A 2012 survey of the Australasian clinical medical physics and biomedical engineering workforce.

A survey of the medical physics and biomedical engineering workforce in Australia and New Zealand was carried out in 2012 following on from similar surveys in 2009 and 2006. 761 positions (equivalent to 736 equivalent full time (EFT) positions) were captured by the survey. Of these, 428 EFT were in radiation oncology physics, 63 EFT were in radiology physics, 49 EFT were in nuclear medicine physics, 150 EFT were in biomedical engineering and 46 EFT were attributed to other activities. The survey reviewed the experience profile, the salary levels and the number of vacant positions in the workforce for the different disciplines in each Australian state and in New Zealand. Analysis of the data shows the changes to the workforce over the preceding 6 years and identifies shortfalls in the workforce.

A Monte Carlo evaluation of three Compton camera absorbers.

We present a quantitative study on the performance of cadmium zinc telluride (CZT), thallium-doped sodium iodide (NaI(Tl)) and germanium (Ge) detectors as potential Compton camera absorbers. The GEANT4 toolkit was used to model the performance of these materials over the nuclear medicine energy range. CZT and Ge demonstrate the highest and lowest efficiencies respectively. Although the best spatial resolution was attained for Ge, its lowest ratio of single photoelectric to multiple interactions suggests that it is most prone to inter-pixel cross-talk. In contrast, CZT, which demonstrates the least positioning error due to multiple interactions, has a comparable spatial resolution with Ge. Therefore, we modelled a Compton camera system based on silicon (Si) and CZT as the scatterer and absorber respectively. The effects of the detector parameters of our proposed system on image resolution were evaluated and our results show good agreement with previous studies. Interestingly, spatial resolution which accounted for the least image degradation at 140.5 keV became the dominant degrading factor at 511 keV, indicating that the absorber parameters play some key roles at higher energies. The results of this study have validated the predictions by An et al. which state that the use of a higher energy gamma source together with reduction of the absorber segmentation to sub-millimetre could achieve the image resolution of 5 mm required in medical imaging.

GEANT4 simulation of the effects of Doppler energy broadening in Compton imaging.

A Monte Carlo approach was used to study the effects of Doppler energy broadening on Compton camera performance. The GEANT4 simulation toolkit was used to model the radiation transport and interactions with matter in a simulated Compton camera. The low energy electromagnetic physics model of GEANT4 incorporating Doppler broadening developed by Longo et al. was used in the simulations. The camera had a 9 × 9 cm scatterer and a 10 × 10 cm absorber with a scatterer to-absorber separation of 5 cm. Modelling was done such that only the effects of Doppler broadening were taken into consideration and effects of scatterer and absorber thickness and pixelation were not taken into account, thus a 'perfect' Compton camera was assumed. Scatterer materials were either silicon or germanium and the absorber material was cadmium zinc telluride. Simulations were done for point sources 10 cm in front of the scatterer. The results of the simulations validated the use of the low energy model of GEANT4. As expected, Doppler broadening was found to degrade the Compton camera imaging resolution. For a 140.5 keV source the resulting full-width-at-half-maximum (FWHM) of the point source image without accounting for Doppler broadening and using a silicon scatterer was 0.58 mm. This degraded to 7.1 mm when Doppler broadening was introduced and degraded further to 12.3 mm when a germanium scatterer was used instead of silicon. But for a 511 keV source, the FWHM was better than for a 140 keV source. The FWHM improved to 2.4 mm for a silicon scatterer and 4.6 mm for a germanium scatterer. Our result for silicon at 140.5 keV is in very good agreement with that published by An et al.

Photon beam convolution using polyenergetic energy deposition kernels.

In photon beam convolution calculations where polyenergetic energy deposition kernels (EDKS) are used, the primary photon energy spectrum should be correctly accounted for in Monte Carlo generation of EDKS. This requires the probability of interaction, determined by the linear attenuation coefficient, mu, to be taken into account when primary photon interactions are forced to occur at the EDK origin. The use of primary and scattered EDKS generated with a fixed photon spectrum can give rise to an error in the dose calculation due to neglecting the effects of beam hardening with depth. The proportion of primary photon energy that is transferred to secondary electrons increases with depth of interaction, due to the increase in the ratio mu ab/mu as the beam hardens. Convolution depth-dose curves calculated using polyenergetic EDKS generated for the primary photon spectra which exist at depths of 0, 20 and 40 cm in water, show a fall-off which is too steep when compared with EGS4 Monte Carlo results. A beam hardening correction factor applied to primary and scattered 0 cm EDKS, based on the ratio of kerma to terma at each depth, gives primary, scattered and total dose in good agreement with Monte Carlo results.

The effect of density on the 10MV photon beam penumbra.

An investigation into the density dependence of the penumbra of the Varian Clinac 18/10 10MV photon beam has been carried out. A water/lung phantom was constructed of polystyrene (r = 1.04 g cm-3) and cork (r = 0.23 g cm-3), in which interfaces exist both parallel and perpendicular to the beam axis. The irradiation of the phantom was also simulated using the EGS4 Monte Carlo system with a cartesian voxel geometry. Experimental (TLD) and Monte Carlo dose profiles are in close agreement, and show a large degree of penumbral broadening in the lung region. This broadening is due primarily to lateral electronic disequilibrium occurring at a larger distance from the geometric beam edge in lung than in water. This disequilibrium can also cause the dose in lung to drop below the dose in water at the same depth and off axis distance, even though the radiological depth is less in lung. Monte Carlo simulations were also performed where the dose is separated into primary and scattered components, for homogeneous media of densities 0.25, 0.50, 0.75 and 1.00 g cm-3. The penumbral width of the primary dose profile was found to be almost constant with depth for a point source of photons (after the initial build-up region), where the lateral distances from the 95-50% and 50-5% dose levels on the dose profile (normalised to the dose at the central axis) are equal in all cases. Also, primary penumbra width was found to be almost inversely proportional to density. The primary penumbra for a unit density material can be fitted accurately by an exponential forming function with empirical determined coefficients. The penumbral shape for the lower densities can then be closely fitted by scaling the coefficients in proportion to density. This scaling method has application in treatment planning, where the predicted primary penumbra shape should take account of inhomogeneities, and is particularly important in matching adjacent fields. When the scattered dose component is added to give the total dose, penumbral width increases because the scattered dose penumbra is wider than that of the primary dose. Also, the inverse proportionality of the penumbra width with density does not hold for the scattered dose. The relative contribution of the scattered dose increases with density. Therefore, the inverse proportionality of penumbra width with density does not hold for the total dose.

Electron contamination in 4 MV and 10 MV radiotherapy x-ray beams.

A thin window parallel-plate ionization chamber was constructed for dose measurement in the build-up region of high energy radiotherapy photon beams. The chamber is an integral part of a perspex block. The entrance window is 12 microns Melinex foil with a thin aluminium surface. Cavity thickness is 1.45 mm. Surface doses for varying field sizes were found to increase almost linearly with the side length of a square field. The surface dose for a 10x10 cm 4 MV photon beam is 12.1% for an open field and this increases to 14.1% with a polycarbonate block tray in the beam. Similarly for a 10 MV photon beam the surface dose is 10.6% for an open field and this increases to 12.4% with a polycarbonate block tray. The difference between the dose for an open field and a field with a polycarbonate block tray inserted becomes more significant for larger field sizes. Electron contamination depth dose curves are determined for a 4 MV and 10 MV photon beam. This is achieved by subtracting a pure photon beam build-up curve generated by an EGS4 Monte Carlo simulation from the experimental build-up curve. The EGS4 curve is a theoretical, electron contamination free curve. The electron contamination curve (of the 10 MV photon beam) has depth dose characteristics similar to that of a broad low energy electron beam.

Beam hardening of 10 MV radiotherapy x-rays: analysis using a convolution/superposition method.

Total and primary polyenergetic dose spread arrays (PDSA) have been generated for a high energy 10 MV radiotherapy photon beam using the electron gamma shower (EGS) Monte Carlo code. By considering the attenuation of fluence per energy interval, PDSA have been produced at radiological depths of 0 cm (the surface PDSA) and 40 cm (the beam hardened PDSA). By comparing primary PDSA produced at these different depths, the effect of beam hardening on the PDSA has been quantified. Calculations show that the mean electron range due to the surface primary PDSA is 6.67 mm and the mean electron range of the beam hardened primary PDSA is 8.24 mm. In comparison, a 3 MeV primary monoenergetic dose spread array (MDSA) has a much smaller mean electron range of 4.81 mm. A radiotherapy x-ray beam computation method is introduced which involves a single superposition of the surface generated PDSA or beam hardened PDSA with a polyenergetic TERMA. The mean percentage difference between depth-dose curves obtained using super-position of surface and beam hardened PDSA is only 0.1%. The mean percentage difference from experimental data for these superposition curves is 2.8% down to 40 cm in a homogeneous phantom. The superposition process is shown to be forgiving to spectral differences when calculating the PDSA, but sensitive to the incident photon energy spectrum used to calculate the TERMA.

Superposition dose calculation in lung for 10MV photons.

Currently available radiotherapy treatment planning systems employ scatter function models such as ETAR and Batho dSAR for dose calculation. Errors using these models for high energy photon irradiation occur in and beyond lung tissue for small fields. For larger fields, central axis dose is correctly predicted but penumbral broadening in lung is underestimated. The major source of error is the assumption that lateral electronic equilibrium is always established. A superposition algorithm has been developed for 10MV photons which calculates the dose by convolving the TERMA (Total Energy Released per unit MAss by primary photons) with a dose spread array formed using the EGS4 Monte Carlo code. TERMA and dose spread arrays are both generated using a 10 component photon energy spectrum. Dose in inhomogeneous media is calculated using dose spread arrays generated for different density media and by scaling dose spread arrays according to density variations. This method ensures that electronic disequilibrium is modelled in situations where it exists. Superposition results in a lung phantom for a 5 x 5 cm field agree with EGS4 Monte Carlo results to within 2% for p = 0.20 gcm-3 and p = 0.30 gcm-3 lung. Profiles generated by superposition for a 10 x 10 cm field at mid-lung and compared with film measurements show that penumbral broadening in low density material is also correctly predicted.