A theoretical study of cavity chamber correction factors for photon beam absorbed dose determination

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Abstract
Theoretical expressions were derived for the contributions to ionization density in cavity chambers from secondary electrons arising from photon absorption in wall, build-up cap and surrounding medium. The method allows the derivation of C.lambda. [absorbed dose conversion factor] and displacement factors, which are evaluated for the Farmer graphite chamber for a range of photon beams. Differences between C.lambda. values calculated for air-walled and water-walled chambers were consistent with those determined by Nahum and Greening. The currently recommended displacement factor for the Farmer chamber with cap was in error by about 0.8%. On the basis of a comparison between theoretically predicted air doses in cavities and the experimental results of Fregene, no further correction to C.lambda. is required for the chamber effective point of measurement.