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Australasian Physical & Engineering Sciences in Medicine: The Official Journal of the Australasian College of Physical Scientists and Engineers in Medicine (v.34, #2)

Editorial: the ACPSEM diagnostic imaging TEAP project by John Cormack (pp. 173-178).

Applications of ICA and fractal dimension in sEMG signal processing for subtle movement analysis: a review by Ganesh R. Naik; Sridhar Arjunan; Dinesh Kumar (pp. 179-193).

The surface electromyography (sEMG) signal separation and decphompositions has always been an interesting research topic in the field of rehabilitation and medical research. Subtle myoelectric control is an advanced technique concerned with the detection, processing, classification, and application of myoelectric signals to control human-assisting robots or rehabilitation devices. This paper reviews recent research and development in independent component analysis and Fractal dimensional analysis for sEMG pattern recognition, and presents state-of-the-art achievements in terms of their type, structure, and potential application. Directions for future research are also briefly outlined.

Keywords: Independent component analysis; Blind source separation; Fractal theory; Fractal dimension; Surface electromyography

Development of a dosimetry inter-comparison for IMRT as part of site credentialing for a TROG multi-centre clinical trial for prostate cancer by B. Healy; J. Frantzis; R. Murry; J. Martin; M. Middleton; C. Catton; T. Kron (pp. 195-202).

A methodology has been developed for a dosimetry inter-comparison of intensity modulated radiation therapy (IMRT) delivery in Australasia. The inter-comparison is part of site credentialing for those sites participating in the prostate fractionated irradiation trial (PROFIT) for intermediate-risk prostate patients developed by the Ontario Clinical Oncology Group and coordinated in Australasia by the Trans Tasman Radiation Oncology Group. Features of the dosimetry inter-comparison design included the use of a dedicated pelvic anthropomorphic phantom, the use of a single CT data set of the phantom including contours and the use of radiochromic film as a dosimeter. Action levels for agreement between measured dose and treatment planning system dose have been proposed based on measurement uncertainty and international experience. A trial run of the dosimetry procedure at the reference centre gave results within the predefined action levels.

Keywords: Multi-centre clinical trials; Quality assurance; IMRT; Radiochromic film dosimetry; Prostate cancer

Simulation of fractionated and continuous irradiation in photodynamic therapy: study the differences between photobleaching and singlet oxygen dose deposition by Nadia Naghavi; Mohammad Hossein Miranbaygi; Ameneh Sazgarnia (pp. 203-211).

This study aims to compare the continuous irradiation with fractionated irradiation for photodynamic therapy (PDT) of solid tumors with intraperitoneally administered 5-aminolaevulinic acid (ALA). Therefore, considering the complex physiology of solid tumors and in order to inform simulations well, we did experiments on Balb/c mice using non-invasive fluorescence spectroscopy to have a feedback of protoporphyrin IX (PpIX) concentration in tumor just before irradiation and during treatment. PDT simulations were performed based on delivery of 36 J cm⁻² total laser energy (630 nm) at the fluence rate of 40 mW cm⁻² either for continuous or fractionated illumination. Based on the calculated amounts of ¹O₂ dose deposition and comparing these amounts with the 5 × 10 ¹⁸ molecules cm⁻³ threshold of reacting ¹O₂, simulation results demonstrate that fractionated illumination with alternating light and dark periods of 60 s improved the tumor response further for PpIX-mediated PDT.

Keywords: Photodynamic therapy (PDT); Simulation; Fractionated light; Singlet oxygen; Photobleaching

Evaluation of effective energy using radiochromic film and a step-shaped aluminum filter by T. Gotanda; T. Katsuda; R. Gotanda; A. Tabuchi; K. Yamamoto; T. Kuwano; H. Yatake; K. Kashiyama; K. Yabunaka; T. Akagawa; Y. Takeda (pp. 213-222).

Although the half-value layer (HVL) is one of the important parameters for quality assurance (QA) and quality control (QC), constant monitoring has not been performed because measurements using an ionization chamber (IC) are time-consuming and complicated. To solve these problems, a method using radiochromic film and step-shaped aluminum (Al) filters has been developed. To this end, GAFCHROMIC EBT2 dosimetry film (GAF-EBT2), which shows only slight energy dependency errors in comparison with GAFCHROMIC XR TYPE-R (GAF-R) and other radiochromic films, has been used. The measurement X-ray tube voltages were 120, 100, and 80 kV. GAF-EBT2 was scanned using a flat-bed scanner before and after exposure. To remove the non-uniformity error caused by image acquisition of the flat-bed scanner, the scanning image of the GAF-EBT2 before exposure was subtracted after exposure. HVL was evaluated using the density attenuation ratio. The effective energies obtained using HVLs of GAF-EBT2, GAF-R, and an IC dosimeter were compared. Effective energies with X-ray tube voltages of 120, 100, and 80 kV using GAF-EBT2 were 40.6, 36.0, and 32.9 keV, respectively. The difference ratios of the effective energies using GAF-EBT2 and the IC were 5.0%, 0.9%, and 2.7%, respectively. GAF-EBT2 and GAF-R proved to be capable of measuring effective energy with comparable precision. However, in HVL measurements of devices operating in the high-energy range (X-ray CT, radiotherapy machines, and so on), GAF-EBT2 was found to offer higher measurement precision than GAF-R, because it shows only a slight energy dependency.

Keywords: Radiochromic film; Effective energy; Half-value layer; Quality assurance; Quality control

ROPES eye plaque brachytherapy dosimetry for two models of ¹⁰³Pd seeds by Pooneh Saidi; Mahdi Sadeghi; Alireza Shirazi; Claudio Tenreiro (pp. 223-231).

Brachytherapy dose distributions are calculated for 15 mm ROPES eye plaque loaded with model Theragenics200 and IR06-¹⁰³Pd seeds. The effects of stainless steel backing and Acrylic insert on dose distribution along the central axis of the eye plaque and at critical ocular structure are investigated. Monte Carlo simulation was carried out with the Version 5 of the MCNP. The dose at critical ocular structure by considering the eye composition was calculated. Results are compared with the calculated data for COMS eye plaque loaded with Theragenics200 palladium-103 seeds and model 6711 iodine-125 seed. The air kerma strength of the IR06-¹⁰³Pd seed to deliver 85 Gy in apex of tumor in water medium was calculated to be 4.10 U/seed. Along the central axis of stainless steel plaque loaded with new ¹⁰³Pd seeds in Acrylic insert, the dose reduction relative to water is 6.9% at 5 mm (apex). Removal of the Acrylic insert from the plaque (replacing with water) did not make significantly difference in dose reduction results (~0.2%). The presence of the stainless steel backing results in dose enhancement near the plaque relative to water. Doses at points of interest are higher for ROPES eye plaque when compared to COMS eye plaque. The dosimetric parameters calculated in this work for the new palladium seed, showed that in dosimetry point of view, the IR06-¹⁰³Pd seed is suitable for use in brachytherapy. The effect of Acrylic insert on dose distribution is negligible and the main effect on dose reduction is due to the presence of stainless steel plaque backing.

Keywords: ¹⁰³Pd; Eye plaque; Brachytherapy; Dosimetry

ROPES eye plaque brachytherapy dosimetry for two models of ¹⁰³Pd seeds by Pooneh Saidi; Mahdi Sadeghi; Alireza Shirazi; Claudio Tenreiro (pp. 223-231).

Keywords: ¹⁰³Pd; Eye plaque; Brachytherapy; Dosimetry

A study into the relationship between the measured penumbra and effective source size in the modeling of the Pinnacle RTPS by Johnson Yuen; Nick Hardcastle; Peter Metcalfe (pp. 233-241).

The effect of detector size in the broadening of the penumbra on the model in the Pinnacle RTPS is investigated. A second order polynomial was devised to correlate the source size parameter with the RTPS-calculated penumbra. The optimal source size parameter was calculated for penumbra measurements based on the diamond detector and a standard ionization chamber (IC). This work was done for Jaw fields, MLC fields with a leaf end radius of 8 cm, and MLC fields with a leaf end radius of 12 cm. The optimum source size of the 8 cm MLC fields matched the jaw fields, and an average (based on field sizes studied) of 1.1 mm for the diamond detector data and 2.4 mm for the ionization chamber was established. The effect of this overestimation of the source size parameter based on detector-induced penumbra broadening was considered for a clinical IMRT prostate plan by using two models (diamond and IC). There were differences in the DVH of the PTV and of OARs but these effects were of negligible clinical significance. Dose difference distributions showed dose difference areas to be in penumbra regions of the segments, with larger dose differences where penumbras intersected and/or there was a significant weighting on the segment. Gamma analysis was also performed between the two plans, and was found to increase the amount of fail rates significantly for both 2%/2 mm and 3%/3 mm criteria. This decreases the sensitivity of IMRT QA in the detection of systematic errors.

Keywords: Source size; RTPS; Detector size; IMRT; Penumbra

Evaluation of the effects of gold nanoparticle shape and size on contrast enhancement in radiological imaging by Price Jackson; Selvakannan Periasamy; Vipul Bansal; Moshi Geso (pp. 243-249).

There has been increasing interest in the use of a nanoparticle-based media as a contrast-enhancement agent in medical imaging, particularly with gold Nanoparticles in radiography. Particularly attractive, is the prospect of modifying the surface of these materials with monoclonal antibodies to preferentially bind the nanoparticles to tumour sites. These materials differ from conventional molecular agents in their ability to be modified with cell specificity, or tailored for size and shape for maximum uptake. We investigated the consideration that quantum confinement electronic effects in nanometre-sized metals might have an effect on the integrated photon attenuation of gold atoms; in the same manner as these materials affect X-ray absorption and scattering as seen in X-ray absorption spectroscopy. This experiment has been designed to identify any effect on contrast enhancement that might result from employing gold nanoparticles with a variety of sizes. Spherical particles and nanorods were synthesised for this application. Image contrast enhancement was quantified by contrast-to-noise ratio in computed radiography. Results are consistent with existing measurements of gold nanoparticle contrast enhancement in radiography. No significant variation in attenuation depending on particle size was observed. Findings indicate that nanoparticle-based contrast agents in the size range 4–30 nm-can be synthesised for maximum stability or cell specificity (directed cellular uptake) without consideration of effect of size on contrast enhancement.

Keywords: Gold nanoparticles; Contrast media

Evaluation of a Gafchromic EBT2 film dosimetry system for radiotherapy quality assurance by T. Aland; T. Kairn; J. Kenny (pp. 251-260).

This study examines the dosimetric accuracy of Gafchromic EBT2 model radiochromic film for use in radiotherapy quality assurance. In this study, film was scanned using an Epson Perfection V700 flatbed scanner in transmission mode at 75 DPI with the subsequent analysis performed using the red and blue colour channels and ImageJ software. Results of this study suggest that the conversion of film optical density to measured dose should, at present, utilise red channel data only, without application of a blue channel correction to the data. For the batch of film examined here, film uniformity and reproducibility appear to have improved compared with published results using older batches. The orientation of the film on the scanner and the side of the film facing the light source were found to have substantial effects on results. Based on the results of this study, it is possible to recommend the use of EBT2 film in routine quality assurance testing for radiotherapy, in situations where a dose uncertainty of up to 2.8% is acceptable.

Keywords: EBT2; Gafchromic film; Dosimetry; Radiotherapy; Quality Assurance

The measurement of backscatter factors of kilovoltage X-ray beams using Gafchromic™ EBT2 film by L. Smith; R. Hill; M. Nakano; J. Kim; Z. Kuncic (pp. 261-266).

Backscatter factors are essential in the determination of radiation dose for kilovoltage X-ray beams. The accurate measurement of backscatter factors in water (B _W) is difficult and published values are based largely on Monte Carlo calculations. A number of studies have found that the measurement of B _W in the energy range from 50 to 300 kVp is possible using Gafchromic EBT film, but this film is no longer commercially available. In this work, we evaluated whether the newer Gafchromic EBT2 film is suitable for the determination of B _W for kilovoltage X-ray beams. B _W were measured with Gafchromic EBT2 film for beam qualities of 50, 100 and 280 kVp and field sizes of 2, 3, 4 and 6 cm diameter and compared with tabulated values published in the AAPM TG-61 protocol. We found that rotation of the film position during readout had a small but non-negligible effect on the optical density readings. The agreement between measured and published B _W was better than 3%, with the largest difference of occurring for the 2 cm diameter field with the 50 kVp X-ray beam. However, these differences are consistent with the total estimated uncertainty for the measurements, as calculated by the ISO GUM. Our results demonstrate that Gafchromic EBT2 film is a suitable dosimeter for B _W measurements for clinical kilovoltage X-ray beams.

Keywords: Backscatter factors; Kilovoltage X-rays; Gafchromic film

The use of a mixed Poisson model for tumour control probability computation in non homogeneous irradiations by Francisco Cutanda Henríquez; Silvia Vargas Castrillón (pp. 267-272).

Tumour control probability (TCP) is the probability of destroying every clonogen in a tumour as a result of a Radiation Therapy treatment. Assuming absorbed dose homogeneity throughout the tumour volume, TCP can be easily derived from a cell survival model. If absorbed dose is non homogeneous, its distribution has to be taken into account, because survival fractions depend on dose. This work presents a method based on mixture probability distributions to introduce absorbed dose heterogeneity using dose volume histograms. Results are close to the ones provided by the standard voxel oriented method usually utilized, but the mixture method makes more robust assumptions about independence between voxels. Therefore, this method is more flexible, and could potentially deal with variations in survival fraction caused by other factors.

Keywords: Tumour control probability; Mixed Poisson models; Dose volume histogram

Monte Carlo study of the energy response and depth dose water equivalence of the MOSkin radiation dosimeter at clinical kilovoltage photon energies by C. P. L. Lian; M. A. R. Othman; D. Cutajar; M. Butson; S. Guatelli; A. B. Rosenfeld (pp. 273-279).

Skin dose is often the quantity of interest for radiological protection, as the skin is the organ that receives maximum dose during kilovoltage X-ray irradiations. The purpose of this study was to simulate the energy response and the depth dose water equivalence of the MOSkin radiation detector (Centre for Medical Radiation Physics (CMRP), University of Wollongong, Australia), a MOSFET-based radiation sensor with a novel packaging design, at clinical kilovoltage photon energies typically used for superficial/orthovoltage therapy and X-ray CT imaging. Monte Carlo simulations by means of the Geant4 toolkit were employed to investigate the energy response of the CMRP MOSkin dosimeter on the surface of the phantom, and at various depths ranging from 0 to 6 cm in a 30 × 30 × 20 cm water phantom. By varying the thickness of the tissue-equivalent packaging, and by adding thin metallic foils to the existing design, the dose enhancement effect of the MOSkin dosimeter at low photon energies was successfully quantified. For a 5 mm diameter photon source, it was found that the MOSkin was water equivalent to within 3% at shallow depths less than 15 mm. It is recommended that for depths larger than 15 mm, the appropriate depth dose water equivalent correction factors be applied to the MOSkin at the relevant depths if this detector is to be used for depth dose assessments. This study has shown that the Geant4 Monte Carlo toolkit is useful for characterising the surface energy response and depth dose behaviour of the MOSkin.

Keywords: Monte Carlo; MOSFET; Geant4; Radiation protection; Kilovoltage energy

Evaluation of EBT radiochromic film using a multiple exposure technique by Karl Roozen; Tomas Kron; Annette Haworth; Rick Franich (pp. 281-289).

Radiochromic film is a self developing two-dimensional dosimeter system that is widely used in radiotherapy. Since its development some 40 years ago many improvements have been made; however the sensitivity can vary across the film and a non-linear dose response remains. The former can be addressed using a double exposure technique; however this technique assumes that the response is linear so that the dose distribution is incorrectly measured. We are proposing the use of two homogenous exposures; one performed prior, the other after the irradiation to be measured. In this ‘multiple exposure technique’ (MET), the first homogenous exposure can be used to correct for variations in response in different parts of the film, while the second homogenous exposure allows correction for non-linearity of response with dose. The MET was tested with a 60° wedged field and an Intensity Modulated Radiation Therapy fluence map produced by a computerised treatment planning system. In the wedge field measurements, the MET profile showed agreement within 0.6 cGy for 80% of the field compared to ionisation chamber dose values. A comparison of a TPS generated fluence map dose distribution with one measured using conventionally calibrated EBT, and another measured with MET calibrated EBT, showed similar agreement. The MET would be particularly useful for the assessment of highly inhomogenous dose distributions with high maximum dose such as encountered in hypofractionated radiation therapy.

Keywords: EBT gafchromic film; EBT2 gafchromic film; Double exposure technique; Multiple exposure technique; Intensity modulated radiation therapy (IMRT); Hypofractionated radiation therapy

Dosimetric impact of systematic MLC positional errors on step and shoot IMRT for prostate cancer: a planning study by N. M. Ung; C. S. Harper; L. Wee (pp. 291-298).

The positional accuracy of multileaf collimators (MLC) is crucial in ensuring precise delivery of intensity-modulated radiotherapy (IMRT). The aim of this planning study was to investigate the dosimetric impact of systematic MLC positional errors on step and shoot IMRT of prostate cancer. A total of 12 perturbations of MLC leaf banks were introduced to six prostate IMRT treatment plans to simulate MLC systematic positional errors. Dose volume histograms (DVHs) were generated for the extraction of dose endpoint parameters. Plans were evaluated in terms of changes to the defined endpoint dose parameters, conformity index (CI) and healthy tissue avoidance (HTA) to planning target volume (PTV), rectum and bladder. Negative perturbations of MLC had been found to produce greater changes to endpoint dose parameters than positive perturbations of MLC (p < 0.01). Negative and positive asynchronised MLC perturbations of −1 mm resulted in median changes in D₉₅ of −1.2 and 0.9% respectively. Negative and positive synchronised MLC perturbations of 1 mm in one direction resulted in median changes in D₉₅ of −2.3 and 1.8% respectively. Doses to rectum were generally more sensitive to systematic MLC errors compared to bladder (p < 0.01). Negative and positive synchronised MLC perturbations of 1 mm in one direction resulted in median changes in endpoint dose parameters of rectum and bladder from 1.0 to 2.5%. Maximum reduction of −4.4 and −7.3% were recorded for conformity index (CI) and healthy tissue avoidance (HTA) respectively due to synchronised MLC perturbation of 1 mm. MLC errors resulted in dosimetric changes in IMRT plans for prostate.

Keywords: Intensity-modulated radiotherapy; Multileaf collimators; Prostate cancer; Systematic errors

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Australasian Physical & Engineering Sciences in Medicine: The Official Journal of the Australasian College of Physical Scientists and Engineers in Medicine (v.34, #2)

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Australasian Physical & Engineering Sciences in Medicine: The Official Journal of the Australasian College of Physical Scientists and Engineers in Medicine (v.34, #2)