Volume 32, Issue 3, March 2005
 EDITORIAL
 OBITUARY


John S. Laughlin (1918–2004)
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 RADIATION THERAPY PHYSICS


Boron neutron capture enhancement of fast neutron radiotherapy utilizing a moderated fast neutron beam
View Description Hide DescriptionAn investigation of the therapeutic potential of boronneutron capture (BNC) enhancement of fast neutron therapy utilizing the Harper University Hospital superconducting cyclotronproduced fast neutron therapy beam is presented. A technique for modification of the fast neutron beam to increase the BNC enhancement is presented along with an evaluation of the effects of beam moderation on the biological effectiveness of the absorbed dose. Characteristics of the photon,neutron, and boronneutron capture components of the absorbed dose are presented. Results demonstrate the possibility of therapeutic gains greater than 50% over conventional fast neutron therapy at depths required to treat brain lesions. This enhancement is estimated assuming currently achievable boron concentrations, and is more than adequate to provide a therapeutic window for the effective treatment of Glioblastoma Multiforme without prohibitive toxicity to the normal brain.

Calculation of photon energy deposition kernels and electron dose point kernels in water
View Description Hide DescriptionEffects of changes in the physics of EGSnrc compared to EGS4/PRESTA on energy deposition kernels for monoenergetic photons and on dose point kernels for beta sources in water are investigated. In the diagnostic energy range, Compton binding corrections were found to increase the primary energy fraction up to 4.5% at with a corresponding reduction of the scatter component of the kernels. Rayleigh scatteredphotons significantly increase the scatter component of the kernels and reduce the primary energy fraction with a maximum 12% reduction also at where the Rayleigh cross section in water has its maximum value. Sampling the photoelectron angular distribution produces a redistribution of the energy deposited by primaries around the interaction site causing differences of up to 2.7 times in the backscattered energy fraction at . Above the pair production threshold, the dose distribution versus angle of the primary dose component is significantly different from the EGS4 results. This is related to the more accurate angular sampling of the electronpositron pair direction in EGSnrc as opposed to using a fixed angle approximation in default EGS4. Total energy fractions for photon beams obtained with EGSnrc and EGS4 are almost the same within 0.2%. This fact suggests that the estimate of the total dose at a given point inside an infinite homogeneous water phantom irradiated by broad beams of photons will be very similar for kernels calculated with both codes. However, at interfaces or near boundaries results can be very different especially in the diagnostic energy range. EGSnrc calculated kernels for monoenergetic electrons (, , and ) and beta spectra ( and ) are in excellent agreement with reported EGS4 values except at where inclusion of spin effects in EGSnrc produces an increase of the effective range of electrons. Comparison at with an ETRAN calculation of the electron dose point kernel shows excellent agreement.

Dosimetric evaluation of a commercial 3D treatment planning system using the AAPM Task Group 23 test package
View Description Hide DescriptionThe accuracy of the dose calculation algorithm is one of the most critical steps in assessing the radiotherapy treatment to achieve the 5% accuracy in dose delivery, which represents the suggested limit to increase the complicationfree local control of tumor. We have used the AAPM Task Group 23 (TG23) test package for clinical photon external beam therapy to evaluate the accuracy of the new version of the PLATO TPS algorithm. The comparison between tabulated values and calculated ones has been performed for 266 and 297 dose values for the 4 and photon beams, respectively. Dose deviations less than 2% were found in the 98.5% and 90.6% analyzed dose points for the two considered energies, respectively. Larger deviations were obtained for both energies, in large dose gradients, such as the buildup region or near the field edges and blocks. As far as the radiological field width is concerned, 64 points were analyzed for both the energies: 53 points (83%) and 64 points (100%) were within for the 4 and photon beams, respectively. The results show the good accuracy of the algorithm either in simple geometry beam conditions or in complex ones, in homogeneous medium, and in the presence of inhomogeneities, for low and high energy beams. Our results fit well the data reported by several authors related to the calculation accuracy of different treatment planning systems (TPSs) (within a mean value of 0.7% and 1.2% for 4 and respectively). The TG23 test package can be considered a powerful instrument to evaluate dose calculation accuracy, and as such may play an important role in a quality assurance program related to the commissioning of a new TPS.

Dosimetric effect of collimating jaws for small multileaf collimated fields
View Description Hide DescriptionThe dosimetric effects from the jaw positioned close to the small field (, , and ) sideedge generated by a singlefocused multileaf collimator(MLC) were measured and studied. The measurement is important in intensity modulated radiotherapy(IMRT) because generally the jaw cannot perfectly cover all the leafends in a segment of irregular field. This leads to additional dose contributed by (1) the end surface of the jaw, (2) the leafend, and (3) the inter and intraleaf leakage/transmissions during the dosimetricmeasurement. Moreover, most of the conventional treatment planning systems ignore these effects in the dose calculation. In this study, measurements were made using a Varian 21 EX linear accelerator with 6 MV photon beam through a MLC containing 120 leaves. Percentage depth dose, beam profile, and output for small fields were measured by varying the jaw at different positions away from the leafends in the field sideedge. Moving the jaw away from the leafends increases the output and penumbra width for the small fields. Such increase is particularly significant when the field size is small and the degree of increase changes quickly when the jawend is at about 1–2 cm from the leafend. It is suggested that measurements should be carried out in the IMRT commissioning to provide information to physicists in reviewing the treatment planning system’s accuracy regarding leaf leakage/transmission and jaw effects.

Comparison of inhomogeneity correction algorithms in small photon fields
View Description Hide DescriptionAlgorithms such as convolution superposition, Batho, and equivalent pathlength which were originally developed and validated for conventional treatments under conditions of electronic equilibrium using relatively large fields greater than are routinely employed for inhomogeneity corrections. Modern day treatments using intensity modulated radiation therapy employ small beamlets characterized by the resolution of the multileaf collimator. These beamlets, in general, do not provide electronic equilibrium even in a homogeneous medium, and these effects are exaggerated in media with inhomogenieties. Monte Carlo simulations are becoming a tool of choice in understanding the dosimetry of small photon fields as they encounter low density media. In this study, depth dose data from the Monte Carlo simulations are compared to the results of the convolution superposition, Batho, and equivalent pathlength algorithms. The central axis dose within the lowdensity inhomogeneity as calculated by Monte Carlo simulation and convolution superposition decreases for small field sizes whereas it increases using the Batho and equivalent pathlength algorithms. The dose perturbation factor (DPF) is defined as the ratio of dose to a point within the inhomogeneity to the same point in a homogeneous phantom. The dose correction factor is defined as the ratio of dose calculated by an algorithm at a point to the Monte Carlo derived dose at the same point, respectively. DPF is noted to be significant for small fields and low density for all algorithms. Comparisons of the algorithms with Monte Carlo simulations is reflected in the DCF, which is close to 1.0 for the convolutionsuperposition algorithm. The Batho and equivalent pathlength algorithms differ significantly from Monte Carlo simulation for most field sizes and densities. Convolution superposition shows better agreement with Monte Carlo data versus the Batho or equivalent pathlength corrections. As the field size increases the DCF’s for all algorithms converge toward 1.0. The largest differences in DCF are at the interface where changes in electron transport are greatest. For a 6 MV photon beam, electronic equilibrium is restored at field sizes above 3 cm diameter and all of the algorithms predict dose in and beyond the inhomogeneous region equally well. For accurate dosimetry of small fields within and near inhomogeneities, however, simple algorithms such as Batho and equivalent pathlength should be avoided.

2Step IMAT and 2Step IMRT: A geometrical approach
View Description Hide DescriptionThe purpose of this paper is to develop a method that reduces the number of segments for intensity modulated arc therapy (IMAT) and intensity modulated radiotherapy(IMRT) for concave target volumes (TV). The aim was to utilize no more than two intensity levels per organ at risk (OAR) and to derive both optimal segment widths and weights from geometric considerations. Brahme’s model of an annular target surrounding a circular OAR was used as test model. Brahme’s solution was substituted by a single segment added to a simple field blocking the OAR. Width and weight of the segment were the free parameters to optimize. The objective function to minimize was the root mean square (rms) error of the dose in the target volume. One boundary condition was—neglecting scatter—“zerodose” to the OAR. The resulting rules for width and weight of the additive segment are referred to as “optimized 2Step IMAT” and “2Step IMRT.” The recommendations were applied to some simplified plans representing clinical cases using a commercial planning system. Optimized 2Step IMAT improved the rms by a factor of 4 with respect to techniques simply blocking the OAR. The additional segment reduced the rms below 3% for cases with gaps between OAR and TV larger than 8% of the TV diameter. The results for 2Step IMAT are applicable to IMRT and aperture modulated arc therapy (AMAT). 2Step IMAT can be utilized for noncylindrical cases and for more than one OAR. A geometrical and topological approach to IMAT and IMRT can be useful to understand fluence profiles. The results could be applied to ameliorate other topologybased procedures used in some planning systems. Basic mechanisms of 2Step IMAT can assist with the creation of rules for adaptive IMRT to compensate for patient motion.

Particle selection and beam collimation system for laseraccelerated proton beam therapy
View Description Hide DescriptionIn a laseraccelerated proton therapy system, the initial protons have broad energy and angular distributions, which are not suitable for direct therapeutic applications. A compact particle selection and collimation device is needed to deliver small pencil beams of protons with desired energy spectra. In this work, we characterize a superconducting magnet system that produces a desired magnetic field configuration to spread the protons with different energies and emitting angles for particle selection. Four magnets are set side by side along the beam axis; each is made of NbTi wires which carry a current density of at 4.2 K, and produces a magnetic field of in the corresponding region. Collimation is applied to both the entrance and the exit of the particle selection system to generate a desired proton pencil beam. In the middle of the magnet system, where the magnetic field is close to zero, a particle selection collimator allows only the protons with desired energies to pass through for therapy. Simulations of proton transport in the presence of the magnetic field show that the selected protons have successfully refocused on the beam axis after passing through the magnetic field with the optimal magnet system. The energy spread for any given characteristic proton energy has been obtained. It is shown that the energy spread is a function of the magnetic field strength and collimator size and reaches the full width at half maximum of 25 MeV for 230 MeV protons.Dose distributions have also been calculated with the GEANT3 Monte Carlo code to study the dosimetric properties of the laseraccelerated proton beams for radiation therapy applications.

Combined X–Y scanning magnet for conformal proton radiation therapy
View Description Hide DescriptionLightion beams have several features that make them very effective in radiation therapy applications. These include favorable depth dose distribution, finite penetration range, and high radiobiological efficiency. Moreover, magnetic scanning methods allow one to spread an ion beam to an exact image of a complex tumor shape. The ion scanning system usually consists of two magnets, each scanning horizontal and vertical directions independently. This paper discusses the design for a novel combined X–Y beam scanning magnet which is under development for the Midwest ProtonRadiotherapy Institute.

Developments in megavoltage cone beam CT with an amorphous silicon EPID: Reduction of exposure and synchronization with respiratory gating
View Description Hide DescriptionWe have studied the feasibility of a lowdose megavoltage cone beam computed tomography (MV CBCT) system for visualizing the gross tumor volume in respiratory gated radiation treatments of nonsmallcell lung cancer. The system consists of a commercially available linear accelerator(LINAC), an amorphous silicon electronic portal imaging device, and a respiratory gating system. The gantry movement and beam delivery are controlled using dynamic beam delivery toolbox, a commercial software package for executing scripts to control the LINAC. A specially designed interface box synchronizes the LINAC,image acquisition electronics, and the respiratory gating system. Images are preprocessed to remove artifacts due to detector sag and LINAC output fluctuations. We report on the output, flatness, and symmetry of the images acquired using different imaging parameters. We also examine the quality of threedimensional (3D) tomographic reconstruction with projection images of anthropomorphic thorax, contrast detail, and motion phantoms. The results show that, with the proper choice of imaging parameters, the flatness and symmetry are reasonably good with as low as three beam pulses per projection image. Resolution of 5% electron density differences is possible in a contrast detail phantom using 100 projections and 30 MU. Synchronization of image acquisition with simulated respiration also eliminated motion artifacts in a moving phantom, demonstrating the system’s capability for imaging patients undergoing gated radiation therapy. The acquisition time is limited by the patient’s respiration (only one image per breathing cycle) and is under 10 min for a scan of 100 projections. In conclusion, we have developed a MV CBCT system using commercially available components to produce 3D reconstructions, with sufficient contrast resolution for localizing a simulated lung tumor, using a dose comparable to portal imaging.
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 RADIATION IMAGING PHYSICS


A novel extension of the parallelbeam projectionslice theorem to divergent fanbeam and conebeam projections
View Description Hide DescriptionThe general goal of this paper is to extend the parallelbeam projectionslice theorem to divergent fanbeam and conebeam projections without rebinning the divergent fanbeam and conebeam projections into parallelbeam projections directly. The basic idea is to establish a novel link between the local Fourier transform of the projection data and the Fourier transform of the image object. Analogous to the two and threedimensional parallelbeam cases, the measured projection data are backprojected along the projection direction and then a local Fourier transform is taken for the backprojected data array. However, due to the loss of the shift invariance of the image object in a single view of the divergentbeam projections, the measured projection data is weighted by a distance dependent weight before the local Fourier transform is performed. The variable in the weighting function is the distance from the backprojected point to the xray source position. It is shown that a special choice of the weighting function, , will facilitate the calculations and a simple relation can be established between the Fourier transform of the image function and the local Fourier transform of the weighted backprojection data array. Unlike the parallelbeam cases, a onetoone correspondence does not exist for a local Fourier transform of the backprojected data array and a single line in the twodimensional (2D) case or a single slice in the 3D case of the Fourier transform of the image function. However, the Fourier space of the image object can be built up after the local Fourier transforms of the weighted backprojection data arrays are shifted and then summed in a laboratory frame. Thus the established relations Eq. (27) and Eq. (29) between the Fourier space of the image object and the Fourier transforms of the backprojected data arrays can be viewed as a generalized projectionslice theorem for divergent fanbeam and conebeam projections. Once the Fourier space of the image function is built up, an inverse Fourier transform could be performed to reconstruct tomographic images from the divergent beam projections. Due to the linearity of the Fourier transform, an image reconstruction step can be performed either when the complete Fourier space is available or in parallel with the building of the Fourier space. Numerical simulations are performed to verify the generalized projectionslice theorem by using a disc phantom in the fanbeam case.

Image reconstruction in peripheral and central regionsofinterest and data redundancy
View Description Hide DescriptionAlgorithms have been developed for image reconstruction within a regionofinterest (ROI) from fanbeam data less than that required for reconstructing the entire image. However, these algorithms do not admit truncated data. In this work, we investigate exact ROIimage reconstruction from fanbeam data containing truncations by use of the socalled fanbeam backprojectionfiltration (BPF) algorithm. We also generalize the fanbeam BPF algorithm to exploit redundant information inherent in the truncated fanbeam data. Because the parallelbeam scan can be interpreted as a special case of the fanbeam scan, based upon the fanbeam BPF algorithm, we derive a parallelbeam BPF algorithm for exactly reconstructing ROI images from truncated parallelbeam data. Furthermore, we investigate image reconstruction within two types of distinctive ROIs, which are referred to as the peripheral and central ROIs, respectively, from fanbeam data containing truncations and discuss their potential clinical applications. The results can readily be generalized to reconstructing 3D ROI images from data acquired in circular and helical conebeam scan. They can also be extended to address ROIimagereconstruction problems in parallel, fan, and conebeam scans with general trajectories. The work not only has significant implications for clinical and animalimaging applications of CT, but also may find applications in other imaging modalities.

Contrastdetail phantom scoring methodology
View Description Hide DescriptionPublished results of medical imaging studies which make use of contrast detail mammography (CDMAM) phantom images for analysis are difficult to compare since data are often not analyzed in the same way. In order to address this situation, the concept of ideal contrast detail curves is suggested. The ideal contrast detail curves are constructed based on the requirement of having the same product of the diameter and contrast (disk thickness) of the minimal correctly determined object for every row of the CDMAM phantom image. A correlation and comparison of five different quality parameters of the CDMAM phantom image determined for obtained ideal contrast detail curves is performed. The image quality parameters compared include: (1) contrast detail curve—a graph correlation between “minimal correct reading” diameter and disk thickness; (2) correct observation ratio—the ratio of the number of correctly identified objects to the actual total number of objects multiplied by 100; (3) image quality figure—the sum of the product of the diameter of the smallest scored object and its relative contrast; (4) figureofmerit—the zero disk diameter value obtained from extrapolation of the contrast detail curve to the origin (e.g., zero disk diameter); and (5) factor—the product of the thickness and the diameter of the smallest correctly identified disks. The analysis carried out showed the existence of a nonlinear relationship between the above parameters, which means that use of different parameters of CDMAM image quality potentially can cause different conclusions about changes in image quality. Construction of the ideal contrast detail curves for CDMAM phantom is an attempt to determine the quantitative limits of the CDMAM phantom as employed for image quality evaluation. These limits are determined by the relationship between certain parameters of a digital mammographysystem and the set of the gold disks sizes in the CDMAM phantom. Recommendations are made on selections of CDMAM phantom regions which should be used for scoring at different image quality and which scoring methodology may be most appropriate. Special attention is also paid to the use of the CDMAM phantom for image quality assessment of digital mammographysystems particularly in the vicinity of the Nyquist frequency.
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 RADIATION MEASUREMENT PHYSICS


The effect of ambient pressure on well chamber response: Experimental results with empirical correction factors
View Description Hide DescriptionFor some aircommunicating welltype chambers used for lowenergy brachytherapysource assay, deviations from expected values of measured air kerma strength were observed at low pressures associated with high altitudes. This effect is consistent with an overcompensation by the air density correction to standard atmospheric temperature and pressure. This work demonstrates that the correction does not fully compensate for the high altitude pressure effects that are seen with aircommunicating chambers at low photon energies in the range of . Deviations of up to 18% at a pressure corresponding to an approximate elevation of for photon energies of are possible. For highenergy photons and for highenergy beta emitters in aircommunicating chambers the factor is applicable. As expected, the ambient pressure does not significantly affect the response of pressurized well chambers (within 1%) to either low or highenergy photons. However, when used with beta emitters, pressurized chambers appear to exhibit a slight dependence on the ambient pressure. Using measured data, the response and correction factors were determined for three models of aircommunicating well chambers for lowenergy photonsources at various pressures corresponding to elevations above sea level. Monte Carlo calculations were also performed which were correlated with the experimental findings. A more complete study of the Monte Carlo calculations is presented in the accompanying paper, “The effect of ambient pressure on well chamber response: Monte Carlo calculated results for the HDR1000 Plus.”

Glass rod detectors for small field, stereotactic radiosurgery dosimetric audit
View Description Hide DescriptionThis paper demonstrates the feasibility of using glass rod detectors for quality assurance audit of radiosurgery units. Five radiosurgery units (3 Gamma Knife model C, 1 Gamma Knife model U and 1 Cyberknife) located in California participated in the study. At each center glass rod detectors were used to measure a number of dosimetric parameters including relative collimator output factor and absolute dose rate. The Gamma Knife data obtained is in excellent agreement with the commissioning data generated by the manufacturer for each unit and the Cyberknife data is in general agreement with the data published by other centers. In particular the output factor of the 4 mm Gamma Knife helmet factor, a subject of abundant debate, was measured in the range 0.863–0.872 with an accuracy of better than 1% across the four participating centers. It is hoped that this pilot study will facilitate a nationwide postal audit of stereotactic radiosurgery units.
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 THERMOTHERAPY PHYSICS


MRIguided interstitial ultrasound thermal therapy of the prostate: A feasibility study in the canine model
View Description Hide DescriptionThe feasibility of MRIguided interstitial ultrasound thermal therapy of the prostate was evaluated in an in vivo canine prostate model. MRI compatible, multielement interstitial ultrasound applicators were developed using diameter cylindrical piezoceramic transducers (7 to ) sectored to provide of angular directional heating. Two in vivo experiments were performed in canine prostate. The first using two interstitial ultrasound applicators, the second using three ultrasound applicators in conjunction with rectal and urethral cooling. In both experiments, the applicators were inserted transperineally into the prostate with the energy directed ventrally, away from the rectum. Electrical power levels of per element ( acoustic output power) were applied for heating periods of 18 and . Phasesensitive gradientecho MR imaging was used to monitor the thermal treatment in realtime on a interventional MRI system. Contrastenhanced T1weighted images and vitalstained serial tissue sections were obtained to assess thermal damage and correlate to realtime thermal contour plots and calculated thermal doses. Results from these studies indicated a large volume of ablated (nonstained) tissue within the prostate, extending 1.2 to from the applicators to the periphery of the gland, with the dorsal margin of coagulation welldefined by the applicator placement and directionality. The shape of the lesions correlated well to the hypointense regions visible in the contrastenhanced T1weighted images, and were also in good agreement with the contours of the threshold temperature and . This study demonstrates the feasibility of using directional interstitial ultrasound in conjunction with MRIthermal imaging to monitor and possibly control thermal coagulation within a targeted tissue volume while potentially protecting surrounding tissue, such as rectum, from thermal damage.
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 TISSUE MEASUREMENTS


Assessment of whole body and regional evaporative heat loss coefficients in very premature infants using a thermal mannequin: Influence of air velocity
View Description Hide DescriptionIn human adults, experimental assessment of the evaporative heat loss coefficient requires a fully wetted skinsurface area implying exposure to severe heat stress. For ethical reasons, this type of experimental situation is impossible to perform on neonates. The aim of the present study was to assess values in clinical situations for the body as a whole and for the different body segments, in particular, in natural and forced convection and using an anthropomorphic, sweating, thermal mannequin to represent a very small premature neonate (body mass 900 g). Skin hydration (i.e., simulated sweating) was performed by two electronic pumping systems, providing a steady adjustable flow of water to the mannequin surface. Experiments were carried out in a closedincubator heated to air temperatures of 33 °C and 36 °C, with air velocities ranging from 0.01 to , and with four levels of air relative humidity (40, 50, 60, and 80%). For the body as a whole, in natural convection, whereas in forced convection was 11.7, 12.4, and for air velocities of 0.2, 0.4, and , respectively. As far as local is concerned, our results showed that the relative values of regional water loss in forced convection differ greatly from those observed under still air conditions. Thus, increasing air velocity enhances the heterogeneity in regional skin cooling, which may contribute to the neonate’s thermal discomfort.
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 RADIATION PROTECTION PHYSICS


Helical tomotherapy radiation leakage and shielding considerations
View Description Hide DescriptionLeakage radiation and room shielding considerations increase significantly for intensitymodulated radiation therapy(IMRT)treatments due to the increased beamon time to deliver modulated fields. Tomotherapy, with its slice by slice approach to IMRT, further exacerbates this increase. Accordingly, additional shielding is used in tomotherapy machines to reduce unwanted radiation. The competing effects of the high modulation and the enhanced shielding were studied. The overall room leakage radiation levels are presented for the continuous gantry rotations, which are always used during treatments. The measured leakage at 4 m from the isocenter is less than relative to calibration output. Primary radiation exposure levels were investigated as well. The effect of forwarddirected leakage through the beamcollimation system was studied, as this is the leakage dose the patient would receive in the course of a treatment. A 12min treatment was calculated to produce only 1% patient leakage dose to the periphery region. Longer treatment times might yield less patient dose if the field width selected is correspondingly narrower. A method for estimating the worstcase leakage dose a patient would receive is presented.
