Radiochromic film dosimetry of a low energy proton beam

In this work some dosimetric characteristics of MD-55-2 GafChromic films we re studied in a low energy proton beam (21.5 MeV) directly in a water phant om. The nonlinearity of the optical density was quantified by a factor P-li n. A correction factor P-en, that accounts for optical density dependence o n the energy, was empirically determined. The effects of detector thickness in depth dose measurements and of the film orientation with respect to bea m direction were investigated. The results show that the MD-55-2 films prov ide dose measurements with the films positioned perpendicularly to the prot on beam. A dosimetric formalizm is proposed to determine the dose to water at depth d, with films oriented perpendicularly to the beam axis. This form alism uses a calibration factor of the radiochromic film determined directl y on the proton beam at a reference depth in water, and the P-lin factor, t hat takes into account the nonlinearity of the calibration curve and the P- en factor that, in turn takes into account the change of proton beam energy in water. The MD-55-2 films with their high spatial resolution and the qua siwater equivalent material are attractive, positioned perpendicularly alon g the beam axis, for the absolute dose determination of very small beam siz es and modulated proton beams. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00)01107-X].