Peter N. Johnston

Multivariate analysis method for energy calibration and improved mass assignment in recoil spectrometry

Abstract Heavy ion recoil spectrometry is rapidly becoming a well established analysis method, but the associated data analysis processing is still not well developed. The pronounced nonlinear response of silicon detectors for heavy ions leads to serious limitation and complication in mass gating, which is the principal factor in obtaining energy spectra with minimal cross talk between elements. To overcome the above limitation, a simple empirical formula with an associated multiple regression method is proposed for the absolute energy calibration of the time of flight-energy dispersive detector telescope used in recoil spectrometry. A radical improvement in mass assignment was realized, which allows a more accurate and improved depth profiling with the important feature of making the data processing much easier.

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.

The accuracy of the pencil beam convolution and anisotropic analytical algorithms in predicting the dose effects due to attenuation from immobilization devices and large air gaps.

When a photon beam passes through the treatment couch or an immobilization device, it may traverse a large air gap (up to 15 cm or more) prior to entering the patient. Previous studies have investigated the ability of various treatment planning systems to calculate the dose immediately beyond small air gaps, typically less than 5 cm thick, such as those within the body. The aim of this study is to investigate the ability of the Eclipse anisotropic analytical algorithm (AAA) and pencil beam convolution (PBC) algorithm to calculate the dose beyond large air gaps. Depth dose data in water for a 6 MV photon beam, 10 x 10 cm2 field size, and 100 cm SSD were measured beyond a range of air gaps (1-15 cm). The thickness of the water equivalent material positioned before the air gap ranged from 0.2 to 4 cm. Dose was calculated with the Eclipse PBC algorithm and AAA. The scattered and primary dose components were calculated from the measurements. The measured results indicate that as the air gap increases (from 1 to 15 cm) the dose reduces at the water surface and that beyond an air gap a secondary buildup region is required to re-establish electronic equilibrium. The dose beyond the air gap is also reduced at depths beyond the secondary buildup region. The PBC algorithm did not predict any reduction in dose beyond the air gap. AAA predicted the secondary buildup region but did not predict the reduction in dose at depths beyond it. The reduction in dose beyond the secondary buildup region was shown to be particularly relevant for air gaps of 5 cm or more when there was a 2 cm of water equivalent material positioned before the air gap. For these cases, where electronic equilibrium is established in the material positioned before the air gap, both algorithms were found to overestimate the dose by 2.0%-5.5%. It was concluded that the dose to depths of up to 15 cm beyond a large air gap is reduced due to a decrease in scattered radiation, produced in the material positioned before the air gap, reaching the point of interest. This effect is not well modeled by the Eclipse AAA and PBC algorithm and may result in dose calculation errors greater than 2.5%. Due to the contribution of other uncertainties in the radiation therapy treatment planning and delivery process, dose calculation errors of this magnitude are not consistent with the recommendation of the International Commission on Radiation Units and Measurements that the absorbed dose to the target volume be delivered with an uncertainty of less than +/- 5%.

An in vivo investigative protocol for HDR prostate brachytherapy using urethral and rectal thermoluminescence dosimetry

PURPOSE To develop an in vivo dosimetry based investigative action level relevant for a corrective protocol for HDR brachytherapy boost treatment. METHODS AND MATERIALS The dose delivered to points within the urethra and rectum was measured using TLD in vivo dosimetry in 56 patients. Comparisons between the urethral and rectal measurements and TPS calculations showed differences, which are related to the relative position of the implant and TLD trains, and allowed shifts of implant position relative to the prostate to be estimated. RESULTS AND CONCLUSIONS Analysis of rectal dose measurements is consistent with implant movement, which was previously only identified with the urethral data. Shift corrected doses were compared with results from the TPS. Comparison of peak doses to the urethra and rectum has been assessed against the proposed corrective protocol to limit overdosing these critical structures. An initial investigative level of 20% difference between measured and TPS peak dose was established, which corresponds to 1/3 of patients which was practical for the caseload. These patients were assessed resulting in corrective action being applied for one patient. Multiple triggering for selective investigative action is outlined. The use of a single in vivo measurement in the first fraction optimizes patient benefit at acceptable cost.

Electron Interaction with Gel Dosimeters: Effective Atomic Numbers for Collisional, Radiative and Total Interaction Processes

Abstract Taylor, M. L., Franich, R. D., Trapp, J. V. and Johnston, P. N. Electron Interaction with Gel Dosimeters: Effective Atomic Numbers for Collisional, Radiative and Total Interaction Processes. Radiat. Res. 171, 123–126 (2009). The effective atomic number is widely employed in radiation studies, particularly for the characterization of interaction processes in dosimeters, biological tissues and substitute materials. Gel dosimeters are unique in that they comprise both the phantom and dosimeter material. In this work, effective atomic numbers for total and partial electron interaction processes have been calculated for the first time for a Fricke gel dosimeter, five hypoxic and nine normally oxygenated polymer gel dosimeters. A range of biological materials are also presented for comparison. The spectrum of energies studied spans 10 keV to 100 MeV, over which the effective atomic number varies by 30%. The effective atomic numbers of gels match those of soft tissue closely over the full energy range studied; greater disparities exist at higher energies but are typically within 4%.

Technical Note: Modeling a complex micro‐multileaf collimator using the standard BEAMnrc distribution

PURPOSE The component modules in the standard BEAMnrc istribution may appear to be insufficient to model micro-multileaf collimators that have trifaceted leaf ends and complex leaf profiles. This note indicates, however, that accurate Monte Carlo simulations of radiotherapy beams defined by a complex collimation device can be completed using BEAMnrcs standard VARMLC component module. METHODS That this simple collimator model can produce spatially and dosimetrically accurate microcollimated fields is illustrated using comparisons with ion chamber and film measurements of the dose deposited by square and irregular fields incident on planar, homogeneous water phantoms. RESULTS Monte Carlo dose calculations for on-axis and off-axis fields are shown to produce good agreement with experimental values, even on close examination of the penumbrae. CONCLUSIONS The use of a VARMLC model of the micro-multileaf collimator, along with a commissioned model of the associated linear accelerator, is therefore recommended as an alternative to the development or use of in-house or third-party component modules for simulating stereotactic radiotherapy and radiosurgery treatments. Simulation parameters for the VARMLC model are provided which should allow other researchers to adapt and use this model to study clinical stereotactic radiotherapy treatments.

Transit dose of an Ir-192 high dose rate brachytherapy stepping source

Clinical dosimetry for high dose rate (HDR) brachytherapy with a single stepping source generally neglects the transit dose. This study investigates the effects of the transit dose in the target volume of an HDR brachytherapy stepping source. A video method was used to analyse the entrance, exit and the interdwell transit speed of the source for different path lengths and step sizes ranging from 2.5 mm to 995 mm. The transit speed was found to vary with the step size and path length. For the travelled distances of 2.5, 5.0, 10.0, 230 and 995 mm, the average transit speeds were 54, 72, 233, 385 and 467 mm s(-1) respectively. The results also show that the manufacturer has attempted to compensate for the effects of interdwell transit dose by reducing the actual dwell time of the source. A well-type chamber was used to determine the dose differences between two sets of measurements, one being the stationary dose only and the other being the sum of stationary and transit doses. Single catheters of active lengths of 20 and 40 mm, different dwell times of 0.5, 1, 2 and 5 s and different step sizes of 2.5, 5 and 10 mm were used in the measurements with the well-type chamber. Most of the measured dose differences between stationary and stationary plus interdwell source movement were within 2%. The additional dose due to the source transit can be as high as 24.9% for the case of 0.5 s dwell time, 10 mm step size and 20 mm active length. The dose difference is mainly due to the entrance and exit source movement but not the interdwell movement.

Adapting a generic BEAMnrc model of the BrainLAB m3 micro-multileaf collimator to simulate a local collimation device

This work is focussed on developing a commissioning procedure so that a Monte Carlo model, which uses BEAMnrcs standard VARMLC component module, can be adapted to match a specific BrainLAB m3 micro-multileaf collimator (microMLC). A set of measurements are recommended, for use as a reference against which the model can be tested and optimized. These include radiochromic film measurements of dose from small and offset fields, as well as measurements of microMLC transmission and interleaf leakage. Simulations and measurements to obtain microMLC scatter factors are shown to be insensitive to relevant model parameters and are therefore not recommended, unless the output of the linear accelerator model is in doubt. Ultimately, this note provides detailed instructions for those intending to optimize a VARMLC model to match the dose delivered by their local BrainLAB m3 microMLC device.

Remote auditing of radiotherapy facilities using optically stimulated luminescence dosimeters

PURPOSE On 1 July 2012, the Australian Clinical Dosimetry Service (ACDS) released its Optically Stimulated Luminescent Dosimeter (OSLD) Level I audit, replacing the previous TLD based audit. The aim of this work is to present the results from this new service and the complete uncertainty analysis on which the audit tolerances are based. METHODS The audit release was preceded by a rigorous evaluation of the InLight® nanoDot OSLD system from Landauer (Landauer, Inc., Glenwood, IL). Energy dependence, signal fading from multiple irradiations, batch variation, reader variation, and dose response factors were identified and quantified for each individual OSLD. The detectors are mailed to the facility in small PMMA blocks, based on the design of the existing Radiological Physics Centre audit. Modeling and measurement were used to determine a factor that could convert the dose measured in the PMMA block, to dose in water for the facilitys reference conditions. This factor is dependent on the beam spectrum. The TPR20,10 was used as the beam quality index to determine the specific block factor for a beam being audited. The audit tolerance was defined using a rigorous uncertainty calculation. The audit outcome is then determined using a scientifically based two tiered action level approach. Audit outcomes within two standard deviations were defined as Pass (Optimal Level), within three standard deviations as Pass (Action Level), and outside of three standard deviations the outcome is Fail (Out of Tolerance). RESULTS To-date the ACDS has audited 108 photon beams with TLD and 162 photon beams with OSLD. The TLD audit results had an average deviation from ACDS of 0.0% and a standard deviation of 1.8%. The OSLD audit results had an average deviation of -0.2% and a standard deviation of 1.4%. The relative combined standard uncertainty was calculated to be 1.3% (1σ). Pass (Optimal Level) was reduced to ≤2.6% (2σ), and Fail (Out of Tolerance) was reduced to >3.9% (3σ) for the new OSLD audit. Previously with the TLD audit the Pass (Optimal Level) and Fail (Out of Tolerance) were set at ≤4.0% (2σ) and >6.0% (3σ). CONCLUSIONS The calculated standard uncertainty of 1.3% at one standard deviation is consistent with the measured standard deviation of 1.4% from the audits and confirming the suitability of the uncertainty budget derived audit tolerances. The OSLD audit shows greater accuracy than the previous TLD audit, justifying the reduction in audit tolerances. In the TLD audit, all outcomes were Pass (Optimal Level) suggesting that the tolerances were too conservative. In the OSLD audit 94% of the audits have resulted in Pass (Optimal level) and 6% of the audits have resulted in Pass (Action Level). All Pass (Action level) results have been resolved with a repeat OSLD audit, or an on-site ion chamber measurement.

RBS and recoil spectrometry analysis of CoSi2 formation on GaAs

Abstract Mass and energy-dispersive recoil spectrometry has recently reached the state of development where it is possible to separately characterise Ga and As in GaAs samples. Since it is possible to simultaneously characterise several elements (light as well as heavy), e.g. C, O, Si, Co, Ga and As, the technique is suited for examining the depth distribution of metallisation contacts on GaAs. In a Swedish-Australian collaboration a recoil detector telescope was attached to a beamline of the FN tandem accelerator “ANTARES”, at Lucas Heights Research Laboratories, Australia. In the measurements presented here, 127 I 10+ at an energy of 77 MeV was employed to analyse GaAs samples with thin film overlayers — Si(220 nm)/Co(50 nm)/〈100〉-GaAs. A reference sample and samples annealed at 300 to 600°C were analysed. The measurements showed that CoSi 2 is formed during annealing at and above 500°C with no detectable reaction between the GaAs-substrate and the CoSi 2 overlayer.