Justifying referrals for paediatric CT.

• The system of radiation protection assumes a linear dose-response relationship with no threshold for low doses and dose rate exposures. This is based on epidemiological evidence at higher doses. • Hence there is a small theoretical risk of carcinogenesis attributable to low doses of ionising radiation. This risk is associated with any diagnostic imaging procedure involving radiation. • Radiosensitivity declines with age, so children are more susceptible to radiation risks than adults. Females are more radiosensitive than males. • The radiation protection system is based on the assumption that radiation risk is cumulative over a lifetime. • For an individual, a justified, optimised computed tomography (CT) scan will result in more benefit than harm. A doctor must justify the necessity for a CT scan before referring an individual for imaging.

The influence of field size on stopping-power ratios in- and out-of-field: quantitative data for the BrainLAB m3 micro-multileaf collimator.

The objective of this work is to quantify the systematic errors introduced by the common assumption of invariant secondary electron spectra with changing field sizes, as relevant to stereotactic radiotherapy and other treatment modes incorporating small beam segments delivered with a linac-based stereotactic unit. The EGSnrc/BEAMnrc Monte Carlo radiation transport code was used to construct a dosimetrically-matched model of a Varian 600C linear accelerator with mounted BrainLAB micro-multileaf collimator. Stopping-power ratios were calculated for field sizes ranging from 6 × 6 mm2 up to the maximum (98 × 98 mm2), and differences between these and the reference field were computed. Quantitative stopping power data for the BrainLAB micro-multileaf collimator has been compiled. Field size dependent differences to reference conditions increase with decreasing field size and increasing depth, but remain a fraction of a percent for all field sizes studied. However, for dosimetry outside the primary field, errors induced by the assumption of invariant electron spectra can be greater than 1%, increasing with field size. It is also shown that simplification of the Spencer-Attix formulation by ignoring secondary electrons below the cutoff kinetic energy applied to the integration results in underestimation of stopping-power ratios of about 0.3% (and is independent of field size and depth). This work is the first to quantify stopping powers from a BrainLAB micro-multileaf collimator. Many earlier studies model simplified beams, ignoring collimator scatter, which is shown to significantly influence the spectrum. Importantly, we have confirmed that the assumption of unchanging electron spectra with varying field sizes is justifiable when performing (typical) in-field dosimetry of stereotactic fields. Clinicians and physicists undertaking precise out-of-field measurements for the purposes of risk estimation, ought to be aware that the more pronounced spectral variation results in stopping powers (and hence doses) that differ more than for in-field dosimetry.

On the use of mercury as a means of locating background sources in ultra low-background HPGe-detector systems.

In low-level gamma-ray spectrometry, it is common to measure large samples in order to obtain low detection limits for the massic activity (in mBq/kg). These samples have significant shielding effects. In order to study whether the background sources in three ultra low-background HPGe detectors were located in the detector or in the shield, Marinelli beakers filled with hyperpure mercury were measured. Although the measurements were hampered by the presence of cosmogenically produced (194)Hg, information regarding the major background location of (40)K, (60)Co, (137)Cs, (210)Pb, (226)Ra, (228)Ra and (228)Th could be obtained.

Cascade summing effects in close geometry gamma-ray spectrometry.

Low-level gamma-ray spectrometry, particularly in underground laboratories, is used to measure radioactivity in diverse close geometries, often placing samples directly on the detector which favours strong cascade summing effects. Monte-Carlo modelling of the efficiency is extended to cascade summing effects from complete decay sequences including positron emitting radionuclides. The effect of geometrical uncertainties on the overall uncertainty is presented. The results reinforce the need for radiography of the detector and inclusion of the detector shielding in the model.

A low-level activation technique for monitoring thermonuclear fusion plasma conditions.

Optimisation of the confinement and sustainability of a thermonuclear plasma requires methods to monitor processes in the plasma. In this work three materials were used as activation targets (Ti, MgF2 and a TiVAl compound). They were placed inside the joint European Torus (JET) vacuum chamber. Certain gamma-ray emitting radionuclides (7Be, 54Mn, 56Co, 57Co, 58Co and 46Sc) were measured using ultra low-level gamma-ray spectrometry in an underground laboratory 1-2 months after activation. They were found to arise from neutron activation of bulk sample material and surface contaminants sputtered from other Tokamak parts. Decision thresholds for some activation products were determined in order to aid in giving upper bounds for the flux of charged particles.

The distribution of 210Pb in human bone and its impact on methods for the retrospective estimation of 222Rn exposure from in vivo measurements.

It is possible to estimate radon exposure to man retrospectively by the in vivo measurement of the decay product (210)Pb, which accumulates in the bones. For in vivo methods, knowledge of the distribution of (210)Pb in the skeleton is needed to determine the optimal site for measurement, the skull or the knee. In this study the activity of (210)Pb in a variety of bone samples from 3 individuals have been measured in vitro using underground gamma-ray spectrometry. The individuals were unlikely to have had elevated intakes of Rn. These measurements give baseline data on the bone massic activity of (210)Pb. They show that the massic activity is similar for each of these people and there are similar massic activities of (210)Pb in the skull and the knee of the 2 individuals for which the skull was measured. Additionally for 2 of the individuals trabecular and cortical bone were separated and massic activities were found to be strongly correlated indicating that the (210)Pb is associated with the hydroxyapatite.

Measurement of low levels of 26Al from meteorite samples.

As part of an intercomparison to resolve discrepancies between accelerator mass spectrometry results and radiometric results, the 26Al activity in four meteorite samples was measured using ultra low-level gamma-ray spectrometry in the underground laboratory HADES. Although reference sources were used, extensive use was made of computer modelling to determine corrections for absorption, coincidence summing between gamma rays in the decays and annihilation radiation following positron emission. Directional correlation corrections were also taken into account. The limiting uncertainties in these measurements arose from counting statistics of 5-9%. Some computer modelling was undertaken to determine optimum geometry for this type of intercomparison involving gamma-ray spectrometry.

Thermal neutron fluence from ultra low-level gamma-ray spectrometry of spoons activated during the JCO criticality accident at Tokai-mura in 1999.

During the JCO-accident in Tokai-mura in 1999, the surrounding village was irradiated by an uncontrolled neutron flux. At some locations in that village, the thermal neutron flux was determined retrospectively by measurement of the very low activity of 51Cr and 60Co in stainless-steel spoons using gamma-ray spectrometry in underground laboratories. Activities determined in the HADES underground facility are presented here, together with calibrations performed using a well-defined thermal neutron flux to directly estimate the fluence of thermal neutrons independent of most assumptions. The results show measurable 51Cr in three samples and 60Co in four samples taken from locations at distances of up to 430m from the accident location despite the elapse of 4 half-lives of 51Cr before measurement. Effects of air transport of the samples were considered and shown to be negligible.

Paediatric CT imaging trends in Australia.

INTRODUCTION: The use of CT has rapidly increased since its introduction. Although an important medical tool for diagnosis and treatment, CT is recognised as being among the highest contributors to population radiation exposure. As the risks associated with exposure are higher for children than for adults, this study assessed the impact of paediatric CT in Australia by analysing imaging trends. METHODS: CT imaging trends were derived from Medicare data. Comparable data from a dedicated paediatric hospital (Royal Children's Hospital Melbourne (RCH)) were analysed to determine the validity of utilising Medicare statistics in the younger age groups. The resulting trends reflect the situation for paediatric CT imaging in Australia. RESULTS: In 2009, 2.1 million CT services were billed to Medicare in Australia for children and adults. The average annual growth in the number of CT services provided since 1994 was 8.5%, compared with population growth of 1.4%. Comparison of RCH and Medicare data revealed that only one third of paediatric CT imaging is captured by Medicare. Combining the data sets showed that over the last 20 years, there has been an average annual increase of 5.1% in the CT imaging rate for 0 to 18-year-olds. However, in recent years, growth in the imaging rate for 11 to 18-year-olds has slowed, while for 5 to 10-year-olds the imaging rate has declined. CONCLUSIONS: The significant growth in CT services is attributable to increased demand from the adult demographic. Conversely, increases in the imaging rate for paediatric patients have slowed overall. In fact, for some age groups the rate has fallen.