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 range: A superposition/convolution algorithm and a Monte Carlo algorithm. The results of these models were compared in phantoms with cavities and low-density inhomogeneities. An idealized geometry was considered with inhomogeneities represented by regions of air and lung equivalent material. Measurements were performed with a parallel plate ionization chamber, thin TLDs (thermoluminescent dosimeters) and film. Dose calculations were done with a generalized Bathe model, the Pinnacle collapsed cone convolution model (CCC), and the Peregrine Monte Carlo dose calculation algorithm. Absolute central axis and off axis dose data at various depths relative to interfaces of inhomogeneities 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 density 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 measurements within the measurement and computational uncertainties. (C) 2000 American Association of Physicists in Medicine. [S0093-2405(00)00906-8].