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

Purpose: To determine experimentally the intrinsic energy response, k , of EBT3 GafChromic radiochromic film with kilovoltage x rays, Cs, and Co in therapeutic and diagnostic dose ranges through direct measurement with an accompanying mathematical approach to describe the physical processes involved.

Methods: The EBT3 film was irradiated with known doses using Co, Cs, and 13 NIST-matched kilovoltage x-ray beams. Seven dose levels, ranging from 57 to 7002 mGy, 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) cm , 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, k , for each dose level was determined relative to 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 Co was also determined.

Results: k was determined for a wide range of doses and energies. The results show a unique variation of k as a function of energy, and agree well with results from other investigations. There was no measurable dose dependence for k within the 500-7002 mGy range outside of the expanded measurement uncertainty of 3.65% (k = 2). For doses less than 500 mGy, the signal-to-noise ratio was too low to determine k accurately. The single-free-parameter, β, fit calculations derived from the single-hit model show a correlation with k that suggests that β, at least in part, characterizes the microdosimetric interactions that determine k .

Conclusions: For the beam qualities investigated, a single energy-dependent k correction can be used for doses between 500 and 7002 mGy. 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 k .