A linear relationship for the LET-dependence of Gafchromic EBT3 film in spot-scanning proton therapy

Radiochromic film (RCF) is a valuable dosimetric tool, primarily due to its sub-millimeter spatial resolution. For accurate proton dosimetry, the dependence of film response on linear energy transfer (LET) must be characterized and calibrated. In this work, we characterized film under-response, or ‘quenching’, as a function of dose-weighted linear energy transfer (LET d ) in several proton fields and established a simple, linear relationship with LET d . We performed measurements as a function of depth in a PMMA phantom irradiated by a spot-scanning proton beam. The fields had energies of 71.3 MeV, 71.3 MeV with filter, and 159.9 MeV. At each depth (measurements taken in depth step sizes of 0.5–1 mm in the Bragg peak), we measured dose with a PTW Markus ion chamber and EBT3 RCF. EBT3 under-response was characterized by the ratio of dose measured with film to that with ion chamber. LET d values for our experimental setup were calculated using in-house clinical Monte Carlo code. Measured film under-response increased with LET d , from approximately 10% under-response for LET d = 5 keV µ m −1 to approximately 20% for LET d = 8 keV µ m −1 . The under-response for all measurements was plotted versus LET d . A linear fit to the data was performed, yielding a function for under-response, , with respect to LET d : . Finally, the linear under-response relationship was applied to a film measurement within a spread-out Bragg peak (SOBP). Without correcting for LET d -dependence in the SOBP measurement, the discrepancy between film and Monte Carlo profiles was greater than 15% at the distal edge. With correction, the corrected film profile was within 2% and 1 mm of the Monte Carlo profile. RCF response depends on LET d , potentially under-responding by >15% in clinically-relevant scenarios. Therefore, it is insufficient to perform only a dose calibration; LET calibration is also necessary for accurate proton film dosimetry. The LET d -dependence of EBT3 can be described by a single, linear function over a range of clinically-relevant proton therapy LET d values.