Attenuation in high-energy x-ray beams.

Attenuation factors in water have been measured by a narrow-beam technique in various portions of x-ray beams with nominal energies of 6 and 25 MV, with and without a wedge in the beam. The results were analyzed in terms of an attenuation coefficient mu for small water thicknesses and a beam-hardening coefficient eta that describes the change in attenuation per unit depth. The variation of mu within the field was significant, about 0.5% per centimeter at 6 MV and 0.8% per centimeter at 25 MV for open beams. The heavy wedge used in these experiments caused significant (about 10%) beam hardening at 6 MV, softened the beam somewhat at 25 MV, and increased the variation of mu within the field to 3%-5%. These effects should be taken into account in dose calculations, and correction factors can be designed based on the variation of mu with off-axis radius for open beams and with off-axis position for wedged beams. The experimental technique, based on two measurements with the beam going through a water tank with either 26- or 50-cm path length, was simple and highly reproducible. The beam hardening with depth in water, i.e., the value of eta, was readily determined but found to be clinically insignificant.