments rarely simulate narrow beam conditions. The effect of broad beams on the attenuation of cobalt‐60 gamma rays and 6‐, 18‐, and 25‐MV x rays by lead is considered both theoretically and experimentally. The theoretical calculations are based on first‐scatter considerations and indicate a dependence on treatment geometry (i.e., maximum scattering angle). Measured attenuation coefficients vary by as much as 16%, comparing narrow and broad beam data for typical treatment conditions. Broad beam attenuation coefficients, calculated from the measured zero‐area values agree with the measured results to within 2% on average with a maximum deviation of 5%. A very simple empirical procedure is proposed to determine attenuation coefficients for a wide range of field sizes. This agrees with the measured data to within 0.5% on average and 4% at worst. With irregularly shaped absorbers, a sector integration technique is considered in combination with a first‐scatter calculation. For a long rectangular field, the agreement between measured and calculated attenuation coefficients is better than 1.5% for all energies. The question of accuracy in attenuation coefficients needed to give an accuracy of 3% in relative transmission is addressed and shows a dependence on absorber thickness. For 75% transmission, the required accuracy in attenuation coefficient is 10%, whereas for 25% transmission, it should be improved to 2%.