(Tuesday, 7/31/2018) 4:30 PM - 6:00 PM
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 “quenching,� 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 0.5—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/μm to 20% for LETd=8 keV/μm. 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.
Keywords
Radiochromic Film, LET, Protons
Taxonomy
TH- Radiation dose measurement devices: Radiochromic film
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