Experimental investigation of GafChromic® EBT3 intrinsic energy dependence with kilovoltage x rays, 137 Cs, and 60 Co.

Abstract

To determine experimentally the intrinsic energy response, kbq , of EBT3 GafChromic® radiochromic film with kilovoltage x rays, 137 Cs, and 60 Co in therapeutic and diagnostic dose ranges through direct measurement with an accompanying mathematical approach to describe the physical processes involved. The EBT3 film was irradiated with known doses using 60 Co, 137 Cs, and 13 NIST-matched kilovoltage x-ray beams. Seven dose levels, ranging from 57 to 7002mGy, were chosen for this work. Monte Carlo methods were used to convert air-kerma rates to dose rates to the film active layer for each energy. A total of 738 film dosimeters, each measuring (1.2×1.2) cm2 , were cut from three film sheets out of the same lot of the latest version of EBT3 film, to allow for multiple dosimeters to be irradiated by each target dose and beam quality as well as unirradiated dosimeters to be used as controls. Net change in optical density in excess of the unirradiated controls was measured using the UWMRRC Laser Densitometry System (LDS). The dosimeter intrinsic energy response, kbq , for each dose level was determined relative to 60 Co, as the ratio of dosimeter response to each beam quality relative to the absorbed dose to the film active volume at the same dose level. A simplified, single-hit mathematical model was used to derive a single-free-parameter, β, which is a proportionality constant that is dependent on beam quality and describes the microdosimetric interactions within the active layer of film. The response of β for each beam quality relative to 60 Co was also determined. kbq was determined for a wide range of doses and energies. The results show a unique variation of kbq as a function of energy, and agree well with results from other investigations. There was no measurable dose dependence for kbq within the 500-7002mGy range outside of the expanded measurement uncertainty of 3.65% (k=2). For doses less than 500mGy, the signal-to-noise ratio was too low to determine kbq accurately. The single-free-parameter, β, fit calculations derived from the single-hit model show a correlation with kbq that suggests that β, at least in part, characterizes the microdosimetric interactions that determine kbq . For the beam qualities investigated, a single energy-dependent kbq correction can be used for doses between 500 and 7002mGy. Using the single-hit model with the single-free-parameter fit to solve for β shows promise in the determination of the intrinsic energy response of film, with β being the mathematical analog of the measured kbq .