A novel calorimeter for synchrotron produced monochromatic x-ray beams.

BACKGROUND: Electron synchrotrons produce x-ray beams with dose rates orders of magnitude greater than conventional x-ray tubes and with beam sizes on the order of a few millimeters. These characteristics put severe challenges on current dosimeters to accurately realize absorbed dose or air kerma. PURPOSE: This work seeks to investigate the suitability of a novel aluminum-based calorimeter to determine absorbed dose to water with an uncertainty significantly smaller than currently possible with conventional detectors. A lower uncertainty in the determination of absolute dose rate would impact both therapeutic applications of synchrotron-produced x-ray beams and research investigations. METHODS: A vacuum-based calorimeter prototype with an aluminum core was built, matching the beam profile of the 140 keV monochromatic x-ray beam, produced by the Canadian Light Source Biomedical Imaging and Therapy beamline. The choice of material and overall calorimeter design was optimized using FEM thermal modeling software while Monte Carlo radiation transport simulations were used to model the impact of interactions of the radiation beam with the detector components. RESULTS: Corrections for both the thermal conduction and radiation transport effects were of the order of 3% and the simplicity of the geometry, combined with the monochromatic nature of the incident x-ray beam, meant that the uncertainty in each correction was ≤0.5%. The calorimeter performance was found to be repeatable over multiple irradiations of 1 Gy at the ± 0.6% level, and no systematic dependence on environmental effects or total dose was observed. CONCLUSION: The combined standard uncertainty in the determination of absorbed dose to aluminum was estimated to be 0.8%, indicating that absorbed dose to water, the ultimate quantity of interest, could be determined with an uncertainty on the order of 1%. This value is an improvement over current techniques used for synchrotron dosimetry and comparable with the state-of-the art for conventional kV x-ray dosimetry.

VATS surgical anatomical resection of bronchopulmonary sequestration presenting as chest sepsis.

BACKGROUND: Bronchopulmonary sequestration (BPS) is a malformation of the lungs resulting in lung tissue lacking direct communication to the tracheobronchial tree. Most cases demonstrate systemic arterial blood supply from the descending thoracic aorta, the abdominal aorta, celiac axis or splenic artery and venous drainage via the pulmonary veins with occasional drainage into azygos vein. BPS is considered a childhood disease and accounts for 0.15-6.40% of congenital pulmonary malformations. BPS is divided into intralobar sequestrations (ILS) and extralobar sequestrations (ELS) with ILS accounting for 75% of all cases. METHODS: Here we present our 11-year experience of dealing with BPS; all cases presented with recurrent chest sepsis in young-late adulthood regardless of the type of pathological sequestration. The surgical technique employed was a minimally invasive video-assisted thoracoscopic anterior approach (VATS). RESULTS: Between May 2010 and September 2021, we have operated on nine adult patients with bronchopulmonary sequestration who presented late with symptoms of recurrent chest sepsis. Most patients in the cohort had lower lobe pathology, with a roughly even split between right and left sided pathology. Moreover, the majority were life-long never smokers and an equal preponderance in males and females. The majority were extralobar sequestrations (56%) with pathological features in keeping with extensive bronchopneumonia and bronchiectasis. There were no major intra-operative or indeed post-operative complications. Median length of stay was 3 days. CONCLUSIONS: Dissection and division of the systemic feeding vessel was readily achievable through a successful anterior VATS approach, regardless of the type of sequestration and without the use of pre-operative coiling of embolization techniques. This approach gave excellent access to the hilar structures yet in this pathology, judicious and perhaps a lower threshold for open approach should be considered.

Validating Fricke dosimetry for the measurement of absorbed dose to water for HDR 192Ir brachytherapy: a comparison between primary standards of the LCR, Brazil, and the NRC, Canada.

Two Fricke-based absorbed dose to water standards for HDR Ir-192 dosimetry, developed independently by the LCR in Brazil and the NRC in Canada have been compared. The agreement in the determination of the dose rate from a HDR Ir-192 source at 1 cm in a water phantom was found to be within the k = 1 combined measurement uncertainties of the two standards: D NRC/D LCR = 1.011, standard uncertainty = 2.2%. The dose-based standards also agreed within the uncertainties with the manufacturer's stated dose rate value, which is traceable to a national standard of air kerma. A number of possible influence quantities were investigated, including the specific method for producing the ferrous-sulphate Fricke solution, the geometry of the holder, and the Monte Carlo code used to determine correction factors. The comparison highlighted the lack of data on the determination of G(Fe3+) in this energy range and the possibilities for further development of the holders used to contain the Fricke solution. The comparison also confirmed the suitability of Fricke dosimetry for Ir-192 primary standard dose rate determinations at therapy dose levels.

The Fricke dosimeter as an absorbed dose to water primary standard for Ir-192 brachytherapy.

The aim of this project was to develop an absorbed dose to water primary standard for Ir-192 brachytherapy based on the Fricke dosimeter. To achieve this within the framework of the existing TG-43 protocol, a determination of the absorbed dose to water at the reference position, D(r0,θ0), was undertaken. Prior to this investigation, the radiation chemical yield of the ferric ions (G-value) at the Ir-192 equivalent photon energy (0.380 MeV) was established by interpolating between G-values obtained for Co-60 and 250 kV x-rays.An irradiation geometry was developed with a cylindrical holder to contain the Fricke solution and allow irradiations in a water phantom to be conducted using a standard Nucletron microSelectron V2 HDR Ir-192 afterloader. Once the geometry and holder were optimized, the dose obtained with the Fricke system was compared to the standard method used in North America, based on air-kerma strength.Initial investigations focused on reproducible positioning of the ring-shaped holder for the Fricke solution with respect to the Ir-192 source and obtaining an acceptable type A uncertainty in the optical density measurements required to yield the absorbed dose. Source positioning was found to be reproducible to better than 0.3 mm, and a careful cleaning and control procedure reduced the variation in optical density reading due to contamination of the Fricke solution by the PMMA holder. It was found that fewer than 10 irradiations were required to yield a type A standard uncertainty of less than 0.5%.Correction factors to take account of the non-water components of the geometry and the volume averaging effect of the Fricke solution volume were obtained from Monte Carlo calculations. A sensitivity analysis showed that the dependence on the input data used (e.g. interaction cross-sections) was small with a type B uncertainty for these corrections estimated to be 0.2%.The combined standard uncertainty in the determination of absorbed dose to water at the reference position for TG-43 (1 cm from the source on the transverse axis, in a water phantom) was estimated to be 0.8% with the dominant uncertainty coming from the determination of the G-value. A comparison with absorbed dose to water obtained using the product of air-kerma strength and the dose rate constant gave agreement within 1.5% for three different Ir-192 sources, which is within the combined standard uncertainties of the two methods.

The inverse-square gamma-irradiation anomaly of the Nuclear Enterprises 2575 large-volume ionisation chamber.

The Nuclear Enterprises (Model 2575) 600 cc ionisation chamber is examined to discover the cause of its anomalous behaviour in inverse-square stability measurements. Measurements and Monte Carlo calculations are employed to isolate the cause of the discrepancy. It is found that most of the effect is due to the long photon attenuation pathlengths in the long side wall of the instrument. A phenomenological procedure, based on measurements, is proposed to correct for the anomaly. The procedure results in inverse-square stability to within 0.5 % over a range of 1-7 m.