Monte Carlo-based Investigation of Absorbed-dose Energy Dependence of Thermoluminescent Dosimeters in Therapeutic Proton and Carbon Ion Beams.

Background: The present study is aimed at calculating relative absorbed-dose energy response correction (R) of commonly used thermoluminescent dosimeters (TLDs) such as LiF, Li₂B₄O₇, and Al₂O₃ as a function of depth in water for protons (50-250 MeV/n) and carbon ion (80-480 MeV/n) beams using Monte Carlo-based FLUKA code.

Materials and methods: On-axis depth-dose profiles in water are calculated for protons (50-250 MeV/n) and carbon ion (80-480 MeV/n) beams using FLUKA code. For the calculation of R, selective depths are chosen based on the depth-dose profiles. In the simulations, the TLDs of dimensions 1 mm × 1 mm × 1 mm are positioned at the flat, dose gradient, and Bragg peak regions of the depth-dose profile. Absorbed dose to detector was calculated within the TLD material. In the second step, TLD voxels were replaced by water voxel of similar dimension and absorbed dose to water was scored.

Results: The study reveals that for both proton and carbon ion beams, the value of R differs from unity significantly at the Bragg peak position and is close to unity at the flat region for the investigated TLDs. The calculated R value is sensitive to depth in water, beam energy, type of ion beam, and type of TLD.

Discussion: For accurate dosimetry of protons and carbon ion beams using TLDs, the response of the TLD should be corrected to account for its absorbed-dose energy dependence.