Surface dose measurements in clinical 6 MeV-X-photon beams : Comparison of Measurements and MC calculations.

For radiotherapy of superfi cial lesions, but also in intensity modulated radiotherapy (IMRT), 6 MeV-X-photon beams are preferred, with relative surface doses, dependent on fi eld size and SSD, of 8 % to 28 %. Depending on absorption material within the beam this values can vary considerable.The infl uence of a block tray, different metals and a new type of transmission detector on surface doses as well as on other beam parameters have been investigated. The surface doses as well as some other fi eld parameters have been checked by means of a plane parallel ionization chamber (Markus chamber, type 23 343, PTW-Freiburg,Germany). Because of specifi c design features the readings have to be corrected, and the results have been verifi ed by Monte Carlo (MC) simulations and calculations: Measurements agreed well with calculations within Δ ≈ 3 %. So it could be shown for the used Markus chamber, type 23 343, that the applied correction function is suitable for open fi elds as well as for blocked fi elds at different clinical used SSDs.