Dose Linearity of LiF:Mg,Ti and LiF:Mg,Cu,P in Radiotherapy: Effect of Time Between Dose and Evaluation

Clinical measurements using LiF dosemeters in radiotherapy are complicated by deviations from a linear dose response at typical doses per fraction (exceeding 1 Gy). In the present study, two TLD materials: LiF:Mg,Ti and LiF:Mg,Cu,P, were utilised to investigate the effect of time between irradiation and evaluation on dose linearity. TLDs were irradiated to doses of 0.01, 0.1, 0.5, 2.5 and 20 Gy using a 6 MV photon beam from a medical linear accelerator and evaluated one hour, one week, one month and six months after irradiation. Assuming linear response at 0.1 Gy, the ratio of the response at 5 Gy and 0.1 Gy, R50.1, was taken as a measure of response variation with dose. LiF:Mg,Ti retained stability of dose response over time; however, LiF:Mg,Cu,P showed a 16% and 13% increase in R50.1 for the slow and fast cool down, respectively. Overestimation could occur in clinical dose evaluation after storage longer than between one week and a month after the measurement was taken. (author)