Empirical studies on the correlation of the proton distal dose fall-off and prompt gamma distribution

Abstract

Prompt-gamma distributions in the proton therapy were found as well correlated with the distal fall-off measured using an ionization chamber in the therapy energy range, although the correlation becomes blurred at higher beam energies. A multi-layered collimator system was used to collimate the gammas emitted from 90 deg of the direction of a proton beam, in which a CsI scintillator specially contoured was used as the detector. The reference dose distributions were measured using a Markus chamber in water phantom. To suppress the background and enhance the signal it was needed to select the gamma energy above 4 MeV. It seems that the prompt gamma measurement is a promising method to locate the proton distal dose fall-off in situ.