Energy correction factors of LiF powder TLDs irradiated in high-energy electron beams and applied to mailed dosimetry for quality assurance networks

Absorbed dose determination with thermoluminescent dosimeters (TLDs) genera lly relies on calibration in Co-60 gamma -ray reference beams. The energy c orrection factor f(Co)(E) for electron beams takes into account the differe nce between the response of the TLD in the beam of energy E and in the Co-6 0 gamma -ray beam. In this work, f(Co)(E) was evaluated for an LiF powder i rradiated in electron beams of 6 to 20 MeV (Varian 2300C/D) and 10 to 50 Me V (Racetrack MM50), and its variation with electron energy, TLD size and na ture of the surrounding medium was also studied for LiF powder. The results have been applied to the ESTRO-EQUAL mailed dosimetry quality assurance ne twork. Monte Carlo calculations (EGS4, PENELOPE) and experiments have been perform ed for the LIF powder (rho = 1.4 g cm(-3)) (DTL937, Philitech, France), rea d on a home made reader and a PCL3 automatic reader (Fimel, France). The TL Ds were calibrated using Fricke dosimetry and compared with three ionizatio n chambers (NE2571, NACP02, ROOS). The combined uncertainties in the experimental f(Co)(E) factors determined in this work; are less than about 0.4% (1 SD), which is appreciably smaller than the uncertainties up to 1.4% (1 SD) reported For other calculated val ues in the literature. Concerning the Varian 2300C/D beams, the measured f( Co)(E) 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; the agreement with Monte Carlo calculations is better than 0.5%. For the Racetrack MM50 pulsed-scan ned beams, the average experimental value of f(Co)(E) is 1.071 +/- 0.005 (1 SD) for a mean electron energy at depth E; ranging from 4.3 to 36.3 MeV: S CE is up to 2% higher for the MM50 beams than for the 2300C/D beams in the range of the tested energies. 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 siz e and nature of the surrounding medium. The f(Co)(E) values obtained for the LiF powder (3 mm diameter and 15 mm le ngth) irradiated in water, have been applied to the EQUAL external audit ne twork, leading to a good agreement between stated and measured doses, with a mean value of 1.002 +/- 0.022 (1 SD), for 170 beam outputs checked (36 el ectron beam energies) in 13 'reference' radiotherapy centres in Europe. Such f(Co)(E) data improve the accuracy of the absorbed dose TLD determinat ion in electron beams, justifying their use for quality control in radiothe rapy.