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, Ra-0.1(5), was taken as a measure of response variations with dose. LiF:Mg,Ti retained stability of dose response over time, however, LiF:Mg,Cu,P showed a 16% and 13% increase in R-0.1(5), 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.