Investigation of stereotactic radiotherapy dose using dosimetry film and Monte Carlo simulations

This study uses dosimetry film measurements and Monte Carlo simulations to investigate the accuracy of type-a (pencil-beam) dose calculations for predicting the radiation doses delivered during stereotactic radiotherapy treatments of the brain. It is shown that when evaluating doses in a water phantom, the type-a algorithm provides dose predictions which are accurate to within clinically relevant criteria, gamma(3%,3mm), but these predictions are nonetheless subtly different from the results of evaluating doses from the same fields using radiochromic film and Monte Carlo simulations. An analysis of a clinical meningioma treatment suggests that when predicting stereotactic radiotherapy doses to the brain, the inaccuracies of the type-a algorithm can be exacerbated by inadequate evaluation of the effects of nearby bone or air, resulting in dose differences of up to 10% for individual fields. The results of this study indicate the possible advantage of using Monte Carlo calculations, as well as measurements with high-spatial resolution media, to verify type-a predictions of dose delivered in cranial treatments

The development of a Monte Carlo system to verify radiotherapy treatment dose calculations

Recent advances in the planning and delivery of radiotherapy treatments have resulted in improvements in the accuracy and precision with which therapeutic radiation can be administered. As the complexity of the treatments increases it becomes more difficult to predict the dose distribution in the patient accurately. Monte Carlo methods have the potential to improve the accuracy of the dose calculations and are increasingly being recognised as the “gold standard” for predicting dose deposition in the patient. In this study, software has been developed that enables the transfer of treatment plan information from the treatment planning system to a Monte Carlo dose calculation engine. A database of commissioned linear accelerator models (Elekta Precise and Varian 2100CD at various energies) has been developed using the EGSnrc/BEAMnrc Monte Carlo suite. Planned beam descriptions and CT images can be exported from the treatment planning system using the DICOM framework. The information in these files is combined with an appropriate linear accelerator model to allow the accurate calculation of the radiation field incident on a modelled patient geometry. The Monte Carlo dose calculation results are combined according to the monitor units specified in the exported plan. The result is a 3D dose distribution that could be used to verify treatment planning system calculations. The software, MCDTK (Monte Carlo Dicom ToolKit), has been developed in the Java programming language and produces BEAMnrc and DOSXYZnrc input files, ready for submission on a high-performance computing cluster. The code has been tested with the Eclipse (Varian Medical Systems), Oncentra MasterPlan (Nucletron B.V.) and Pinnacle3 (Philips Medical Systems) planning systems. In this study the software was validated against measurements in homogenous and heterogeneous phantoms. Monte Carlo models are commissioned through comparison with quality assurance measurements made using a large square field incident on a homogenous volume of water. This study aims to provide a valuable confirmation that Monte Carlo calculations match experimental measurements for complex fields and heterogeneous media

Establishing the impact of temporary tissue expanders on electron and photon beam dose distributions

Purpose: This study investigates the effects of temporary tissue expanders (TTEs) on the dose distributions in breast cancer radiotherapy treatments under a variety of conditions. Methods: Using EBT2 radiochromic film, both electron and photon beam dose distribution measurements were made for different phantoms, and beam geometries. This was done to establish a more comprehensive understanding of the implant’s perturbation effects under a wider variety of conditions. Results: The magnetic disk present in a tissue expander causes a dose reduction of approximately 20% in a photon tangent treatment and 56% in electron boost fields immediately downstream of the implant. The effects of the silicon elastomer are also much more apparent in an electron beam than a photon beam. Conclusions: Evidently, each component of the TTE attenuates the radiation beam to different degrees. This study has demonstrated that the accuracy of photon and electron treatments of post-mastectomy patients is influenced by the presence of a tissue expander for various beam orientations. The impact of TTEs on dose distributions establishes the importance of an accurately modelled high-density implant in the treatment planning system for post-mastectomy patients

Women in medical physics: A preliminary analysis of workforce and research participation in Australia and New Zealand

Although the participation of women within the science, technology, engineering and mathematics workforces has been widely discussed over recent decades, the recording and analysis of data pertaining to the gender balance of medical physicists in Australia and New Zealand remains rare. This study aimed to provide a baseline for evaluating future changes in workforce demographics by quantifying the current level of representation of women in the Australasian medical physics workforce and providing an indication of the relative contribution made by those women to the local research environment. The 2015 <i>Australasian College of Physical Scientists and Engineers in Medicine (ACPSEM</i>) member directory and list of chief physicists at <i>ACPSEM</i>-accredited radiation oncology and diagnostic imaging training centres were interrogated to identify the gender balance of medical physicists working in Australia and New Zealand. A specific investigation of the employment levels of all medical physicists in Queensland was undertaken to provide an example of the gender balance at different levels of seniority in one large Australian state. Lists of authors of medical physics presentations at <i>ACPSEM</i> annual conferences and authors of publications in the <i>ACPSEM</i>’s official journal, were used to provide an indication of the gender balance in published research within Australia and New Zealand. The results of this study showed that women currently constitute approximately 28 % of the medical physics workforce in Australia and New Zealand, distributed disproportionally in junior roles; there is a decrease in female participation in the field with increasing levels of seniority, which is particularly apparent in the stratified data obtained for the Queensland workforce. Comparisons with older data suggest that this situation has changed little since 2008. Examination of <i>ACPSEM</i> conference presentations suggested that there are similar disparities between the gender-balance of proffered and invited or keynote speakers (28 % and 13 % from female authors) and the gender balance of certified and chief physicists (28 % and 21 % female). The representation of women in the <i>ACPSEM</i> journal does not differ substantially between authorship of proffered versus invited work (22 % and 19 % from female authors). While this work was limited to evaluating the membership, annual conference and official journal of the <i>ACPSEM</i> (rather than evaluating the entire medical physics workforce and the contributions of male and female physicists to international conferences and publications), this study nonetheless led to the following recommendations: that a longitudinal study analysing correlations between age, period of service, seniority and gender should be undertaken and that future <i>ACPSEM</i> workforce surveys should include analyses of gender representation

Calibration seed sampling for iodine-125 prostate brachytherapy

Use of iodine-125 seeds for intraoperative planning and delivery of low dose rate brachytherapy treatments for prostate cancer requires that the air kerma strength of the seeds be checked against the vendor-supplied calibration certificate at the beginning of the surgical procedure. In practise, activity checks of multiple sources are difficult to achieve in surgery. This study therefore investigated the reliability of a calibration method that sampled only one seed, or a small number of seeds, by evaluating the consistency the air kerma strengths of all seeds in three small batches of 10, 20 and 30 iodine-125 seeds and calculating the probability of achieving results representative of each batch, within different levels of uncertainty. For the cartridges containing 10, 20 and 30 seeds, the mean differences between the source strengths identified by physical measurement and their decay-corrected calibration certificate values were respectively 5.0%±4.4%, −2.7%±3.7%, and 0.4%±2.8%. Assays of 5 randomly sampled seeds were shown to produce results within 3% of the mean air kerma strength of each batch, with larger assays producing less uncertainty. For these seeds, there was a greater than 30% chance that a randomly selected seed would have an activity that differed by more than 3% from the mean activity of all seeds in the cartridge. Although attractive as an efficiency measure, the testing of just one seed from a cartridge of iodine-125 seeds has a significant probability of producing an activity measurement that is not representative of the activity of the other seeds in the cartridge, potentially leading to substantial inaccuracies in implant dosimetry

A feasibility study of multislice X-ray CT imaging of gel dosimeters using the 'zero scan' method

This study extends the ‘zero scan’ method for CT imaging of polymer gel dosimeters to include multi-slice acquisitions. Multi slice CT images consisting of 24 slices of 1.2 mm thickness were acquired of an irradiated polymer gel dosimeter, and processed with the zero scan technique. The results demonstrate that zero scan based gel readout can be successfully applied to generate a three dimensional image of the irradiated gel field. Compared to the raw CT images the processed figures and cross gel profiles demonstrated reduced noise and clear visibility of the penumbral region. Moreover these improved results further highlight the suitability of this method in volumetric reconstruction with reduced CT data acquisition per slice. This work shows that 3D volumes of irradiated polymer gel dosimeters can be acquired and processed with x-ray CT

Photon beam dose distributions for patients with implanted temporary tissue expanders

This study examines the effects of temporary tissue expanders (TTEs) on the dose distributions of photon beams in breast cancer radiotherapy treatments. EBT2 radiochromic film and ion chamber measurements were taken to quantify the attenuation and backscatter effects of the inhomogeneity. Results illustrate that the internal magnetic port present in a tissue expander causes a dose reduction of approximately 25% in photon tangent fields immediately downstream of the implant. It was also shown that the silicone elastomer shell of the tissue expander reduced the dose to the target volume by as much as 8%. This work demonstrates the importance for an accurately modelled high-density implant in the treatment planning system for post-mastectomy breast cancer patients

Medical physics publishing in a changing research environment: the Australasian Physical and Engineering Sciences in Medicine 40th anniversary editorial

Australasian Physical &amp; Engineering Sciences in Medicine (APESM, or “the college journal”) is now 40 years old, making it one of the oldest medical physics journals in the world [1]. In fact, the history of this journal stretches back an additional 18 years; APESM replaced the Australasian Bulletin of Medical Physics, which in turn replaced the Australasian Newsletter of Medical Physics, which was first published in December 1959...

Molecular-dynamics simulation of model polymer nanocomposite rheology and comparison with experiment

The shear-rate dependence of viscosity is studied for model polymer melts containing various concentrations of spherical filler particles by molecular-dynamics simulations, and the results are compared with the experimental results for calcium-carbonate-filled polypropylene. Although there are some significant differences in scale between the simulated model polymer composite and the system used in the experiments, some important qualitative similarities in shear behavior are observed. The trends in the steady-state shear viscosities of the simulated polymer-filler system agree with those seen in the experimental results; shear viscosities, zero-shear viscosities, and the rate of shear thinning are all seen to increase with filler content in both the experimental and simulated systems. We observe a significant difference between the filler volume fraction dependence of the zero-shear viscosity of the simulated system and that of the experimental system that can be attributed to a large difference in the ratio of the filler particle radius to the radius of gyration of the polymer molecules. In the simulated system, the filler particles are so small that they only have a weak effect on the viscosity of the composite at low filler volume fraction, but in the experimental system, the viscosity of the composite increases rapidly with increasing filler volume fraction. Our results indicate that there exists a value of the ratio of the filler particle radius to the polymer radius of gyration such that the zero-shear-rate viscosity of the composite becomes approximately independent of the filler particle volume fraction