Peak Temperature Ratio Analysis of TLD Glow Curves to Characterize Orthovoltage X-Rays in Terms of Their LET
C Reft1*, (1) Univ Chicago, Chicago, ILSU-E-T-47 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: Investigate using the ratio of the two high temperature peaks (HTR) to analyze the glow curves of TLD700 to characterize orthovoltage x-rays in terms of their linear energy transfer (LET) as well as well as their HVL. LET is related to relative biological effectiveness (RBE) and studies have demonstrated RBE values from 1.1 to 2 for orthovoltage x-rays compared to megavoltage x-rays which have implications for radiobiological studies.
Methods: TLD700 chips .015cm and .038cm thick were irradiated to about 0.20 Gy with x-rays with photon effective energies from 13 KeV to 6 MeV with an orthovoltage therapy unit, a linac and gamma rays from I-125, Ir-192 and Cs-137 radioactive isotopes. The different thickness detectors are used to correct for possible self-attenuation effects in the detectors at the lower energies. Since the 280 C high temperature peak is more sensitive to high LET radiation than the 210 C lower temperature peak, the ratio of the areas under the two temperature peaks for the TLDs irradiated to the different beam qualities normalized to the peak ratio for 6 MV is used to determine their HTR. These HTR values are compared to the calculated LET values obtained from the effective photon energies of the irradiations.
Results: The data show that the HTR generally increases with decreasing photon energy from 1 for megavoltage x-rays to 1.94 for 13 KeV x-rays with a local maximum of 1.65 occurring at about 100 KeV. This dependence is similar to that of the calculated LET values which also exhibit a local maximum of 1.85 KeV/micron around 100 KeV. No measurable difference is observed for different thickness TLDs.
Conclusion: The HTR analysis of the TLD glow curves provides another method to characterize orthovoltage x-rays in terms of their LET that provides information on their biological effectiveness.