On the Correction of Dose Profile Discrepancies by Introducing Air in the Derivation of an Electron Spectrum

Abstract

Knowledge of the energy spectrum of an electron beam is relevant for accurate dose calculation in radiotherapy. In previous works, it has been possible to reconstruct the electron spectrum of various clinical energies (6, 9, 12 and 15 MeV) within the typical percentage of clinical acceptance (Pa > 95 %) according to the gamma index (GI) (2 %/2mm), for both depth dose percentages (PDD) and dose profiles (DP), except for 6 MeV profiles. Therefore, the purpose of this work was to introduce air between the radiation source and the phantom surface to simulate both the monoenergetic PDDs necessary in the reconstruction of the spectrum of a 6 MeV beam and to obtain the PDD of this spectrum. Validation was performed using the gamma index with the typical threshold for clinical acceptance. The results showed that the PDP of the vacuum spectrum had a better agreement than the PDP of the air spectrum (Pa = 100 %), with respect to the measured PDD (Pa = 97 %). Regarding the PD, the introduction of air improved the agreement in clinical interest but not enough to reach the acceptance percentage. It is concluded that this technique does not seem to be a good alternative to correct the discrepancies in the field edges between the DP of an inversely reconstructed spectrum and the measured DP.