The energy response of standard (TLD-100) and high-sensitivity (TLD-100H) LiF thermoluminescence dosemeters (TLDs) has been studied for photon beams with mean energies from about 25 keV to 1100 keV. Canadian primary standards for air kerma were used to establish the air kerma rates for each of the photon beams. TLDs were mounted in a PMMA holder and the air kerma response was measured as a function of energy. The EGSnrc Monte Carlo code was used to model the TLD holder and calculate the absorbed dose to the TLD chip per unit air kerma for each beam. The measured and calculated results were combined to obtain the intrinsic dose response of the TLD chip. Broadly, our results are consistent with existing data, which show a marked difference in the energy dependence of the two materials. However, the precision of our measurements (standard uncertainty of about 0.6%) has permitted the identification of features that have not been noted before. In particular, the energy dependence of the two materials is quite different in the important energy region delimited by 137Cs and 60Co gamma rays.

The quality dependence of LiF TLD in megavoltage photon beams with qualities from 60Co gamma-rays to 25 MV x-rays has been studied experimentally against ion chamber measurements and theoretically by Monte Carlo simulation using the EGS4 Monte Carlo code system. The experimental findings are that the energy dependence of 1 mm thick TLD-100 (micro-rods and chips) on average decreases slowly from 1.0 for 60Co gamma-rays to 0.989 +/- 1.3% for 6 MV x-rays (TPR20/10 = 0.685) and to 0.974 +/- 1.3% for 25 MV x-rays (TPR20/10 = 0.800) relative to 60Co gamma-rays. The Monte Carlo results vary from 0.991 +/- 0.9% for 6 MV x-rays to 0.978 +/- 0.8% for 25 MV x-rays. Differences between chips and micro-rods were negligible and there was no difference in the energy dependence between TLDs irradiated in water or Perspex (PMMA). The Monte Carlo simulation shows that the contribution to the total absorbed dose from photon interactions in the 1 mm diameter and 6 mm long TLD material varies from 50% for 60Co gamma-rays to 10% for 25 MV x-rays. When the diameter of the TLD micro-rod was increased from 1 mm to 5 mm there was no significant change in response computed by Monte Carlo even though the dose contribution to the total dose scored in the TLD material from photon interactions in the cavity increased to 85% for 60Co gamma-rays and 30% for 25 MV x-rays.

Monte Carlo simulations with the EGS4 code system have been performed to determine the quality dependence of diamond TLDs in photon beams ranging from 25 kV to 25 MV x-rays and also in megavoltage electron beams. It has been shown that diamond TLDs in the form of discs of thickness 0.3 mm and diameter 5.64 mm show no significant dependence on the incident energy in clinical electron beams when irradiated close to dmax, but require an energy correction factor of 1.050 +/- 0.008 compared with diamond TLDs irradiated in 60Co gamma-rays. The correction factor increases with depth of irradiation and this effect is greater for thicker detectors. The Monte Carlo predicted sensitivity in x-ray beams is constant within 2.5% over the energy range 250 kV to 25 MV. However the sensitivity decreases by about 60% for 25 kV x-rays compared with 60Co gamma-rays.

A Monte Carlo study of the energy-response factor of aluminium oxide (Al2O3) and lithium fluoride (LiF) TLDs in kilovoltage and megavoltage photon beams relative to 60Co gamma rays has been performed using EGSnrc Monte Carlo simulations. The sensitive volume of the detector was simulated as a disc of diameter 2.85 mm and thickness 1 mm. The phantom material was water and the irradiation depth was 2.0 cm in kilovoltage photon beams and 5.0 cm for megavoltage photon beams. The results show that the energy-response of the Al2O3 and LiF-TLDs is constant within 3% for photon beam energies in the energy range of 60Co gamma rays to 25 MV X rays. However, both detectors show an enhanced response for kilovoltage photon beams, which in the case of 50 kV X rays is 3.2 times higher than that for 60Co gamma rays. The energy-response factor was 1.46 for LiF irradiated in 50 kV X rays. The Al2O3 detector has an energy-response that is 2.2 times higher than that of LiF in 50 kV X rays decreasing to 1.19 for 250 kV X rays. The results show that the addition of 0.1 or 1% of carbon by weight (as dopant) into the Al2O3 does not change the Monte Carlo determined energy-response factor by more than 1%.

A Monte Carlo simulation of the quality dependence of different TL materials, in the form of discs 3.61 mm in diameter and 0.9 mm thick, in radiotherapy photon beams relative to 60Co gamma-rays has been performed. The beam qualities ranged from 50 kV to 25 MV x-rays. The TL materials were: CaF2, CaSO4, LiF and Li2B4O7. The effects of the dopants on energy deposition in the TL material have also been determined for the highly sensitive LiF:Mg:Cu:P (TLD-100H) and for CaF2:Mn. It was found that there was a significant difference in the quality dependence factor derived from Monte Carlo simulations between LiF and LiF:Mg:Cu:P but not between CaF2 and CaF2:Mn. The quality dependence factors for Li2B4O7 varied from 0.990 +/- 0.008 (1 sd) for 25 MV x-rays to 0.940 +/- 0.009 (1 sd) for 50 kV x-rays relative to 60Co gamma-rays; Monte Carlo simulations were also performed for Li2B4O7 in megavoltage electron beams. For CaF2, the quality dependence factor varied from 0.927 +/- 0.008 (1 sd) for 25 MV x-rays to 10.561 +/- 0.008 (1 sd) for 50 kV x-rays. The figure for CaSO4 ranged from 0.943 +/- 0.008 (1 sd) for 25 MV x-rays to 9.010 +/- 0.008 (1 sd) for 50 kV x-rays. The quality dependence factor for CaF2 increases by up to 5% with depth and by up to 15% with field size for the kilovoltage x-ray beams. For LiF-TLD, however, there was no significant dependence on the field size or depth of irradiation in the kilovoltage energy range.

The energy correction factor of LiF thermoluminescent dosemeters (TLDs) calibrated in Co-60 gamma-rays and used for measurements in megavoltage electron beams has been determined experimentally and theoretically using Monte Carlo simulations. The experiments show that the energy correction factor of 1 mm thick TLD-100 has an average for both rods and chips which varies from 1.036 +/- 1.3% (1 SD) for 4 MeV electron beams to 1.021 +/- 1.3% (1 SD) for 20 MeV electron beams for measurement performed at dmax in PMMA (Perspex). The results of the Monte Carlo simulations were within 0.6% of the experimental results and ranged from 1.041 +/- 0.9% (1 SD) for 2 MeV electrons to 1.028 +/- 0.8% (1 SD) for 20 MeV electron beams. There was no significant difference in the energy correction factors of LiF TLDs irradiated in PMMA or water by Monte Carlo simulation and experiments. Differences in the energy correction factors between rods and chips of the same thickness were negligible both in the experiments and in Monte Carlo calculation. When the diameter of the LiF TLD micro-rod was increased from 1 to 5 mm, the simulated energy correction factors increased by as much as 5% over this energy range. The energy correction factors changed by up to 4% for irradiation of TLD at depths other than at dmax for a 5 MeV mono-energetic electron beam.

EGS4 Monte Carlo simulations have been performed to examine general cavity theory for a number of TLD cavity materials irradiated in megavoltage photon and electron beams. The TLD materials were LiF, Li2B4O7, CaF2 and CaSO4 irradiated in Perspex, water, Al, Cu and Pb phantoms. For megavoltage photon beams, this has been done by determining the dose component (1-d) resulting from photon interactions in the cavity compared with the dose component resulting from photon interactions in the phantom material (d) by Monte Carlo simulations and analytical techniques. The results indicate that the Burlin exponential attenuation technique can overestimate the dose contribution from photon interactions in a 1 mm thick LiF cavity by up to 100% compared with the Monte Carlo results for LiF TLDs irradiated in a water or Perspex phantom. However, there is agreement to within 1% in the quality dependence factor, determined from Burlin's cavity theory, Monte Carlo simulations and experimental measurements for LiF and Li2B4O7 TLDs irradiated in a Perspex or a water phantom. The agreement was within 3% for CaF2 TLDs. However there was disagreement between Monte Carlo simulations and Burlin's theory of 6 and 12% for LiF TLDs irradiated in copper and lead phantoms respectively. The adaptation of Burlin's photon cavity theory and other modifications to his photon general cavity theory for electrons have been shown to be seriously flawed.

EGSnrc Monte Carlo simulation was used to investigate dose perturbation effects in prostate seed implant brachytherapy using 125I radioactive seeds used in implant brachytherapy. Dose perturbation effects resulting from the seed mutual attenuation in a prostate seed implant consisting of 27 seeds were investigated. The results showed that for 125I seeds implanted into the prostate at 1.00 cm, 0.75 cm and 0.50 cm apart (uniform spacing), the dose perturbation effects are up to 10%. The volume of the target occupied by the 10% dose difference between the full Monte Carlo simulation and the single seed superposition model decreases with increasing seed spacing. Despite the differences between the Monte Carlo simulation and the simple superposition, there was no significant change in the dose volume histogram for 1 cm and 0.75 cm seed spacing. However, there was a significant change in the dose volume histogram when the seed spacing was 0.5 cm. An analysis of the external volume index (EI), coverage index (CI) and homogeneity index (HI) also showed that there is no difference in these indexes for the 1.00 cm and 0.75 cm seed spacing between the simple superposition model and the full Monte Carlo simulation. Compared to the full Monte Carlo simulations, the simple superposition model overestimated EI, CI and HI by 7%, 5% and 4% respectively for the 0.50 cm seed spacing.