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

Abstract

Purpose To establish a method of accurate dosimetry required to quantify the expected linear energy transfer (LET) quenching effect of EBT3 film used to benchmark the dose distribution for a given treatment field and specified measurement depth. In order to facilitate this technique, a full analysis of film calibration which considers LET variability at the plane of measurement and as a function of proton beam quality is demonstrated. Additionally, the corresponding uncertainty from the process was quantified for several measurement scenarios. Materials and methods The net change in optical density (OD) from a single version of GafchromicTM EBT3 film was measured using an Epson flatbed scanner and NIST‐traceable OD filters. Film OD response was characterized with respect to the known dose to water at the point of measurement for both a NIST‐traceable 60Co beam at the UWADCL and several clinical single‐energy and spread‐out Bragg peak (SOBP) proton beam qualities at the Northwestern Medicine Chicago Proton Center. Increasing proton LET environments were acquired by placing film at increasing depths of Gammex HE Solid Water® whose water‐equivalent thickness was characterized prior to measurement. Results A strong LET dependence was observed near the Bragg peak (BP) consistent with previous studies performed with earlier versions of EBT3 film. The influence of range straggling on the film's LET response appears to have a uniform effect toward the BP regardless of the nominal beam energy. Proximal to this depth, the film's response decreased with decreasing energy at the same dose‐average LET. The opposite trend was observed for depths past the BP. Changes in the SOBP energy modulation showed a linear relationship between the film's relative response and dose‐averaged LET. Relative effectiveness factors (RE) were observed to range between 2%–7% depending on the width of the SOBP and depth of the film. Using the field‐specific calibration technique, a total k = 1 uncertainty in the absorbed dose to water was estimated to range from 4.68%–5.21%. Conclusion While EBT3 film's strong LET dependence is a common problem in proton beam dosimetry, this work has shown that the LET dependence can be taken into account by carefully considering the depth and energy modulation across the film using field‐specific corrections. RE factors were determined with a combined k = 1 uncertainty of 3.57% for SOBP environments and between 3.17%–4.69% for uniform, monoenergetic fields proximal to the distal 80% of the BP.

Dose‐ rather than fluence‐averaged LET should be used as a single‐parameter descriptor of proton beam quality for radiochromic film dosimetry

“…The relative effectiveness (RE) was introduced to quantify the LET quenching effect . In literature RE is mostly defined either as the ratio of the absorbed dose to water, , and the film dose, , required to yield the same effect or as the ratio of and the apparent film dose, , applying the parameters obtained at Those two definitions are fundamentally different and cannot be mutually exchanged due to the nonlinear dependency of dose and net optical density [Eq. ].…”

Experimental and Monte Carlo characterization of a dynamic collimation system prototype for pencil beam scanning proton therapy

“…The signal linearity to increasing doses was assessed by the percent difference of the maximum residual following a linear fit optimized using a trust‐region method default to MATLAB’s fit function. The relative profile from the EBT3 film was converted from netΔOD following the methods of Smith et al 20 and is plotted in Fig. 10.…”

“…Therefore, a secondary check was performed to first benchmark the lateral profile measurements of the Lynx to the profiles measured with Gafchromic ® EBT3 film. While EBT3 film has been shown to have a non‐linear sensitivity quenching response for high LET radiations, 18,19 previous work has shown the quenching effect to be constant with change in optical density and thus a suitable dosimeter for relative dosimetry 20 …”

“…While EBT3 film has been shown to have a non-linear sensitivity quenching response for high LET radiations, 18,19 previous work has shown the quenching effect to be constant with change in optical density and thus a suitable dosimeter for relative dosimetry. 20 The dose response of the Lynx scintillator was benchmarked across multiple beam energies and residual depths in Gammex Solid Water â . For each irradiation, a 10 cm 9 10 cm uniform field was delivered with beam spots placed 2.5 mm apart at the isocenter.…”

Characterization of a Low-Cost Plastic Fiber Array Detector for Proton Beam Dosimetry

“…Several detector types used in proton dosimetry for therapeutic applications, such as plastic and inorganic scintillators, gel detectors and Gafchromic films, have a response that shows some level of dependence on the proton energy [ 18 ]. For example, LET response variability of Gafchromic EBT3 film from a Co-60 calibration in clinical proton beam qualities is demonstrated in [ 19 , 20 ]. The effect is particularly visible when such detectors are employed to measure the dose-depth curves of quasi-mono-energetic proton beams, which then exhibit a less pronounced peak dose compared to ionization chamber measurements.…”