Calibration of a TLD-100 powder dosimetric system to verify the absorbed dose to water imparted by 137Cs sources in low dose rate brachytherapy at the oncology unit in the Hospital General de Mexico

A thermoluminescence dosimetry (TLD) system was characterised at SSDL-ININ to verify the air-kerma strength (S(K)) and dose-to-water (D(W)) values for (137)Cs sources used in low dose rate (LDR) brachytherapy treatments at the Hospital General de Mexico (HGM). It consists of a Harshaw 3500 reader and a set of TLD-100 powder capsules. The samples of TLD-100 powder were calibrated in terms of D(W) vs. nC or nC mg(-1), and their dose response curves were corrected for supralinearity. The D(W) was calculated using the AAPM TG-43 formalism using S(K) for a CDCSM4 (137)Cs reference source. The S(K) value was obtained by using a NE 2611 chamber, and with two well chambers. The angular anisotropy factor was measured with the NE 2611 chamber for this source. The HGM irradiated TLD-100 powder capsules to a reference dose D(W) of 2 Gy with their (137)Cs sources. The percent deviations between the imparted and reference doses were 1.2% < or = Delta < or = 6.5%, which are consistent with the combined uncertainties: 5.6% < or = u(c) < or = 9.8% for D(W).

Regression models for the determination of the absorbed dose rate with an extrapolation chamber for flat ophthalmic applicators

The average surface absorbed dose rate, given by flat ophthalmic applicators (90Sr/90Y, 925 MBq) is determined in equivalent soft tissue using an extrapolation chamber with two flat parallel electrodes of variable separation; the input electrode is fixed in relation to the collector electrode of constant area. When estimating the extrapolation curve slope using a linear regression model, it has been observed that average surface dose rate values were underestimated by up to 19%, as compared to estimations of these values by means of a second degree polynomial regression model, while an improvement of up to 37% is observed in the standard error of the slope in the quadratic model, as compared to that of the linear model. With the aim of validating the results of these models, goodness of fit tests to a Normal (the Shapiro-Wilk test) as well as homogeneity tests on treatment variance (the Bartlett test) were applied. The analysis of variance (ANOVA) tables of fit and residual error breakdown are given: table 3a and 3b for linear fit; 7a and 7b for quadratic fit, and table 10 to error breakdown. Also presented is the global uncertainty of the average dose rate, taking into account the reproducibility of the experimental set-up. It may be inferred that by using this type of measurement for the extrapolation curve slope, quadratic regression models allow for a greater degree of accuracy and precision in determining surface dose rate values. The effective area of the collector electrode and the effective electrode separation in the chamber are also determined by measuring the chamber's electric capacity. Finally, there is an attempt to relate the use of the regression models to the experimental conditions during the measurement of ionization currents (diameter of collector electrode, electrical field gradient, radiation field uniformity, radiation field intensity, etc.). In this particular case, deviations in the distance inverse square law and the "screening" effects during the collection of negative charges (both for primary radiation and the ionization generated by it), are presented as necessary, but insufficient, conditions to explain thoroughly the quadratic behavior of ionizing currents.