Inter-centre variability of CT-based stopping-power prediction in particle therapy: Survey-based evaluation.

BACKGROUND AND PURPOSE: Stopping-power ratios (SPRs) are used in particle therapy to calculate particle range in patients. The heuristic CT-to-SPR conversion (Hounsfield Look-Up-Table, HLUT), needed for treatment planning, depends on CT-scan and reconstruction parameters as well as the specific HLUT definition. To assess inter-centre differences in these parameters, we performed a survey-based qualitative evaluation, as a first step towards better standardisation of CT-based SPR derivation. MATERIALS AND METHODS: A questionnaire was sent to twelve particle therapy centres (ten from Europe and two from USA). It asked for details on CT scanners, image acquisition and reconstruction, definition of the HLUT, body-region specific HLUT selection, investigations of beam-hardening and experimental validations of the HLUT. Technological improvements were rated regarding their potential to improve SPR accuracy. RESULTS: Scan parameters and HLUT definition varied widely. Either the stoichiometric method (eight centres) or a tissue-substitute-only HLUT definition (three centres) was used. One centre combined both methods. The number of HLUT line segments varied widely between two and eleven. Nine centres had investigated influence of beam-hardening, often including patient-size dependence. Ten centres had validated their HLUT experimentally, with very different validation schemes. Most centres deemed dual-energy CT promising for improving SPR accuracy. CONCLUSIONS: Large inter-centre variability was found in implementation of CT scans, image reconstruction and especially in specification of the CT-to-SPR conversion. A future standardisation would reduce time-intensive institution-specific efforts and variations in treatment quality. Due to the interdependency of multiple parameters, no conclusion can be drawn on the derived SPR accuracy and its inter-centre variability.

Ion recombination and polarity correction factors for a plane-parallel ionization chamber in a proton scanning beam.

PURPOSE: To evaluate the effect on charge collection in the ionization chamber (IC) in proton pencil beam scanning (PBS), where the local dose rate may exceed the dose rates encountered in conventional MV therapy by up to three orders of magnitude. METHODS: We measured values of the ion recombination (ks ) and polarity (kpol ) correction factors in water, for a plane-parallel Markus TM23343 IC, using the cyclotron-based Proteus-235 therapy system with an active proton PBS of energies 30-230 MeV. Values of ks were determined from extrapolation of the saturation curve and the Two-Voltage Method (TVM), for planar fields. We compared our experimental results with those obtained from theoretical calculations. The PBS dose rates were estimated by combining direct IC measurements with results of simulations performed using the FLUKA MC code. Values of ks were also determined by the TVM for uniformly irradiated volumes over different ranges and modulation depths of the proton PBS, with or without range shifter. RESULTS: By measuring charge collection efficiency versus applied IC voltage, we confirmed that, with respect to ion recombination, our proton PBS represents a continuous beam. For a given chamber parameter, e.g., nominal voltage, the value of ks depends on the energy and the dose rate of the proton PBS, reaching c. 0.5% for the TVM, at the dose rate of 13.4 Gy/s. For uniformly irradiated regular volumes, the ks value was significantly smaller, within 0.2% or 0.3% for irradiations with or without range shifter, respectively. Within measurement uncertainty, the average value of kpol , for the Markus TM23343 IC, was close to unity over the whole investigated range of clinical proton beam energies. CONCLUSION: While no polarity effect was observed for the Markus TM23343 IC in our pencil scanning proton beam system, the effect of volume recombination cannot be ignored.

Abdominal obesity, metabolic syndrome in type 1 diabetic children and adolescents.

INTRODUCTION: The rapid rising prevalence of childhood obesity is related to the increased risk of cardiovascular morbidities. The type 1 diabetic patients are the group at special risk of macroangiopathy. The aim of the study was to estimate the prevalence of abdominal obesity in type 1 diabetic children compared to the general population, and to estimate the prevalence of metabolic syndrome diagnosed according to IDF criteria in type 1 diabetic children and adolescents. MATERIAL AND METHODS: 163 patients with type 1 diabetes mellitus (91 male) aged from 10 to 18 years were included into the study. The diabetes duration ranged from 0.5 to 15.2 years. The height, weight, waist circumference, body fat and blood pressure were measured. HbA1c and plasma lipids concentrations were examined. Body mass index, waist/height ratio (WHtR) and daily dose of insulin were calculated. Estimated glucose disposal rate (eGDR) as indicator of insulin resistance was calculated according to own formula. RESULTS: Abdominal obesity diagnosed as WHtR > 0.5 was found in 19 patients (11.7%, CI 95% 6.7-16.6). Associations between WHtR and HbA1c (r = 0.18; p = 0.027), cholesterol-HDL (r = -0.22; p = 0.004), systolic (3 = 0.37; p < 0.001) and diastolic blood pressure (beta = 0.19, p = 0.046) and Body Fat% (r = 0.59; p < 0.001) were observed. The prevalence of abdominal obesity in diabetic children was higher than in the general population (12.2 vs. 6.8%; p = 0.030). The metabolic syndrome was found in 12 individuals (7.4%, CI 95% 3.4-11.4). Patients with metabolic syndrome were older (16.6 vs. 14.7 years; p = 0.006) and their Body Fat% was higher (21 vs. 30%; p < 0.001) compared to patients not fulfilling the metabolic syndrome criteria. CONCLUSIONS: The prevalence of abdominal obesity in diabetic children is higher than in the general population. WHtR is associated with components of metabolic syndrome. Patients with metabolic syndrome are older and they are characterized by increased body fat.