Absolute dose measurement and energy dependence of LiF dosimeters in proton therapy beam dosimetry

Abstract

Background: For the purpose of patient dose monitoring, thermoluminescent dosimeters (TLDs) can report any dose point of interest as in vivo dosimetry. However, energy dependence of TLDs may perturb proton beam dosimetry, particularly for low-energy proton beams. The purpose of this study was to evaluate the energy dependence of TLDs for proton beams and influence on dosimetry. Methods: Two types of TLD chips, the TLD-100 (LiF: Mg, Ti) and MCP-100 (LiF: Mg, Cu, P), inserted into high-density polyethylene (HDPE) phantom at a depth of 2 cm, were irradiated with 70-230 MeV of proton beams, and at different depths with 230 MeV of proton beams for lower energy proton (<70 MeV) irradiation. The energy dependence was evaluated in terms of relative efficiency, which is the ratio of the emitted luminesce light intensity per unit dose irradiated with proton beam to 6 MV X-ray beam. The proton mean energy at irradiation depths were calculated by a Geant4-based Monte Carlo simulation platform, the particle therapy simulation framework (PT-Sim). The correlation between the relative efficiency and proton mean energy was noted. Results: The relative efficiency of the TLD-100 and MCP-100 with 30-230 MeV proton beams ranged from 1.13 to 0.95 and 0.50 to 0.93, respectively. This study revealed that absolute dose measurement can be achieved by the TLD-100 and MCP-100 with a dose uncertainty of 4.67% and 8.16% for high-energy proton beams and a dose uncertainty of 15.18% and 28.52% for low-energy proton beams, that of mean energy lower than 80 MeV, respectively. Conclusions: As an absolute dosimeter, TLD-100 is a suitable dosimeter for high energy proton beams. MCP-100 presents a larger dose discrepancy than TLD-100 due to proton energy dependence. Applying energydependence correction, the dose difference is smaller at a well-known proton energy spectrum; however, it should be used carefully in clinical, or the dose difference may increase. The energy dependence of MCP-100 become the potential to measurement the linear energy transfer (LET) in particle beams in the future.