Room: AAPM ePoster Library
Purpose: relative biological effect (RBE) increases as the proton linear energy transfer (LET). Currently, a fixed RBE of 1.1 is used in clinic but it has been proposed to include LET in treatment planning. The value of LET is either calculated by Monte Carlo (MC) simulation or measured via some surrogates. In the latter case, MC is needed to calibrate the relationship between the surrogate and LET. We propose a novel method to measure LET that need not be calibrated by MC.
Methods: count energy loss from all the protons at the measurement voxel of interest, a collimator with an open at the voxel was placed in front of a multilayer ion chamber (MLIC), which recorded the proton integral depth dose (IDD). A novel algorithm was developed to extract the proton energy spectrum from the acquired IDD. According to the physics definition, track- and dose-averaged LETs were calculated from the energy spectra at the entrance and exit of the voxel. This method was also applied to IDDs acquired by a Bragg peak ion chamber (BPIC) during proton beam commissioning.
Results: increased slowly in the IDD’s plateau and rapidly after the Bragg peak. For the IDD of a 160 MeV pencil beam scanned by the BPIC, the track- and dose-averaged LETs (in unit of keV/µm) were 0.6 and 1.1 respectively at the water surface, and 8.7 and 14.3 respectively at depth 18.2 cm (the Bragg peak was at depth 17.3 cm) where the IDD was about 6% of the peak. For the same energy pencil beam but whose IDD was acquired by MLIC with the collimator, we obtained similar LET except that right behind the collimator. The dose-averaged LET in MC is step-size dependent, but in our method is independent.
Conclusion: method provides an independent and reliable measurement of proton LET.