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

Abstract

Purpose: To determine experimentally the intrinsic energy response, k bq , of EBT3 GafChromic â 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. Methods: 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 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 9 1.2) cm 2 , 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 bq , 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, b, 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 b for each beam quality relative to 60 Co was also determined. Results: k bq was determined for a wide range of doses and energies. The results show a unique variation of k bq as a function of energy, and agree well with results from other investigations. There was no measurable dose dependence for k bq 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 bq accurately. The single-free-parameter, b, fit calculations derived from the single-hit model show a correlation with k bq that suggests that b, at least in part, characterizes the microdosimetric interactions that determine k bq . Conclusions: For the beam qualities investigated, a single energy-dependent k bq 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 b shows promise in the determination of the intrinsic energy response of film, with b being the mathematical analog of the measured k bq .

LET response variability of Gafchromic EBT3 film from a Co calibration in clinical proton beam qualities

“…The results from this work agree with previous studies indicating that a 20% or larger under‐response compared to Co is observed near the BP. A similar maximum under‐response with EBT3 film has also been reported for the RE from a 115.2 MeV/u carbon ion beam and for the film's intrinsic energy response from a M20 x‐ray beam quality . It also appears from the results shown in Fig.…”

“…Such a behavior has been previously observed in the work of Hammer et al. where the low‐dose (i.e., 500 mGy) regime response showed evidence of increasing intrinsic film response than that of higher dose regimes across photon energies ranging from 10 to 1250 keV …”

An investigation into the INTRABEAM miniature x‐ray source dosimetry using ionization chamber and radiochromic film measurements

“…19,21,22 Accurate dosimetry with these films is challenging, as they are known to have an energy-dependent response at low photon energies. [23][24][25] Eaton and Duck reported differences of up to 6.9% between percent depth dose curves measured with an ionization chamber and Gafchromic EBT film, 19 and up to 8.8% local difference (<10 mm from applicator surface) and 4.8% absolute (>10 mm from applicator surface) difference from the manufacturer (i.e., TARGIT) depth dose curve. They attributed these differences to other experimental uncertainties, such as positioning, rather than film energy dependence.…”

“…Radiochromic films, such as Gafchromic models, have been used in previous studies to investigate the relative and absolute dosimetry of the INTRABEAM system . Accurate dosimetry with these films is challenging, as they are known to have an energy‐dependent response at low photon energies . Eaton and Duck reported differences of up to 6.9% between percent depth dose curves measured with an ionization chamber and Gafchromic EBT film, and up to 8.8% local difference (<10 mm from applicator surface) and 4.8% absolute (>10 mm from applicator surface) difference from the manufacturer (i.e., TARGIT) depth dose curve.…”

Response characterization of EBT‐XD radiochromic films in megavoltage photon and electron beams

“…Radiochromic films due to their high spatial resolution, near tissue‐equivalence, and ability to measure two‐dimensional (2D) planar dose are readily used in radiotherapy dosimetry and quality assurance . The latest of EBT‐series radiochromic films, EBT‐XD (Ashland Inc, Bridgewater, NJ, USA) was specifically designed for high‐dose radiotherapy applications, such as stereotactic radiosurgery and stereotactic body radiation therapy.…”