Reconsideration of the backscatter components in the intermediate cavity theory for photon beams

Cavity theories are used to relate the dose in a medium to the dose in a cavity. No satisfactory analytical approach exists for the case of intermediate cavities, such as TLDs, when interface effects are important. Burlin proposed a general cavity theory that accounts for the cavity size but which ignores interface effects between cavity material and the surrounding materials. Horowitz modified Burlin's theory to include a more systematic account for the contribution of electrons generated in the cavity. Both Kearsley and Haider proposed new models that consider backscattering effects at cavity interfaces. However, these last models introduce additional parameters whose values are not easily available. By using a pure analytical approach, higher orders of backscatter terms were considered and a photon cavity expression was derived. The proposed formula depends on backscattering and attenuation coefficients of the cavity material and the surrounding media as well as the size of the cavity and angular effects. This expression has the Burlin and Bragg-Gray formulae as limiting cases and follows better the pattern of Ogunleye's experimental results. (author)