Room: Karl Dean Ballroom C
Purpose: Gafchromic film is a valuable dosimetric tool, primarily due to its sub-millimeter resolution. For accurate proton dosimetry, the dependence of film response on linear energy transfer must be characterized and calibrated. In this work, we characterized film under-response, or â€œquenching,â€? as a function of dose-weighted linear energy transfer (LETd) in several proton fields and established a simple, linear relationship with LETd.
Methods: We performed measurements as a function of depth in a PMMA phantom irradiated by a spot-scanning proton beam. The fields had energies of 71.3MeV, 71.3MeV with filter, and 159.9MeV. At each depth (increments of 0.5â€”1mm in the Bragg peak), we measured dose with an ion chamber and EBT3 film. EBT3 under-response was characterized by the ratio of dose measured with film to that with ion chamber. LETd values for our experimental setup were calculated using in-house clinical Monte Carlo code. The under-response for all measurements was plotted versus LETd and fit to a linear function. Finally, we applied the linear under-response relationship to a film measurement within a spread-out Bragg peak (SOBP).
Results: Film under-response increased with LETd, from 10% under-response for LETd=5keV/Î¼m to 20% for LETd=8 keV/Î¼m. Results for all film measurements were fit to a linear function of under-response, U, with respect to LETd: U=-0.0251*LETd+1.017. Without correcting for LETd-dependence in the SOBP measurement, the discrepancy between film and Monte Carlo profiles was 25% at the distal edge. With correction, the profiles were nearly identical.
Conclusion: EBT3 response depends significantly on LETd, potentially under-responding by 25% in clinically-relevant scenarios. Therefore, it is insufficient to perform only a dose calibration; LET calibration is also necessary for accurate proton dosimetry. The LETd-dependence of EBT3 can be described by a single, linear function over a range of clinically-relevant LETd values. Such a function is straightforward for clinical implementation.