Bragg-Gray theory and ion chamber dosimetry for photon beams

The validity of the Bragg-Gray cavity theory in photon radiation dosimetry for photon energies from 10 keV to 10 MeV has been investigated quantitatively. A crucial assumption for Bragg-Gray cavity theory to be valid is that the dose from photon interactions in the detector material is negligible. The ratio, F_air, of the absorbed dose resulting from photon interactions in an air cavity, D_air ^PA, to that in air under the condition of charged-particle equilibrium, D_air ^CPE, has been used as a parameter to determine if the air cavity can be classified as a Bragg-Gray cavity. Monte Carlo calculated results show that, for monoenergetic photon beams of energies above about 220 keV, the dose ratio, F_air, is smaller than 0.05 for an air cavity of 6 mm thickness and 6 mm diameter in vacuum. Furthermore, it is shown that the Burlin general cavity theory seriously overestimates the departure from Bragg-Gray behaviour. For clinical photon beams the dose ratio, F_air, is 0.29 for a 150 kV_p beam and 0.27 for a 240 kV_p beam compared to 0.006 for a ⁶⁰Co beam if the cavity is placed at a depth of 5 cm in water. This study confirms that typical air-filled ionization chambers cannot be considered to be Bragg-Gray cavities for low- and medium-energy photon radiation.