Energy correction factors of LiF powder TLDs irradiated in high-energyelectron beams and applied to mailed dosimetry for quality assurancenetworks

Abstract

Absorbed dose determination with thermoluminescent dosimeters (TLDs) generallyrelies on calibration in 60Co γ-ray reference beams. Theenergy correction factor fECo for electron beams takes into accountthe difference between the response of the TLD in the beam of energy E andin the 60Co γ-ray beam. In this work, fECo wasevaluated for an LiF powder irradiated in electron beams of 6 to 20 MeV(Varian 2300C/D) and 10 to 50 MeV (Racetrack MM50), and its variation withelectron energy, TLD size and nature of the surrounding medium was alsostudied for LiF powder. The results have been applied to the ESTRO-EQUALmailed dosimetry quality assurance network.Monte Carlo calculations (EGS4, PENELOPE) and experiments have been performedfor the LiF powder (ρ = 1.4 g cm-3) (DTL937, Philitech,France), read on a home made reader and a PCL3 automatic reader (Fimel,France). The TLDs were calibrated using Fricke dosimetry and compared withthree ionization chambers (NE2571, NACP02, ROOS).The combined uncertainties in the experimental fECo factorsdetermined in this work are less than about 0.4% (1 SD), which is appreciablysmaller than the uncertainties up to 1.4% (1 SD) reported for othercalculated values in the literature. Concerning the Varian 2300C/D beams, themeasured fECo values decrease from 1.065 to 1.049±0.004 (1 SD)when the energy at depth in water increases from 2.6 to 14.1 MeV; theagreement with Monte Carlo calculations is better than 0.5%. For theRacetrack MM50 pulsed-scanned beams, the average experimental value offECo is 1.071±0.005 (1 SD) for a mean electron energy at depthĒz ranging from 4.3 to 36.3 MeV: fECo is up to 2% higherfor the MM50 beams than for the 2300C/D beams in the range of the testedenergies.The energy correction factor for LiF powder (3 mm diameter and 15 mm length)varies with beam quality and type (pulsed or pulsed-scanning), cavity size andnature of the surrounding medium.The fECo values obtained for the LiF powder (3 mm diameter and 15 mmlength) irradiated in water, have been applied to the EQUAL external auditnetwork, leading to a good agreement between stated and measured doses, with amean value of 1.002±0.022 (1 SD), for 170 beam outputs checked (36electron beam energies) in 13 `reference' radiotherapy centres in Europe.Such fECo data improve the accuracy of the absorbed dose TLDdetermination in electron beams, justifying their use for quality control inradiotherapy.