A Monte Carlo evaluation of beam characteristics for total body irradiation at extended treatment distances
The aim is to study beam characteristics at large distances when focusing on the electron component. In particular, to investigate the utility of spoilers with various thicknesses as an electron source, as well as the effect of different spoiler‐to‐surface distances (STSD) on the beam characteristics and, consequently, on the dose in the superficial region. A MC model of a 15 MV Varian accelerator, validated earlier by experimental data at isocenter and extended distances used in large‐field total body irradiation, is applied to evaluate beam characteristics at distances larger than 400 cm. Calculations are carried out using BEAMnrc/DOSXYZnrc code packages and phase space data are analyzed by the beam data processor BEAMdp. The electron component of the beam is analyzed at isocenter and extended distances, with and without spoilers as beam modifiers, assuming vacuum or air surrounding the accelerator head. Spoiler thickness of 1.6 cm is found to be optimal compared to thicknesses of 0.8 cm and 2.4 cm. The STSD variations should be taken into account when treating patients, in particular when the treatment protocols are based on a fixed distance to the patient central sagittal plane, and also, in order to maintain high dose in the superficial region. PACS numbers: 87.55.D‐, 87.55.de, 87.55.K‐
Monte Carlo dosimetry of total body irradiation
The total body irradiation (TBI) technique at the Vancouver Cancer Centre uses a sweeping Cobalt-60 beam with patient-specific lung compensators and a stationary flattening filter, with the patient lying in supine and prone positions during each fraction. The dose calculations for this technique are limited to the dose delivered to a point at patient mid-separation at the level of the umbilicus, and the mean dose to the body must fall within a 10% tolerance of prescription. While the current technique is effective, it has been in place for over 23 years, and detailed dose data to critical organs is necessary before the technique can be upgraded to a more conformal technique that would offer an improved patient experience. For this purpose, a Monte Carlo simulation technique has been developed and applied to collect organ dose data from TBI treatments based on retrospective data of patients recently treated with 12 Gy in 6 fractions. 20 patients, including adults and pediatric patients, are simulated by constructing Monte Carlo phantoms in each of the supine and prone positions based on planning CT images. The supine and prone dose distributions are summed with a deformable registration tool and the doses to the lungs, kidneys, thyroid, and liver are analyzed as well as the dose delivered to the body. It is determined that while all doses fall within prescription, there is a trend where smaller patients receive lower mean body doses and viceversa (mean body dose range: 10.93-12.01 Gy). For most patients, the lungs and liver consistently receive doses below the mean body dose, and the thyroid and kidneys consistently receive higher doses than the mean body dose. This thesis presents an overview of the background physics and biology of TBI as well as a comprehensive survey of different techniques described in the literature. The Monte Carlo simulation technique used for the retrospective study is described with calibration, validation, and optimization details, and the organ dose results of the retrospective study are shown.
The qualitative research interview
Background  Interviews are among the most familiar strategies for collecting qualitative data. The different qualitative interviewing strategies in common use emerged from diverse disciplinary perspectives resulting in a wide variation among interviewing approaches. Unlike the highly structured survey interviews and questionnaires used in epidemiology and most health services research, we examine less structured interview strategies in which the person interviewed is more a participant in meaning making than a conduit from which information is retrieved.