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

Abstract

Purpose: The dose response of Gafchromic EBT3 films exposed to proton beams depends on the dose, and additionally on the beam quality, which is often quantified with the linear energy transfer (LET) and, hence, also referred to as LET quenching. Fundamentally different methods to determine correction factors for this LET quenching effect have been reported in literature and a new method using the local proton fluence distribution differential in LET is presented. This method was exploited to investigate whether a more practical correction based on the dose-or fluence-averaged LET is feasible in a variety of clinically possible beam arrangements. Methods: The relative effectiveness (RE) was characterized within a high LET spread-out Bragg peak (SOBP) in water made up by the six lowest available energies (62.4-67.5 MeV, configuration "b 1 ") resulting in one of the highest clinically feasible dose-averaged LET distributions. Additionally, two beams were measured where a low LET proton beam (252.7 MeV) was superimposed on "b 1 ", which contributed either 50% of the initial particle fluence or 50% of the dose in the SOBP, referred to as configuration "b 2 " and "b 3 ," respectively. The proton LET spectrum was simulated with GATE/ Geant4 at all measurement positions. The net optical density change differential in LET was integrated over the local proton spectrum to calculate the net optical density and therefrom the beam quality correction factor. The LET dependence of the film response was accounted for by an LET dependence of one of the three parameters in the calibration function and was determined from inverse optimization using measurement "b 1 ." This method was then validated on the measurements of "b 2 " and "b 3 " and subsequently used to calculate the RE at 900 positions in nine clinically relevant beams. The extrapolated RE set was used to derive a simple linear correction function based on dose-averaged LET (L d ) and verify the validity in all points of the comprehensive RE set. Results: The uncorrected film dose deviated up to 26% from the reference dose, whereas the corrected film dose agreed within 3% in all three beams in water ("b 1 ", "b 2 " and "b 3 "). The LET dependence of the calibration function started to strongly increase around 5 keV/lm and flatten out around 30 keV/lm. All REs calculated from the proton fluence in the nine simulated beams could be approximated with a linear function of dose-averaged LET (RE = 1.0258À0.0211 lm/keV L d ). However, no functional relationship of RE-and fluence-averaged LET could be found encompassing all beam energies and modulations. Conclusions: The film quenching was found to be nonlinear as a function of proton LET as well as of the dose-averaged LET. However, the linear relation of RE on dose-averaged LET was a good approximation in all cases. In contrast to dose-averaged LET, fluence-averaged LET could not describe the RE when multiple beams were applied.

Improved simultaneous LET and dose measurements in proton therapy

“…( 2 ). In contrast to the results for radiochromic film 31 , where only the dose-averaged LET could describe the relative detector efficiency for different beams, both the dose- and fluence-averaged LET are here found to correlate well with the detector efficiency for the , in agreement with a previous study 16 . The OSLDs are here only calibrated against the fluence-averaged LET although similar results can be obtained using the dose-averaged LET.…”

Towards harmonizing clinical linear energy transfer (LET) reporting in proton radiotherapy: a European multi-centric study

“…This definition supports unambiguous LET reporting and coincides with the averaging technique, secondary particles and scoring medium most commonly specified in radiation biology experiments in PT [17]. LET d is thought to reflect the LET-RBE relationship better than trackaveraging [11,39,40] and is used as input parameter in several in-vitro data-based variable proton RBE models [41]. However, there is an active debate on which particles to include in LET d scoring in PT [14,15,39] and the biological relevance of heavier secondaries on proton RBE remains to be quantified more accurately [17].…”

Empirical quenching correction in radiochromic silicone-based three-dimensional dosimetry of spot-scanning proton therapy

“…Consequently, the LET in each point is described by a spectrum, instead of a single quantity. Dose- and fluence-averaged LET have been successfully implemented in quenching correction models for a variety of other dosimetric systems: the study by Robertson et al used the Birks model for scintillator light emission and LET calculations with Monte Carlo to correct 2D scintillator measurements [19] , Resch et al used LET d as a descriptor of beam quality for radiochromic films [32] , and Herrmann et al used track structure theory to create a correction model for alanine dosimeters [33] . The QCF curve of our study was similar to relative effectiveness curves for other dosimeters estimated with track structure theory [20] , [21] , [34] : a nearly constant dosimeter response for low LET and rapid decrease for higher values.…”