Evaluation of detector material and radiation source position on Compton camera's ability for multitracer imaging.

We present a study on the effects of detector material, radionuclide source and source position on the Compton camera aimed at realistic characterization of the camera's performance in multitracer imaging as it relates to brain imaging. The GEANT4 Monte Carlo simulation software was used to model the physics of radiation transport and interactions with matter. Silicon (Si) and germanium (Ge) detectors were evaluated for the scatterer, and cadmium zinc telluride (CZT) and cerium-doped lanthanum bromide (LaBr(3):Ce) were considered for the absorber. Image quality analyses suggest that the use of Si as the scatterer and CZT as the absorber would be preferred. Nevertheless, two simulated Compton camera models (Si/CZT and Si/LaBr(3):Ce Compton cameras) that are considered in this study demonstrated good capabilities for multitracer imaging in that four radiotracers within the nuclear medicine energy range are clearly visualized by the cameras. It is found however that beyond a range difference of about 2 cm for (113m)In and (18)F radiotracers in a brain phantom, there may be a need to rotate the Compton camera for efficient brain imaging.

AFOMP policy statement no. 4: recommendations for continuing professional development systems for medical physicists in AFOMP countries.

This policy statement, which is the fourth of a series of documents being prepared by the Asia-Oceania Federation of Organizations for Medical Physics Committees Professional Development Committee, gives guidance on how member countries could develop a continuing professional development system for ensuring that its clinical medical physicists are up-to-date in their knowledge and practice. It is not intended to be prescriptive as there are already several CPD systems successfully operated by AFOMP member countries and elsewhere that vary considerably in scope and structure according to local culture, practice and legislation but all of which are capable of ensuring that physicists are up-to-date. It is intended to be advisory and set out options for member countries to develop their individual CPD systems.

Certification and licensing of clinical medical physicists in AFOMP countries.

In many countries the education and training of medical physicists has changed over the last few decades from being rather ad hoc to becoming structured with residency programs becoming quite common. These are often followed by formal certification by independent bodies, and medical physics professional organizations (MPPOs) affirming the quality of certified physicists and recognizing their competence and ability to practice independently. Policies on the training and education of medical physicists have been developed by the International Organization for Medical Physics (IOMP) and by the Asia-Oceania Federation of Organizations for Medical Physics (AFOMP). Currently nine countries in Asia-Oceania operate systems for the certification of clinical medical physicists and four others are planning or implementing such systems. The existing systems, which are described in this paper, generally conform to the policies. Licensing of medical physicists, which is primarily the responsibility of government bodies, is almost unknown in that region.

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.

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.

AFOMP Policy Statement No. 3: recommendations for the education and training of medical physicists in AFOMP countries.

AFOMP recognizes that clinical medical physicists should demonstrate that they are competent to practice their profession by obtaining appropriate education, training and supervised experience in the specialties of medical physics in which they practice, as well as having a basic knowledge of other specialties. To help its member countries to achieve this, AFOMP has developed this policy to provide guidance when developing medical physicist education and training programs. The policy is compatible with the standards being promoted by the International Organization for Medical Physics and the International Medical Physics Certification Board.

AFOMP POLICY STATEMENT No. 2: recommended clinical radiation oncology medical physicist staffing levels in AFOMP countries.

This document is the second of a series of policy statements being issued by the Asia-Oceania Federation of Organizations for Medical Physics (AFOMP). The document was developed by the AFOMP Professional Development Committee (PDC) and was released by the AFOMP Council in 2009. The main purpose of the document is to give guidance as to how many medical physicists are required to staff a radiation oncology department. Strict guidelines are difficult to define as work practices vary from country-to-country and from hospital-to-hospital. A calculation scheme is presented to aid in estimating medical physics staffing requirements that is primarily based on equipment levels and patient numbers but also with allowances for staff training, professional development and leave requirements.

AFOMP policy number 6: code of ethics for medical physicists in AFOMP Countries.

This policy statement, which is the sixth of a series of documents prepared by the Asia-Oceania Federation of Organizations for Medical Physics (AFOMP) Professional Development Committee, gives guidance on how medical physicists in AFOMP countries should conduct themselves in an ethical manner in their professional practice (Ng et al. in Australas Phys Eng Sci Med 32:175-179, 2009; Round et al. in Australas Phys Eng Sci Med 33:7-10, 2010; Round et al. in Australas Phys Eng Sci Med 34:303-307, 2011; Round et al. in Australas Phys Eng Sci Med 35:393-398, 2012; Round et al. in Australas Phys Eng Sci Med 38:217-221, 2015). It was developed after the ethics policies and codes of conducts of several medical physics societies and other professional organisations were studied. The policy was adopted at the Annual General Meeting of AFOMP held in Jaipur, India, in November 2017.

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

A survey of the medical physics and biomedical engineering workforce was carried out in 2006. 495 positions (equivalent to 478 equivalent full time (EFT) positions) were captured by the survey. Of these 268 EFT were in radiation oncology physics, 36 EFT were in radiology physics, 44 were in nuclear medicine physics, 101 EFT were in biomedical engineering and 29 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 identifies staffing shortfalls in the various disciplines and demonstrates the difficulties that will occur in trying to train sufficient physicists to raise staffing to an acceptable level.

Radiation exposure to caregivers from patients undergoing common radionuclide therapies: a review.

The contribution of radionuclide therapies (RNTs) to effective patient treatment is widely appreciated. The administration of high doses has necessitated investigating the potential radiation hazard to caregivers from patients undergoing RNTs. This work aimed to review the literature regarding measured effective doses to caregivers from such patients. The main selection criterion was the presence of real radiation exposure measurements. The results were categorised according to the treatment protocol and dose parameters. Analysis of the collected data demonstrated that the measured effective dose values were within the dose constraints defined by the International Commission on Radiological Protection, provided that the radiation protection instructions were followed by both patients and caregivers. In conclusion, the radiation risk for caregivers was almost negligible. In this context, treatments could be administered more often on an outpatient basis, once cost-effectiveness criteria were established and radiation protection training and procedures were appropriately applied.

Patient-specific dosimetry in peptide receptor radionuclide therapy: a clinical review.

Neuroendocrine tumours (NETs) belong to a relatively rare class of neoplasms. Nonetheless, their prevalence has increased significantly during the last decades. Peptide receptor radionuclide therapy (PRRT) is a relatively new treatment approach for inoperable or metastasised NETs. The therapeutic effect is based on the binding of radiolabelled somatostatin analogue peptides with NETs' somatostatin receptors, resulting in internal irradiation of tumours. Pre-therapeutic patient-specific dosimetry is essential to ensure that a treatment course has high levels of safety and efficacy. This paper reviews the methods applied for PRRT dosimetry, as well as the dosimetric results presented in the literature. Focus is given on data concerning the therapeutic somatostatin analogue radiopeptides (111)In-[DTPA(0),D-Phe(1)]-octreotide ((111)In-DTPA-octreotide), (90)Y-[DOTA(0),Tyr(3)]-octreotide ((90)Y-DOTATOC) and (177)Lu-[DOTA(0),Tyr(3),Thr(8)]-octreotide ((177)Lu-DOTATATE). Following the Medical Internal Radiation Dose (MIRD) Committee formalism, dosimetric analysis demonstrates large interpatient variability in tumour and organ uptake, with kidneys and bone marrow being the critical organs. The results are dependent on the image acquisition and processing protocol, as well as the dosimetric imaging radiopharmaceutical.

AFOMP Policy No 5: career progression for clinical medical physicists in AFOMP countries.

This policy statement, which is the fifth of a series of documents being prepared by the Asia-Oceania Federation of Organizations for Medical Physics Professional Development Committee, gives guidance on how clinical medical physicists' careers should progress from their initial training to career end. It is not intended to be prescriptive as in some AFOMP countries career structures are already essentially defined by employment awards and because such matters will vary considerably from country to country depending on local culture, employment practices and legislation. It is intended to be advisory and set out options for member countries and employers of clinical medical physicists to develop suitable career structures.

Brief histories of medical physics in Asia-Oceania.

The history of medical physics in Asia-Oceania goes back to the late nineteenth century when X-ray imaging was introduced, although medical physicists were not appointed until much later. Medical physics developed very quickly in some countries, but in others the socio-economic situation as such prevented it being established for many years. In others, the political situation and war has impeded its development. In many countries their medical physics history has not been well recorded and there is a danger that it will be lost to future generations. In this paper, brief histories of the development of medical physics in most countries in Asia-Oceania are presented by a large number of authors to serve as a record. The histories are necessarily brief; otherwise the paper would quickly turn into a book of hundreds of pages. The emphasis in each history as recorded here varies as the focus and culture of the countries as well as the length of their histories varies considerably.

Superposition on a multicomputer system.

Superposition (convolution using a noninvariant kernel) has been shown to be a highly promising technique for use in calculating dose distributions in radiotherapy treatment planning. However, one major difficulty that currently prevents use in routine planning is the computational effort required to perform the calculation in three dimensions. To help solve this problem the superposition technique has been implemented on a parallel processor multicomputer in order to examine the performance characteristics of such a system. Up to eight elements have been connected in a pipeline (linear array), and tree networks of three and seven processors have also been constructed (using INMOS T800 transputers). The significant results obtained with these networks are: (1) Both topologies provide near-linear speedup with increasing processor number (8 processors provide 7.81 times the computing power of a single processor when using an optimal communication packet size); (2) increasing communication packet size from 1 voxel to an optimum of approximately 40 voxels significantly reduces communication overhead per processor. Overhead per processor for a 7-element linear array is 6.9% when using 1-voxel packets, but only 1.8% when using 40-voxel packets; (3) the topology of the network has some effect on communication overhead: Arranging 7 processors in a 1-2-4 binary tree reduces overhead to 80.1% of that encountered using a 7-element linear array (with packet size of 1 voxel).

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.

A new method for optimum dose distribution determination taking tumour mobility into account.

A method for determining the optimum dose distribution in the planning target volume is proposed when target volumes are deliberately enlarged to account for tumour mobility in external beam radiotherapy. The optimum dose distribution is a dose distribution that will result in an acceptable level of tumour control probability (TCP) in most of the arising cases of tumour dislocation. An assumption is made that the possible shifts of the tumour are subject to a Gaussian distribution with mean zero and known variance. The idea of a reduced (mean in ensemble) tumour cell density is introduced. On this basis, the target volume and dose distribution in it are determined. The tumour control probability as a function of the shift of the tumour has been calculated. The Monte Carlo method has been used to simulate TCP distributions corresponding to tumour mobility characterized by different variances. The obtained TCP distributions are independent of the variance of the mobility because the dose distribution in the planning target volume is prescribed so that the mobility variance is taken into account. For simplicity a one-dimensional model is used but three-dimensional generalization can be done.

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.

Investigation of ultrasound axially traversing the human eye.

A ray tracing model for ultrasonic propagation through the human eye, including the lens, has been developed on the assumptions of lossless media and non-reflecting interfaces. Measurement of the distribution of an ultrasonic beam before and after traversing specimens of human eyes in vitro, and of the velocity of ultrasound in the various dissected media, has permitted some comparison of the predictions of the model with experiment. The agreement is good although there are significant limitations involved and these are discussed. For imaging systems the effect of the eye arises largely from the lens which acts as a defocussing lens of focal length approx. 13.5 cm. Although the experiments were performed at approx. 4 MHz, the validity of the ray tracing model is largely frequency independent and will be appropriate at the higher frequencies commonly used in ophthalmology.

Ultrasonic beam-plotting with very small spheres.

A method of pulse-echo ultrasonic beam plotting is described. It differs from traditional pulse-echo beam plotting in that the ultrasonic pulses are scattered off a totally isolated sphere rather than a sphere suspended on a wire. The method also allows extremely small spheres to be used thus providing greater resolution. It is demonstrated that pulse-echo beam plotting using spheres of different size produces different iso-echo amplitude curves.

Fourier coefficient description of left ventricular shape.

A method of quantifying the shape of the left ventricle of the heart as seen in 2D echocardiograms was developed. It is based on describing the shape in terms of the coefficients a fifth-order trigonometric Fourier series. Such a series has eleven Fourier coefficients which is too large a number for clinical application so pairs of coefficients are combined to give six coefficients (alpha 0, alpha 1, ... , alpha 5). A trial was conducted to test the ability of the coefficient description to classify subjects as having normal right ventricles or ventricles with an apical abnormality. The tests showed that one of the coefficients (alpha 2) was higher for the subjects with an apical abnormality and that this difference increased with exercise. This is as was expected. However, it was found to be difficult to get a reliable estimate of alpha 2 from a single scan of a patient and that it is therefore probably necessary to average data from several scans to obtain a reliable alpha 2 value for a single patient.