The impact of electron transport on the accuracy of computed dose

The aim of this work was to investigate the accuracy of dose predicted by a Bathe power law correction, and two models which account for electron rang e: A superposition/convolution algorithm and a Monte Carlo algorithm. The r esults of these models were compared in phantoms with cavities and low-dens ity inhomogeneities. An idealized geometry was considered with inhomogeneit ies represented by regions of air and lung equivalent material. Measurement s were performed with a parallel plate ionization chamber, thin TLDs (therm oluminescent dosimeters) and film. Dose calculations were done with a gener alized Bathe model, the Pinnacle collapsed cone convolution model (CCC), an d the Peregrine Monte Carlo dose calculation algorithm. Absolute central ax is and off axis dose data at various depths relative to interfaces of inhom ogeneities were compared. Our results confirm that for a Bathe correction, dose errors in the calculated depth dose arise from the neglect of electron transport. This effect increases as the field size decreases, as the densi ty of the inhomogeneity decreases, and with the energy of incident photons. The CCC calculations were closer to measurements than the Bathe model, but significant discrepancies remain. Monte Carlo results agree with measureme nts within the measurement and computational uncertainties. (C) 2000 Americ an Association of Physicists in Medicine. [S0093-2405(00)00906-8].