AN EGS4 MONTE-CARLO EXAMINATION OF GENERAL CAVITY THEORY

EGS4 Monte Carlo simulations have been performed to examine general ca vity theory for a number of TLD cavity materials irradiated in megavol tage photon and electron beams. The TLD materials were LiF, Li2B4O7, C aF2 and CaSO4 irradiated in Perspex, water, Al, Cu and Pb phantoms. Fo r megavoltage photon beams, this has been done by determining the dose component (1 - d) resulting from photon interactions in the cavity co mpared with the dose component resulting from photon interactions in t he phantom material (df by Monte Carlo simulations and analytical tech niques. The results indicate that the Burlin exponential attenuation t echnique can overestimate the dose contribution from photon interactio ns in a I mm thick LiF cavity by up to 100% compared with the Monte Ca rlo results for LF TLDs irradiated in a water or Perspex phantom. Howe ver, there is agreement to within 1% in the quality dependence factor, determined from Burlin's cavity theory, Monte Carlo simulations and e xperimental measurements for LiF and Li2B4O7 TLDs irradiated in a Pers pex or a water phantom. The agreement was within 3% for CaF2 TLDs. How ever there was disagreement between Monte Carlo simulations and Burlin 's theory of 6 and 12% for LiF TLDs irradiated in copper and lead phan toms respectively. The adaptation of Burlin's photon cavity theory and other modifications to his photon general cavity theory for electrons have been shown to be seriously flawed.