Dose rate constants for 125I, 103Pd, 192Ir and 169Yb brachytherapy sources: an EGS4 Monte Carlo study

An exhaustive revision of dosimetry data for 192Ir, 125I, 103Pd and 169Yb brachytherapy sources has been performed by means of the EGS4 simulation system. The DLC-136/PHOTX cross section library, water molecular form factors, bound Compton scattering and Doppler broadening of the Compton-scattered photon energy were considered in the calculations. The absorbed dose rate per unit contained activity in a medium at 1 cm in water and air-kerma strength per unit contained activity for each seed model were calculated, allowing the dose rate constant (DRC) A to be estimated. The influence of the calibration procedure on source strength for low-energy brachytherapy seeds is discussed. Conversion factors for 125I and 103Pd seeds to obtain the dose rate in liquid water from the dose rate measured in a solid water phantom with a detector calibrated for dose to water were calculated. A theoretical estimate of the DRC for a 103Pd model 200 seed equal to 0.669 +/- 0.002 cGy h(-1) U(-1) is obtained. Comparison of obtained DRCs with measured and calculated published results shows agreement within 1.5% for 192Ir, 169Yb and 125I sources.     

Investigation of the chamber correction factor (k(ch)) for the UK secondary standard ionization chamber (NE2561/NE2611) using medium-energy x-rays

This paper evaluates the characteristics of ionization chambers for the measurement of absorbed dose to water for medium-energy x-rays. The values of the chamber correction factor, k(ch), used in the IPEMB code of practice for the UK secondary standard (NE2561/NE2611) ionization chamber are derived and their constituent factors examined. The comparison of the chambers' responses in air revealed that of the chambers tested only the NE2561, NE2571 and NE2505 exhibit a flat (within 5%) energy response in air. Under no circumstances should the NACP, Sanders electron chamber, or any chamber that has a wall made of high atomic number material, be used for medium-energy x-ray dosimetry. The measurements in water reveal that a chamber that has a substantial housing, such as the PTW Grenz chamber, should not be used to measure absorbed dose to water in this energy range. The value of k(ch) for an NE2561 chamber was determined by measuring the absorbed dose to water and comparing it with that for an NE2571 chamber, for which k(ch) data have been published. The chamber correction factor varies from 1.023 +/- 0.03 to 1.018 +/- 0.001 for x-ray beams with HVL between 0.15 and 4 mm Cu. The values agree with that for an NE2571 chamber within the experimental uncertainty. The corrections due to the stem, waterproof sleeve and replacement of the phantom material by the chamber for an NE2561 chamber are described.     

Effect of adjuvant iridium-192 brachytherapy on the survival of patients with malignant gliomas

The effect of iridium-192 brachytherapy (BRTX) on the survival of patients with malignant gliomas was evaluated in 83 patients with malignant gliomas (42 astrocytoma grade III, 41 glioblastoma multiforme) over a period of 8.5 years. Fifty patients (Group 1) received only standard external beam radiotherapy (EBRT) (mean dose 51.5 +/- 12.4 Gy in 2.0 Gy fractions), and 33 patients (Group 2) received EBRT (mean dose 51.0 +/- 10.8 Gy) combined with BRTX (mean dose 50.2 +/- 13.2 Gy, dose rate of 0.3-0.4 Gy/hr). The median survival periods for patients in Groups 1 and 2 were 12.2 and 23.7 months, respectively (p = 0.0145). The median survival for 17 patients in Group 2 with glioblastoma multiforme was 21.9 months. Using BRTX as an adjuvant to EBRT appeared to confer survival benefits compared to only EBRT (p = 0.0284). Univariate and multivariate analysis identified the variables of histological diagnosis, location, Karnofsky performance status, and BRTX as relevant risk factors for survival time (p < 0.05 for each factor). Among these factors, BRTX was the most important for prolonging survival (p = 0.0015). Adjuvant iridium-192 BRTX and conventional EBRT appears to greatly improve the survival time of patients with malignant gliomas compared to only EBRT and may be the treatment of choice in selected patients with tumors located in deep-seated or eloquent areas.     

Numerical solutions of differential equations of an ionization chamber: plane-parallel and spherical geometry

Despite the widespread availability of MR imaging, CT remains a vital modality for the evaluation of disorders of the spine, offering superior assessment of bone abnormalities, as well as calcification. Advances in technology have enhanced the capabilities of CT, providing exciting new options for data rendering and display. Depending on the clinical indication, either CT or MR may be the modality of choice in the evaluation of spinal disease.     

Prognostic factors in patients with carcinoma of the uterine cervix treated with external beam irradiation and IR-192 high-dose-rate brachytherapy

The spectral properties of the protochlorophyllide forms in the epicotyls of dark-grown pea seedlings have been studied in a temperature range, from 10 to 293 K with conventional fluorescence emission and excitation spectroscopy as well as by fluorescence line narrowing (FLN) at cryogenic temperatures. The conventional fluorescence techniques at lower temperatures revealed separate bands at 628, 634-636, 644 and 655 nm. At room temperature (293 K) the 628 and 634-636 nm emission bands strongly overlapped and the band shape was almost independent of the excitation wavelength. Under FLN conditions, vibronically resolved fluorescence spectra could be measured for the 628 and 634-636 nm bands. The high resolution of this technique excluded the excitonic nature of respective excited states and made it possible to determine the pure electronic (0,0) range of the spectra of the two components. Thus it was concluded that the 628 and 634-636 nm (0,0) emission bands originate from two monomeric forms of protochlorophyllide and the spectral difference is interpreted as a consequence of environmental effects of the surrounding matrix. On the basis of earlier results and the data presented here, a model is discussed in which the 636 nm form is considered as an enzyme-bound protochlorophyllide and the 628 nm form as a protochlorophyllide pool from which the substrate is replaced when the epicotyl is illuminated with continuous light.     

Heating of the packing in a blast-furnace air heater with an internal combustion chamber

For a blast-furnace air heater, two methods of heating the packing by blast-furnace gas enriched with high-caloric fuel are analyzed on the basis of a computer model: (1) supply of the gas mixture to the air heater from a mixer; (2) separate supply of natural gas and blast-furnace gas to the air heater. Several algorithms for controlling the heating of the packing with variable calorific content of the fuel are compared. The economic benefits of each algorithm are calculated.     

Transscleral ciliary sulcus fixation of a posterior chamber lens in an eye with congenital aniridia

A 30-year-old man with bilateral congenital aniridia presented with a subluxated cataract in the right eye. He wore a soft contact lens for aphakic correction in the fellow eye after lensectomy for a subluxated cataract. We performed pars plana lensectomy and vitrectomy with implantation of a transscleral ciliary sulcus fixated posterior chamber intraocular lens in the right eye. Good postoperative visual acuity was obtained.     

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.     

A novel contextual dimension for use with an operant chamber: From simple to hierarchical forms of learning.

A novel procedure is described in which the floor temperatures (warm and cool) in an operant chamber are used as contextual cues in 2 experiments with rats. Experiment 1 demonstrated that rats learn the relationship between these thermal contexts and auditory stimuli that have been paired with them. Experiment 2 showed that thermal contexts can serve as a conditional function that (a) reflects the operation of a mechanism that is common to conventional, visual contexts and (b) is bound to these contexts' ability to retrieve the nature of the relationship between the auditory stimuli and food. (PsycINFO Database Record (c) 2010 APA, all rights reserved)