Table of contents

2006 Roberts Prize

The publishers of Physics in Medicine and Biology (PMB) in association with the Institute of Physics and Engineering in Medicine (IPEM) jointly award an annual prize for an article published in PMB during the previous year. The following ten articles, listed below in chronological order, were rated the best of 2006 based on the (two or three) referees' assessments:

• D W Mundy et al 2006 Radiation binary targeted therapy for HER-2 positive breast cancers: assumptions, theoretical assessment and future directionsPhys. Med. Biol.51 1377–91

• Y Yang et al 2006 Investigation of optical coherence tomography as an imaging modality in tissue engineeringPhys. Med. Biol.51 1649–59

• M Krämer and M Scholz 2006 Rapid calculation of biological effects in ion radiotherapyPhys. Med. Biol.51 1959–70

• P Crespo et al 2006 On the detector arrangement for in-beam PET for hadron therapy monitoringPhys. Med. Biol.51 2143–63

• R J Senden et al 2006 Polymer gel dosimeters with reduced toxicity: a preliminary investigation of the NMR and optical dose-response using different monomersPhys. Med. Biol.51 3301–14

• J Wang et al 2006 FDTD calculation of whole-body average SAR in adult and child models for frequencies from 30 MHz to 3 GHzPhys. Med. Biol.51 4119–27

• C A T Van den Berg et al 2006 The use of MR B⁺₁ imaging for validation of FDTD electromagnetic simulations of human anatomiesPhys. Med. Biol.51 4735–46

• S Qin and K W Ferrara 2006 Acoustic response of compliable microvessels containing ultrasound contrast agentsPhys. Med. Biol.51 5065–88

• R Kramer et al 2006 Skeletal dosimetry in the MAX06 and the FAX06 phantoms for external exposure to photons based on vertebral 3D-microCT imagesPhys. Med. Biol.51 6265–89

• R Leiderman et al 2006 Coupling between elastic strain and interstitial fluid flow: ramifications for poroelastic imagingPhys. Med. Biol.51 6291–313

An IPEM college of jurors then assessed and rated these papers in order to choose a winner. We have much pleasure in advising readers that the 2006 Roberts Prize is awarded to:

• M Krämer and M Scholz 2006 Rapid calculation of biological effects in ion radiotherapyPhys. Med. Biol.51 1959–70

2007 Prize for the Highest Cited Paper

The annual prize for the most highly cited paper is awarded by the journal publishers (IOP Publishing) to the article published in PMB that has received the most citations¹ in the previous 5 years (in this case for the period 2002 to 2006 inclusive). We have much pleasure in advising readers that the 2007 prize is awarded to:

• S S Vedam, P J Keall, V R Kini, H Mostafavi, H P Shukla and R Mohan 2003 Acquiring a four-dimensional computed tomography dataset using an external respiratory signalPhys. Med. Biol.48 45–62

For more information on the Citations Prize, see medicalphysicsweb.org

Simon Harris, Publisher

Steve Webb, Editor-in-Chief

1 Figures taken from Thomson/ISI

Superparamagnetic nanoparticles can be attached in great numbers to pathogenic cells using specific antibodies so that the magnetically-labeled cells themselves become superparamagnets. The cells can then be manipulated and drawn out of biological fluids, as in a biopsy, very selectively using a magnetic needle. We examine the origins and uncertainties in the forces exerted on magnetic nanoparticles by static magnetic fields, leading to a model for trajectories and collection times of dilute superparamagnetic cells in biological fluids. We discuss the design and application of such magnetic needles and the theory of collection times. We compare the mathematical model to measurements in a variety of media including blood.

For more information on this article, see medicalphysicsweb.org

Dose reduction efforts in diagnostic CT have brought the tradeoff of dose versus image quality to the forefront. The need for meaningful characterization of image noise beyond that offered by pixel standard deviation is becoming increasingly important. This work aims to study the implementation of the noise power spectrum (NPS) and noise equivalent quanta (NEQ) on modern, multislice diagnostic CT scanners. The details of NPS and NEQ measurement are outlined and special attention is paid to issues unique to multislice CT. Aliasing, filter design and effects of acquisition geometry are investigated. While it was found that both metrics can be implemented in modern CT, it was discovered that NEQ cannot be aptly applied with certain non-traditional reconstruction filters or in helical mode. NPS and NEQ under a variety of conditions are examined. Extensions of NPS and NEQ to uses in protocol standardization are also discussed.

Balancing dose and image quality requires signal-to-noise (SNR) metrics which incorporate both the variance and the spatial frequency characteristics of noise. In this study, the non-prewhitening matched filter SNR metric is calculated for 2 mm slices of a 1 cm diameter sphere under three different conditions: (1) constant pixel standard deviation, (2) constant dose and (3) constant reconstruction filter. For the constant pixel standard deviation condition, an increase of 260% in SNR was found with increasing filter sharpness. For constant dose, the SNR remained level for smooth to medium filters, then declined by up to 55% with increasing filter sharpness. For a constant reconstruction filter, the SNR increased with dose, but not as high as photon statistics would predict. However, when structured noise was removed from the noise power spectrum, the SNR did vary with quanta statistics. These results offer protocol design guidance for low-frequency-dominated objects.

Robust indices of regional and global cardiac function are a key factor in detection and treatment of heart disease as well as understanding of the fundamental mechanisms of a healthy heart. Myocardial elastography provides a noninvasive method for imaging and measuring displacement and strain of the myocardium for the early detection of cardiovascular disease. However, two-dimensional in-plane axial and lateral strains measured depend on the sonographic view used. This becomes especially critical in a clinical setting and may induce large variations in the measured strains, potentially leading to false diagnoses. A novel method in myocardial elastography is proposed for eliminating this view dependence by deriving the polar, principal and classified principal strains. The performance of the proposed methodology is assessed by employing 3D finite-element left-ventricular models of a control and an ischemic canine heart. Although polar strains are angle-independent, they are sensitive to the selected reference coordinate system, which requires the definition of a centroid of the left ventricle (LV). In contrast, principal strains derived through eigenvalue decomposition exhibit the inherent characteristic of coordinate system independence, offering view (i.e., angle and centroid)-independent strain measurements. Classified principal strains are obtained by assigning the principal components in the physical ventricular coordinate system. An extensive strain analysis illustrates the improvement in interpretation and visualization of the full-field myocardial deformation by using the classified principal strains, clearly depicting the ischemic and non-ischemic regions. Strain maps, independent of sonographic views and imaging planes, that can be used to accurately detect regional contractile dysfunction are demonstrated.

When treating mobile tumors using techniques such as beam gating or beam tracking, precise localization of tumor position is required, which is often realized by fluoroscopically tracking implanted fiducial markers. Multiple markers placed inside or near a tumor are often preferred to a single marker for the sake of accuracy. In this work, we propose a marker tracking system that can track multiple markers simultaneously, without confusing them, and that is also robust enough to continue tracking even when the markers are moving behind bony anatomy. The integrated radiotherapy imaging system (IRIS), developed at the Massachusetts General Hospital (MGH), was used to take fluoroscopy videos for marker tracking. The tracking system integrates marker detection with a multiple object tracking process, inspired by the multiple hypothesis marker tracking (MHT) process. It also utilizes breathing pattern information to help tracking. Four criteria are used to identify tracking failure, and when tracking failure occurs, the system can immediately inform the user. (In the clinical environment, the system would immediately disable the treatment beam.) In this paper, two liver patients with implanted fiducial markers were studied, and the studies were performed retrospectively to assess the effectiveness of the new tracking system. For both patients, LAT and AP fluoroscopic videos were studied. In order to better test the proposed tracking system, artificial markers were added around the real markers to disturb the tracking of the real markers. The performance of the proposed system was compared to that of a conventional tracking system (one that did not use multiple object tracking). The performance of the new system was also investigated with and without consideration of the breathing pattern information. We found that the conventional tracking system can easily miss tracking markers in the presence of artificial markers, and it cannot detect the tracking failures. On the other hand, our proposed system can track markers well and can also successfully detect tracking failures. Failure rate was calculated on a per-frame-per-marker basis for the proposed tracking system. When the system considered breathing pattern information, it had a 0% failure rate 75% of the time and 0.4% failure rate 25% of the time. However, when the system did not consider breathing patterns, it had a much higher failure rate, in the range of 1.2%–12%. Both examples of the proposed system yielded low e₉₅ (the maximum marker tracking error at 95% confidence level)—less than 1.5 mm.

For more information on this article, see medicalphysicsweb.org

In this paper, we propose an optimization concept for a rotation therapy technique which is referred to as arc-modulated cone beam therapy (AMCBT). The aim is a reduction of the treatment time while achieving a treatment plan quality equal to or better than that of IMRT. Therefore, the complete dose is delivered in one single gantry rotation and the beam is modulated by a multileaf collimator. The degrees of freedom are the field shapes and weights for a predefined number of beam directions. In the new optimization loop, the beam weights are determined by a gradient algorithm and the field shapes by a tabu search algorithm. We present treatment plans for AMCBT for two clinical cases. In comparison to step-and-shoot IMRT treatment plans, it was possible by AMCBT to achieve dose distributions with a better dose conformity to the target and a lower mean dose for the most relevant organ at risk. Furthermore, the number of applied monitor units was reduced for AMCBT in comparison to IMRT treatment plans.

Manipulation of interstitial fluid pressure (IFP) has a clinical potential when used in conjunction with near-infrared spectroscopy for the detection of breast cancer. In order to better interpret how the applied pressure alters the vascular space and interstitial water volumes in breast tissue, a study on tissue-mimicking, gelatin phantoms was carried out to mimic the translation of external force into internal pressures. A complete set of three-dimensional (3D) pressure maps were obtained for the interior volumes of phantoms as an external force of 10 mmHg was applied, using mixtures of elastic moduli 19 and 33 kPa to simulate adipose and fibroglandular values of breast tissue. Corresponding linear elastic finite element analysis (FEA) cases were formulated. Shear stress, nonlinear mechanical properties, gravity and tissue geometry were all observed to contribute to internal pressure distribution, with surface shear stresses increasing internal pressures near the surface to greater than twice the applied external pressure. Average pressures by depth were predicted by the linear elastic FEA models. FEA models were run for cases mimicking a 93 kPa tumor inclusion within regions of adipose, fibroglandular tissue, and a composite of the two tissue types to illustrate the localized high fluid pressures caused by a tumor when an external force is applied. The conclusion was that external contact forces can generate potentially clinically useful fluid pressure magnitudes in regions of sharp effective elastic modulus gradients, such as tumor boundaries.

While ART has been studied for years, the specific quantitative implementation details have not. In order for this new scheme of radiation therapy (RT) to reach its potential, an effective ART treatment planning strategy capable of taking into account the dose delivery history and the patient's on-treatment geometric model must be in place. This paper performs a theoretical study of dynamic closed-loop control algorithms for ART and compares their utility with data from phantom and clinical cases. We developed two classes of algorithms: those Adapting to Changing Geometry and those Adapting to Geometry and Delivered Dose. The former class takes into account organ deformations found just before treatment. The latter class optimizes the dose distribution accumulated over the entire course of treatment by adapting at each fraction, not only to the information just before treatment about organ deformations but also to the dose delivery history. We showcase two algorithms in the class of those Adapting to Geometry and Delivered Dose. A comparison of the approaches indicates that certain closed-loop ART algorithms may significantly improve the current practice. We anticipate that improvements in imaging, dose verification and reporting will further increase the importance of adaptive algorithms.

Near-infrared fluorescence optical imaging has the unique opportunity of differentiating diseased lesions from normal lesions based upon environmentally indicated changes in the lifetime of a fluorescent imaging agent. In this paper, we demonstrate three-dimensional lifetime tomography using the gradient-based penalty modified barrier function with simple bounds truncated Newton with trust region method to reconstruct lifetime maps in a clinically relevant, single breast-shaped (∼1081 cm³) phantom from point-frequency-domain photon migration measurements at 100 MHz. A reverse differentiation technique is used to calculate the gradients. This algorithm is desirable because the storage benefit from the use of the truncated Newton method and the reverse differentiation technique increase the speed. Two fluorescent contrast agents, indocyanine green and 3-3'-diethylthiatricarbocyanine iodide which differed in their fluorescence lifetimes by 0.62 ns, were used. Images of targets at a depth of 2.0 cm and target-to-background ratios (T:B) of 212:1 and 70:1 in fluoroscence absorption and 1:2.1 and 2.1:1 in lifetimes are successfully reconstructed. Our results show that image reconstruction is possible when there is (i) a longer lifetime in a target than the background and (ii) a shorter lifetime in a target than the background.

Analyser-based phase contrast imaging can provide radiographs of exceptional contrast at high resolution (<100 µm), whilst quantitative phase and attenuation information can be extracted using just two images when the approximations of geometrical optics are satisfied. Analytical phase retrieval can be performed by fitting the analyser rocking curve with a symmetric Pearson type VII function. The Pearson VII function provided at least a 10% better fit to experimentally measured rocking curves than linear or Gaussian functions. A test phantom, a hollow nylon cylinder, was imaged at 20 keV using a Si(1 1 1) analyser at the ELETTRA synchrotron radiation facility. Our phase retrieval method yielded a more accurate object reconstruction than methods based on a linear fit to the rocking curve. Where reconstructions failed to map expected values, calculations of the Takagi number permitted distinction between the violation of the geometrical optics conditions and the failure of curve fitting procedures. The need for synchronized object/detector translation stages was removed by using a large, divergent beam and imaging the object in segments. Our image acquisition and reconstruction procedure enables quantitative phase retrieval for systems with a divergent source and accounts for imperfections in the analyser.

A model predicting the reflection of ultrasound from multiple layers of small scattering spheres is developed. Predictions of the reflection coefficient, which takes into account the interferences between the different sphere layers, are compared to measurements performed in the 10–80 MHz and 15–35 MHz frequency range with layers of glass beads and spherical acute myeloid leukemia (AML) cells, respectively. For both types of scatterers, the reflection coefficient increases as a function of their density on the surface for less than three superimposed layers, at which point it saturates at 0.38 for glass beads and 0.02 for AML cells. Above three layers, oscillations of the reflection coefficient due to constructive or destructive interference between layers are observed experimentally and are accurately predicted by the model. The use of such a model could lead to a better understanding of the structures observed in layered tissue images.

We suggest that to best reproduce the interaction between a handheld wireless transceiver and the hand holding it, the dielectric properties of the hand phantom should be similar to those of the palm of the hand. The proposed target permittivity and conductivity values are presented and tabulated at a number of frequencies of interest. We have developed suitable phantom hand materials based on carbon-loaded silicones and demonstrated that it was possible to match the proposed dielectric properties in the frequency range 600–6000 MHz.

The suitability of radiochromic EBT film was studied for high-precision clinical quality assurance (QA) by identifying the dose response for a wide range of irradiation parameters typically modified in highly-conformal treatment techniques. In addition, uncertainties associated with varying irradiation conditions were determined. EBT can be used for dose assessment of absorbed dose levels as well as relative dosimetry when compared to absolute absorbed dose calibrated using ionization chamber results. For comparison, a silver halide film (Kodak EDR-2) representing the current standard in film dosimetry was included. As an initial step a measurement protocol yielding accurate and precise results was established for a flatbed transparency scanner (Epson Expression 1680 Pro) that was utilized as a film reading instrument. The light transmission measured by the scanner was found to depend on the position of the film on the scanner plate. For three film pieces irradiated with doses of 0 Gy, ∼1 Gy and ∼7 Gy, the pixel values measured in portrait or landscape mode differed by 4.7%, 6.2% and 10.0%, respectively. A study of 200 film pieces revealed an excellent sheet-to-sheet uniformity. On a long time scale, the optical development of irradiated EBT film consisted of a slow but steady increase of absorbance which was not observed to cease during 4 months. Sensitometric curves of EBT films obtained under reference conditions (SSD = 95 cm, FS = 5 × 5 cm², d = 5 cm) for 6, 10 and 25 MV photon beams did not show any energy dependence. The average separation between all curves was only 0.7%. The variation of the depth d (range 2–25 cm) in the phantom did not affect the dose response of EBT film. Also the influence of the radiation field size (range 3 × 3–40 × 40 cm²) on the sensitometric curve was not significant. For EDR-2 films maximum differences between the calibration curves reached 7–8% for X6MV and X25MV. Radiochromic EBT film, in combination with a flatbed scanner, presents a versatile system for high-precision dosimetry in two dimensions, provided that the intrinsic behaviour of the film reading device is taken into account. EBT film itself presents substantial improvements on formerly available models of radiographic and a radiochromic film and its dosimetric characteristics allow us to measure absorbed dose levels in a large variety of situations with a single calibration curve.

An effective fluence concept was employed to make forward dose calculations to investigate the effects of a distorted fluence map on dose plans. Fluence changes caused by organ motion were calculated using Chui's algorithm (2003 Med. Phys.30 1736). In two test cases with various fluence maps, the effects of motion were simulated using a maximal displacement from 5 mm to 25 mm; 108 fluence maps that were calculated from 16 IMRT plans for eight liver cancer patients were analyzed and compared with and without gating. Fluoroscopic measurements were made of a moving diaphragm in this study. Fluence changes associated with superior–inferior organ motion, perpendicular to the moving MLC, were also examined. The effects of motion on the fluence maps were evaluated from both the fluence differences between static and motion and the χ function. The maximum displacements of the organs in all of these cases were analyzed and correlated with the change in fluence generated from the liver IMRT plans. The dosimetric effects on the target coverage were evaluated for each plan. The results indicate that, for the same fluence map, the mean fluence intensity error or the percentage of the fluence points that have an unacceptable error is linearly related to the extent of motion. For different fluence maps, the degree to which the fluence is distorted by motion is strongly related to the product of the motion extent and the fluence gradient in the direction of diaphragm motion. For eight liver patients and 16 IMRT plans in this work (with gated technique, motion extent from 0.5 cm to 1.0 cm; without gated technique, motion extent from 0.9 cm to 1.8 cm), the fluence modulations are mild, such that the respiratory motion of each patient did not strongly affect the CTV coverage. The mean dose error is 1.5% for free motion (0.9–1.8 cm) and is around 1% for gated motion (0.5–1 cm).

In our previous publication (Mundy et al 2006 Phys. Med. Biol.51 1377) we have described the theoretical assessment of our novel approach in radiation binary targeted therapy for HER-2 positive breast cancers and summarized the future directions in this area of research. In this paper we advanced the numerical analysis to show the detailed radiation dose distribution for various neutron sources in combination with the required boron concentration and allowed radiation skin doses. We once again proved the feasibility of the concept and will use these data and conclusions to start with the experimental verifications.

The purpose of this work was to simulate with the Monte Carlo (MC) code PENELOPE the dose distribution in lung tumours including breathing motion in stereotactic body radiation therapy (SBRT). Two phantoms were modelled to simulate a pentagonal cross section with chestwall (unit density), lung (density 0.3 g cm⁻³) and two spherical tumours (unit density) of diameters respectively of 2 cm and 5 cm. The phase-space files (PSF) of four different SBRT field sizes of 6 MV from a Varian accelerator were calculated and used as beam sources to obtain both dose profiles and dose–volume histograms (DVHs) in different volumes of interest. Dose distributions were simulated for five beams impinging on the phantom. The simulations were conducted both for the static case and including the influence of respiratory motion. To reproduce the effect of breathing motion different simulations were performed keeping the beam fixed and displacing the phantom geometry in chosen positions in the cranial and caudal and left–right directions. The final result was obtained by combining the different position with two motion patterns. The MC results were compared with those obtained with three commercial treatment planning systems (TPSs), two based on the pencil beam (PB) algorithm, the TMS-HELAX (Nucletron, Sweden) and Eclipse (Varian Medical System, Palo Alto, CA), and one based on the collapsed cone algorithm (CC), Pinnacle³ (Philips). Some calculations were also carried out with the analytical anisotropic algorithm (AAA) in the Eclipse system. All calculations with the TPSs were performed without simulated breathing motion, according to clinical practice. In order to compare all the TPSs and MC an absolute dose calibration in Gy/MU was performed. The analysis shows that the dose (Gy/MU) in the central part of the gross tumour volume (GTV) is calculated for both tumour sizes with an accuracy of 2–3% with PB and CC algorithms, compared to MC. At the periphery of the GTV the TPSs overestimate the dose up to 10%, while in the lung tissue close to the GTV PB algorithms overestimate the dose and the CC underestimates it. When clinically relevant breathing motions are included in the MC simulations, the static calculations with the TPSs still give a relatively accurate estimate of the dose in the GTV. On the other hand, the dose at the periphery of the GTV is overestimated, compared to the static case.

The MRI-linear accelerator system, currently being developed, is designed such that the patient is irradiated in the presence of a magnetic field. This influences the dose distribution due to the Lorentz force working on the secondary electrons. Simulations have shown that the following dose effects occur: the build-up distance is reduced, the lateral profile becomes asymmetric in the direction orthogonal to the magnetic field and at tissue–air interfaces the dose increases due to returning electrons. In this work, GafChromic film measurements were performed in the presence of a magnetic field to experimentally quantify these dose effects. Depth–dose curves were measured in a PMMA–air–PMMA phantom and the lateral profiles were measured in a homogeneous PMMA phantom with the photon beam protruding over the edges of the phantom. The measurement results confirmed the magnetic field dose effects that were predicted by simulations. This enabled us to verify Geant4 Monte Carlo simulations of these MRI-linac specific dose effects: the relative agreement for the depth–dose curves between measurements and simulations was within 2.2%/1.8 mm. The relative agreement for the lateral profiles was 2.3%/1.7 mm. Overall, the magnetic field dose effects that are expected for irradiation with the MRI-linac can be modelled using Geant4 Monte Carlo simulations within measurement accuracy.

We are developing novel insert devices for existing whole body PET scanners to achieve better resolution in selected regions of interest such as the head, neck, breast or abdomen. The insert considered here is a full ring of high resolution detectors, which can be placed around the object of interest. Adding the insert inside the scanner leads to three different types of coincidences: insert–insert, insert–scanner and scanner–scanner. The insert–insert and scanner–scanner coincidences are similar to the coincidences obtained in a traditional PET system. The insert–scanner coincidences have an inherent fan-beam geometry for which a spatially variant system matrix is proposed. The system matrix is computed using the intersection of a fan beam with a pixel. A filtered back-projection (FBP) algorithm for the insert–scanner geometry is presented. This FBP algorithm yields images with significantly reduced artifacts compared to FBP reconstructions on insert–scanner data rebinned into parallel beams. This is demonstrated using simulated point source data acquired using SimSET. It is proposed to use a penalized ML-EM (PML-EM) algorithm using a log-cosh roughness penalty function to reconstruct a single activity distribution from all three data sets. This is demonstrated qualitatively using simulated point source data. A quantitative comparison of PML-EM and FBP was performed on data acquired from insert–scanner coincidences using a phantom with hot and cold tumors imaged in an experimental setup. The quantitative studies demonstrate that the resolution/noise tradeoff of PML-EM is improved relative to FBP.

Our study aimed to quantitatively evaluate blood flow in the left ventricle (LV) of apical hypertrophic cardiomyopathy (APH) by combining wall thickness obtained from cardiac magnetic resonance imaging (MRI) and myocardial perfusion from single-photon emission computed tomography (SPECT). In this study, we considered paired MRI and myocardial perfusion SPECT from ten patients with APH and ten normals. Myocardial walls were detected using a level set method, and blood flow per unit myocardial volume was calculated using 3D surface-based registration between the MRI and SPECT images. We defined relative blood flow based on the maximum in the whole myocardial region. Accuracies of wall detection and registration were around 2.50 mm and 2.95 mm, respectively. We finally created a bull's-eye map to evaluate wall thickness, blood flow (cardiac perfusion) and blood flow per unit myocardial volume. In patients with APH, their wall thicknesses were over 10 mm. Decreased blood flow per unit myocardial volume was detected in the cardiac apex by calculation using wall thickness from MRI and blood flow from SPECT. The relative unit blood flow of the APH group was 1/7 times that of the normals in the apex. This normalization by myocardial volume distinguishes cases of APH whose SPECT images resemble the distributions of normal cases.

Lambda tomography (LT) is a well-known local reconstruction technology to reduce the radiation dose or accommodate a limited imaging geometry. After a theoretical analysis of the so-called Calderon operator (CO), the necessary conditions for exact LT reconstruction are presented in terms of the 2D and 3D COs. Based on our previous results on LT, a general scheme is proposed to construct exact LT formulae in terms of the 2D CO with multiple segment trajectories. Every 2D formula has a corresponding 3D cone-beam formula in the Feldkamp framework in terms of the 2D CO which was illustrated in a triple-segment case. Our simulation results verify the correctness and demonstrate the merits of the proposed scheme.

It is well known that the use of a phase space in Monte Carlo simulation introduces a baseline level of variance that cannot be suppressed through the use of standard particle recycling techniques. This variance (termed latent phase-space variance by Sempau et al) can be a significant limiting factor in achieving accurate, low-uncertainty dose scoring results, especially near the surface of a phantom. A BEAMnrc component module (MCTWIST) has been developed to reduce the presence of latent variance in phase-space-based Monte Carlo simulations by implementing azimuthal particle redistribution (APR). For each recycled use of a phase-space particle a random rotation about the beam's central axis is applied, effectively utilizing cylindrical symmetry of the particle fluence and therefore providing a more accurate representation of the source. The MCTWIST module is unique in that no physical component is actually added to the accelerator geometry. Beam modifications are made by directly transforming particle characteristics outside of BEAMnrc/EGSnrc particle transport. Using MCTWIST, we have demonstrated a reduction in latent phase-space variance by more than a factor of 20, for a 10 × 10 cm² field, when compared to standard phase-space particle recycling techniques. The reduction in latent variance has enabled the achievement of dramatically smoother in-water dose profiles. This paper outlines the use of MCTWIST in Monte Carlo simulation and quantifies for the first time the latent variance reduction resulting from exploiting cylindrical phase-space symmetry.

The objective of this study was to investigate the potential of using polycrystalline lithium formate for EPR (electron paramagnetic resonance) dosimetry of clinical electron beams, with the main focus on the dose-to-water energy response. Lithium formate dosimeters were irradiated using ⁶⁰Co γ-rays and 6–20 MeV electrons in a PMMA phantom to doses in the range of 3–9 Gy. A plane-parallel ion chamber was used for water-based absolute dosimetry. In addition, the electron/photon transport was simulated using the EGSnrc Monte Carlo code. From the EPR measurements, the standard deviation of single dosimeter readings was 1.2%. The experimental energy response (the lithium formate dosimeter reading per absorbed dose to water for electrons relative to that for ⁶⁰Co γ rays) was nearly independent of the electron energy and on average 0.99 ± 0.03. The Monte Carlo calculated energy response was on average 0.5% higher than the experimental energy response, the difference being not significant. Simulations with water and polystyrene as irradiation media indicated that the energy response of lithium formate dosimeters was nearly independent of the phantom materials. In conclusion, lithium formate EPR dosimetry of clinical electron beams provides precise dose measurements with low dependence on the electron energy.

This short paper investigates highly constrained organ-motion deconvolution in the context of the delivery of radiotherapy. Whereas unconstrained and unphysical deconvolution of regular intrafraction motion can generate a dose distribution exactly matching the plan (made on a supposedly static object), the addition of fluence-positivity, fluence-smoothing and fluence-capping constraints to create constrained, physically valid, deconvolution reduces the goodness of fit. However, the generated modulations are physically acceptable and essentially are 'for free', no added delivery technology being needed and they improve on the 'do nothing about motion' strategy. This technique cannot ever rival gating, breath-hold or tracking and is not intended to.