Simulation of photon energy spectra from Varian 2100C and 2300C/D Linacs: simplified estimates with PENELOPE Monte Carlo models.

Photon energy spectra of Varian Clinac 2100C and 2300C/D treatment heads were calculated with PENELOPE-2006, using simplified Monte Carlo models. These spectra constitute the basis of beam models used for the calculation of correction factors required for the Austrian absorbed dose to water primary standard. To validate the models, simulated depth dose curves in water and calculated radiation quality factors (TPR(20,10)) were compared with ionization chamber measurements. In addition the results are discussed in context with detailed Monte Carlo studies from the literature in reference to mean spectral photon energy and degree of equivalence between simulated and calculated percentage-depth-dose curves. Mean absolute deviations between measured and simulated depth dose curves smaller than 2.0% were found. The differences between simulation and measurement of TPR(20,10) were smaller than 2.6%. This shows the usefulness of the simplified beam models in applications since their dependence on energy spectra and radiation quality is small in comparison to its energy range.