Penumbral Distributions For High-energy Radiotherapy Photon Beams: Experimental And Theoretical Determinations

A penumbral model is adopted which considers that two distributions exist in the penumbral region of high-energy radiotherapy units namely, the photon-fluence and the dose distributions. The difference between these two distributions is associated with the lateral spread of the second electrons and is strongly dependent on the beam energy and electron density of the medium of interaction.;Specially-designed cylindrical ionization chambers of varying inside diameters were used to determine the primary penumbral dose distributions in media of varying electron density for Co-60, 6- and 31-MV x-ray beams. The results indicate that the measured penumbral dose distributions must be corrected for the inside diameter of the ionization chamber. The penumbral primary dose distributions in lung-equivalent media are significantly broader than those in soft tissue. The difference increases with the beam energy.;Primary penumbral dose distributions in media of varying electron density were also calculated using convolution techniques. Primary dose spread functions were calculated using a modified electron transport model involving both the Klein-Nishina differential collision formula and stopping powers. The primary dose spread functions (or kernels) were convolved with the appropriate primary penumbral photon-fluence distributions to obtain the corresponding penumbral primary dose distributions in homogeneous phantoms. There was satisfactory agreement between theory and experiment.;The results of the project indicate that the lateral spread of the secondary electrons set in motion by photon interactions increases with the beam energy and decreasing electron density of the medium. This increase in the lateral spread of the secondary electrons results in the broadening of the penumbral dose distributions of high-energy photon beams. The study of these effects should lead to a better understanding of the approximations involved in ignoring electron-fluence effects in the irradiated patient