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 μ for small water thicknesses and a beam‐hardening coefficient η that describes the change in attenuation per unit depth. The variation of μ 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 μ 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 μ 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 η, was readily determined but found to be clinically insignificant.