Volume 32, Issue 1, January 2005
- letters to the editor
- radiation therapy physics
- radiation imaging physics
- radiation measurement physics
- magnetic resonance physics
- nuclear medicine physics
- optical measurement physics
- infrared and microwave imaging
- thermotherapy physics
- thermoacoustic physics
- anatomy and physiology
- non-ionizing radiation physics
- radiation protection physics
- ph. d. theses abstracts
- radiation measurement
Index of content:
- LETTERS TO THE EDITOR
- RADIATION THERAPY PHYSICS
The use of film dosimetry of the penumbra region to improve the accuracy of intensity modulated radiotherapy32(2005); http://dx.doi.org/10.1118/1.1829246View Description Hide Description
Accurate measurements of the penumbra region are important for the proper modeling of the radiation beam for linear accelerator-based intensity modulated radiation therapy. The usual data collection technique with a standard ionization chamber artificially broadens the measured beam penumbrae due to volume effects. The larger the chamber, the greater is the spurious increase in penumbra width. This leads to inaccuracies in dose calculations of small fields, including small fields or beam segments used in IMRT. This source of error can be rectified by the use of film dosimetry for penumbra measurements because of its high spatial resolution. The accuracy of IMRT calculations with a pencil beam convolution model in a commercial treatment planning system was examined using commissioning data with and without the benefit of film dosimetry of the beam penumbrae. A set of dose-spread kernels of the pencil beam model was calculated based on commissioning data that included beam profiles gathered with a 0.6-cm-i.d. ionization chamber. A second set of dose-spread kernels was calculated using the same commissioning data with the exception of the penumbrae, which were measured with radiographic film. The average decrease in the measured width of the 80%–20% penumbrae of various square fields of size 3–40 cm, at 5 cm depth in water-equivalent plastic was 0.27 cm. Calculations using the pencil beam model after it was re-commissioned using film dosimetry of the penumbrae gave better agreement with measurements of IMRT fields, including superior reproduction of high dose gradient regions and dose extrema. These results show that accurately measuring the beam penumbrae improves the accuracy of the dose distributions predicted by the treatment planning system and thus is important when commissioning beam models used for IMRT.
32(2005); http://dx.doi.org/10.1118/1.1823571View Description Hide Description
The purpose of this work is to compare the efficacy of mathematical models in predicting the occurrence of radiotherapy-induced left ventricular perfusion defects assessed using single-photon emission computed tomography(SPECT). The basis of this study is data from 73 left-sided breast/chestwall patients treated with tangential photon fields. The mathematical models compared were three commonly used parametric models [Lyman normal tissue complication probability (LNTCP), relative serialty (RS), generalized equivalent uniform dose (gEUD)] and a nonparametric model (Linear discriminant analysis—LDA). Data used by the models were the left ventricular dose—volume histograms, or SPECT-based dose–function histograms, and the presence/absence of SPECT perfusion defects postradiation therapy (21 patients developed defects). For the parametric models, maximum likelihood estimation and F-tests were used to fit the model parameters. The nonparametric LDAmodel step-wise selected features (volumes/function above dose levels) using a method based on receiver operating characteristics (ROC) analysis to best separate the groups with and without defects. Optimistic (upper bound) and pessimistic (lower bound) estimates of each model’s predictive capability were generated using ROC curves. A higher area under the ROC curve indicates a more accurate model (a model that is always accurate has ). The areas under these curves for different models were used to statistically test for differences between them. Pessimistic estimates of areas under the ROC curve using dose–volume histogram/dose–function histogram inputs, in order of increasing prediction accuracy, were LNTCP , RS , gEUD , and LDA. Only the LDAmodel benefited from SPECT-based regional functional information. In general, the LDAmodel was statistically superior to the parametric models. The LDAmodel selected as features the left ventricular volumes above approximately , essentially volume in field, and , as best separating the groups with and without defects. In conclusion, the nonparametric LDAmodel appears to be a more accurate predictor of radiotherapy-induced left ventricular perfusion defects than commonly used parametric models.
32(2005); http://dx.doi.org/10.1118/1.1834835View Description Hide Description
In this study, complete dosimetric datasets for the CSM2 and CSM3 Cs-137 sources were obtained using the Monte Carlo GEANT4 code. The application of this calculation method was experimentally validated with thermoluminescent dosimetry(TLD). Functions and parameters following the TG43 formalism are presented: the dose rate constant, the radial dose functional, and the anisotropy function. In addition, to aid the quality control process on treatment planning systems, a two-dimensional (2D) rectangular dose rate table (the traditional along-away table), coherent with the TG43 dose calculation formalism, is given. The data given in this study complement existing information for both sources on the following aspects: (i) the source asymmetries were considered explicitly in the Monte Carlo calculations, (ii) TG43 data were derived directly from Monte Carlo calculations, (iii) the radial range of the different tables was increased as well as the angular resolution in the anisotropy function, including angles close to the longitudinal source axis. The CSM2 source TG-43 data of Liu et al. [Med. Phys.31, 477–483 (2004)] are not consistent with the Williamson 2D along-away data [Int. J. Radiat. Oncol., Biol., Phys.15, 227–237 (1988)] at distances closer than approximately from the source. The data presented here for this source are consistent with this 2D along-away table, and are suitable for use in clinical practice.
The role of nonelastic reactions in absorbed dose distributions from therapeutic proton beams in different medium32(2005); http://dx.doi.org/10.1118/1.1824194View Description Hide Description
Many new techniques for delivering radiation therapy are being developed for the treatment of cancer. One of these, proton therapy, is becoming increasingly popular because of the precise way in which protons deliver dose to the tumor volume. In order to achieve this level of precision, extensive treatment planning needs to be carried out to determine the optimum beam energies, energy spread (which determines the width of the spread-out Bragg peak), and angles for each patient’s treatment. Due to the level of precision required and advancements in computer technology, there is increasing interest in the use of Monte Carlo calculations for treatment planning in proton therapy. However, in order to achieve optimum simulation times, nonelastic nuclear interactions between protons and the target nucleus within the patient’s internal structure are often not accounted for or are simulated using less accurate models such as analytical or ray tracing. These interactions produce high LET particles such as neutrons, alpha particles, and recoil protons, which affect the dose distribution and biological effectiveness of the beam. This situation has prompted an investigation of the importance of nonelastic products on depth dose distributions within various materials including water, A-150 tissue equivalent plastic, ICRP (International Commission on Radiological Protection) muscle, ICRP bone, and ICRP adipose. This investigation was conducted utilizing the GEANT4.5.2Monte Carlo hadron transport toolkit.
32(2005); http://dx.doi.org/10.1118/1.1828251View Description Hide Description
We describe an approach for external beam radiotherapy of breast cancer that utilizes the three-dimensional (3D) surface information of the breast. The surface data of the breast are obtained from a 3D optical camera that is rigidly mounted on the ceiling of the treatment vault. This 3D camera utilizes light in the visible range therefore it introduces no ionization radiation to the patient. In addition to the surface topographical information of the treated area, the camera also captures gray-scale information that is overlaid on the 3D surfaceimage. This allows us to visualize the skin markers and automatically determine the isocenter position and the beam angles in the breast tangential fields. The field sizes and shapes of the tangential, supraclavicular, and internal mammary gland fields can all be determined according to the 3D surfaceimage of the target. A least-squares method is first introduced for the tangential-field setup that is useful for compensation of the target shape changes. The entire process of capturing the 3D surface data and subsequent calculation of beam parameters typically requires less than 1 min. Our tests on phantom experiments and patient images have achieved the accuracy of 1 mm in shift and 0.5° in rotation. Importantly, the target shape and position changes in each treatment session can both be corrected through this real-time image-guided system.
32(2005); http://dx.doi.org/10.1118/1.1829402View Description Hide Description
Conventional radiotherapytreatment planning systems rely on a static computed tomography(CT)image for planning and evaluation. Intra/inter-fraction patient motions may result in significant differences between the planned and the delivered dose. In this paper, we develop a method to incorporate the knowledge of intra/inter-fraction patient motion directly into the dose calculation. By decomposing the motion into a parallel (to beam direction) component and perpendicular (to beam direction) component, we show that the motion effects can be accounted for by simply modifying the fluence distribution (sinogram). After such modification, dose calculation is the same as those based on a static planning image. This method is superior to the “dose-convolution” method because it is not based on “shift invariant” assumption. Therefore, it deals with material heterogeneity and surface curvature very well. We test our method using extensive simulations, which include four phantoms, four motion patterns, and three plan beams. We compare our method with the “dose-convolution” and the “stochastic simulation” methods (gold standard). As for the homogeneous flat surface phantom, our method has similar accuracy as the “dose-convolution” method. As for all other phantoms, our method outperforms the “dose-convolution.” The maximum motion encoded dose calculation error using our method is within 4% of the gold standard. It is shown that a treatment planning system that is based on “motion-encoded dose calculation” can incorporate random and systematic motion errors in a very simple fashion. Under this approximation, in principle, a planning target volume definition is not required, since it already accounts for the intra/inter-fraction motion variations and it automatically optimizes the cumulative dose rather than the single fraction dose.
Reference photon dosimetry data and reference phase space data for the photon beam from Varian Clinac 2100 series linear accelerators32(2005); http://dx.doi.org/10.1118/1.1829172View Description Hide Description
The current study presents the reference photondosimetry data (RPDD) and reference phase space data (RPSD) for the photon beam from Varian 2100 series linear accelerators. The RPDD provide the basic photondosimetry data, typically collected during the initial commissioning of a new linear accelerator, including output factors, depth dose data, and beam profile data in air and in water. The RPSD provide the full phase space information, such as position, direction, and energy for each particle generated inside the head of any particular linear accelerator in question. The dosimetric characteristics of the photon beam from the majority of the aforementioned accelerators, which are unaltered from the manufacturer’s original specifications, can be fully described with these two data sets within a clinically acceptable uncertainty . The current study also presents a detailed procedure to establish the RPDD and RPSD using measured data and Monte Carlo calculations. The RPDD were constructed by compiling our own measured data and the average data based on the analysis of more than 50 sets of measured data from the Radiological Physics Center (RPC) and 10 sets of clinical dosimetry data obtained from 10 different institutions participating in the RPC’s quality assurance monitoring program. All the measured data from the RPC and the RPC-monitored institutions were found to be within a statistically tight range (i.e., or less) for each dosimetric quantity. The manufacturer’s standard data, except for in-air off-axis factors that are available only from the current study, were compared with the RPDD, showing that the manufacturer’s standard data could also be used as the RPDD for the photon beam studied in this study. The RPSD were obtained from Monte Carlo calculations using the BEAMnrc/DOSXYZnrc code system with (a spread of 3% full width at half maximum) and full width at half maximum as the values of the energy and radial spread of a Gaussian electron pencil beam incident on the target, respectively. The RPSD were capable of generating Monte Carlo data that agreed with the RPDD within the acceptance criteria adopted in the current study (e.g., 1% or for depth dose). A complete set of the RPDD and RPSD from the current study is available from the RPC website (http://rpc.mdanderson.org) or via mass storage media such as DVD or CD-ROM upon request.
32(2005); http://dx.doi.org/10.1118/1.1829171View Description Hide Description
We investigated the feasibility of detecting intensity modulated radiotherapy delivery errors automatically using a scalar evaluation of two-dimensional (2D) transverse dose measurement of the complete treatment delivery. Techniques using the gamma index and the normalized agreement test (NAT) index were used to parametrize the agreement between measured and computed dose distributions to seven different scalar metrics. Simulated verifications with delivery errors calculated using a commercially available treatment planning system for 9 prostate and 7 paranasal sinus cases were compared to 433 clinical verifications. The NAT index with 5% and criteria that included cold areas outside the planning target volume detected the largest percent of delivery errors. Assuming a false positive rate of 5%, it was able to detect 88% of beam energy changes, 94% of a different patient’s plan being delivered, 25% of plans with one beam’s collimator rotated by 90°, 81% of rotating one beam’s gantry angle by 10°, and 100% of omitting the delivery of one beam. However, no instances of changing one beam’s monitor unit setting by 10% or shifting the isocenter by were detected. Although the phantom shift could not be detected by the small change it made in the dose distribution, our autopositioning algorithm clearly identified the spatial anomaly. Using tighter criteria or combining dose and distance disagreements in an either/or fashion resulted in poorer delivery error detection. The mean value of the 2D gamma index distribution was less sensitive to delivery errors than the other scalar metrics studied. Although we found that scalar metrics do not have sufficient delivery error detection rates to be used as the sole clinical analysis technique, manually examining 2D dose comparison images would result in a near 100% detection rate while performing an ion chamber measurement alone would only detect 54% of these errors.
Sliding slice: A novel approach for high accuracy and automatic 3D localization of seeds from CT scans32(2005); http://dx.doi.org/10.1118/1.1833131View Description Hide Description
We present a conceptually novel principle for 3D reconstruction of prostate seed implants. Unlike existing methods for implant reconstruction, the proposed algorithm uses raw CT data (sinograms) instead of reconstructedCT slices. Using raw CT data solves several inevitable problems related to the reconstruction from CT slices. First, the sinograms are not affected by reconstruction artifacts in the presence of metallic objects and seeds in the patient body. Second, the scanning axis is not undersampled as in the case of CT slices; as a matter of fact the scanning axis is the most densely sampled and each seed is typically represented by several hundred samples. Moreover, the shape of a single seed in a sinogram can be modeled exactly, thus facilitating the detection. All this allows very accurate 3D reconstruction of both position and the orientation of the seeds. Preliminary results indicate that the seed position can be estimated with accuracy (average), while the orientation estimate accuracy is within on average. Although the main contribution of the paper is to present a new principle of reconstruction, a preliminary implementation is also presented as a proof of concept. The implemented algorithm has been tested on a phantom and the obtained results are presented to validate the proposed approach.
32(2005); http://dx.doi.org/10.1118/1.1836332View Description Hide Description
Respiratory motion causes movement of internal structures in the thorax and abdomen, making accurate delivery of radiation therapy to tumors in those areas a challenge. To reduce the uncertainties caused by this motion, we have developed feedback-guided breath hold (FGBH), a novel delivery technique in which radiation is delivered only during a voluntary breath hold that is sustained for as long as the patient feels comfortable. Here we present the technical aspects of FGBH, which involve (1) fabricating the hardware so the respiratory trace can be displayed to the patient, (2) assembling a delay box to be used as a breath-hold detector, and (3) performing quality control tests to ensure that FGBH can be delivered accurately and safely. A commercial respiratory tracking system that uses an external fiducial to monitor abdominal wall motion generates and displays the breathing trace and specific positions in the breathing cycle where a breath hold needs to occur. Hardware was developed to present this display to the patient in the treatment position. Patients view the presentation either on a liquid crystal display or through a pair of virtual reality goggles. Using the respiratory trace as a visual aid, the patient performs a breath hold so that the position representing the location of a fiducial is held within a specified gating window. A delay box was fabricated to differentiate between gating signals received during free breathing and those received during breath hold, allowing radiation delivery only when the fiducial was within the breath-hold gating window. A quality control analysis of the gating delay box and the integrated system was performed to ensure that all of the hardware and components were ready for clinical use.
Patient specific optimization of the relation between CT-Hounsfield units and proton stopping power with proton radiography32(2005); http://dx.doi.org/10.1118/1.1833041View Description Hide Description
The purpose of this work is to show the feasibility of using in vivoprotonradiography of a radiotherapy patient for the patient individual optimization of the calibration from CT-Hounsfield units to relative protonstopping power. Water equivalent tissue (WET) calibrated protonradiographs of a dog patient treated for a nasal tumor were used as baseline in comparison with integrated protonstopping power through the calibrated CT of the dog. In an optimization procedure starting with a stoichiometric calibration curve, the calibration was modified randomly. The result of this iteration is an optimized calibration curve which was used to recalculate the dose distribution of the patient. One result of this experiment was that the mean value of the deviations between WET calculations based on the stoichiometric calibration curve and the measurements was shifted systematically away from zero. The calibration produced by the optimization procedure reduced this shift to around . Another result was that the precision of the calibration, reflected as the standard deviation of the normally distributed deviations between WET calculation and measurement, could be reduced from 7.9 to with the optimized calibration. The dose distributions based on the two calibration curves showed major deviations at the distal end of the target volume.
Dose perturbation of a novel cobalt chromium coronary stent on intravascular brachytherapy: A Monte Carlo study32(2005); http://dx.doi.org/10.1118/1.1833592View Description Hide Description
Intravascular brachytherapy has been adopted for the indication of in-stent restenosis on the basis of results of clinical trials using mainly stainless steel stents. Recently, a new stent made of cobalt-chromium L-605 alloy (, ) (MULTI-LINK VISION™) was introduced as an alternative to the 316L stainless steel stent design (SS, ) (MULTI-LINK PENTA™). In this work, we used the Monte Carlo code MCNPX to compare the dose distribution for the GALILEO™ source in and SS stent models. The dose perturbation factor (DPF), defined as the ratio of the dose in water with the presence of a stent to the dose without a stent, was used to compare results. Both stent designs were virtually expanded to diameters of 2.0, 3.0, and using finite element models. The complicated strut shapes of both the and SS stents were simplified using circular rings with an effective width to yield a metal-to-tissue ratio identical to that of the actual stents. The mean DPF at a tissue depth, over the entire stented length of , was 0.935 for the stent and 0.911 for the SS stent. The mean DPF at the intima ( radial distance from the strut outer surface), over the entire stented length of , was 0.950 for , and 0.926 for SS. The maximum DPFs directly behind the and SS struts were 0.689 and 0.644, respectively. All DPF estimates have a standard deviation of , approximating the 95% confidence interval. Although the stent has a higher effective atomic number and greater density than the SS stent, the DPFs for the two stents are similar, probably because the metal-to-tissue ratio and strut thickness of the stent are lower than those of the SS stent.
32(2005); http://dx.doi.org/10.1118/1.1829401View Description Hide Description
Recent work has shown that there is significant uncertainty in measuring build-up doses in megavoltage photon beams especially at high energies. In this present investigation we used a phantom-embedded extrapolation chamber (PEEC) made of Solid Water™ to validate Monte Carlo (MC)-calculated doses in the dose build-up region for 6 and x-ray beams. The study showed that the percentage depth ionizations (PDIs) obtained from measurements are higher than the percentage depth doses (PDDs) obtained with Monte Carlo techniques. To validate the MC-calculated PDDs, the design of the PEEC was incorporated into the simulations. While the MC-calculated and measured PDIs in the dose build-up region agree with one another for the beam, a non-negligible difference is observed for the x-ray beam. A number of experiments and theoretical studies of various possible effects that could be the source of this discrepancy were performed. The contribution of contaminating neutrons and protons to the build-up dose region in the x-ray beam is negligible. Moreover, the MC calculations using the XCOM photon cross-section database and the NIST bremsstrahlung differential cross section do not explain the discrepancy between the MC calculations and measurement in the dose build-up region for the . A simple incorporation of triplet production events into the MCdose calculation increases the calculated doses in the build-up region but does not fully account for the discrepancy between measurement and calculations for the x-ray beam.
- RADIATION IMAGING PHYSICS
Filtered backprojection formula for exact image reconstruction from cone-beam data along a general scanning curve32(2005); http://dx.doi.org/10.1118/1.1828673View Description Hide Description
Recently, Katsevich proved a filtered backprojection formula for exact image reconstruction from cone-beam data along a helical scanning locus, which is an important breakthrough since 1991 when the spiral cone-beam scanning mode was proposed. In this paper, we prove a generalized Katsevich’s formula for exact image reconstruction from cone-beam data collected along a rather flexible curve. We will also give a general condition on filtering directions. Based on this condition, we suggest a natural choice of filtering directions, which is more convenient than Katsevich’s choice and can be applied to general scanning curves. In the derivation, we use analytical techniques instead of geometric arguments. As a result, we do not need the uniqueness of the PI lines. In fact, our formula can be used to reconstructimages on any chord as long as a scanning curve runs from one endpoint of the chord to the other endpoint. This can be considered as a generalization of Orlov’s classical theorem. Specifically, our formula can be applied to (i) nonstandard spirals of variable radii and pitches (with PI- or -PI-windows), and (ii) saddlelike curves.
32(2005); http://dx.doi.org/10.1118/1.1829247View Description Hide Description
The high-voltage condensers in a polarity-inversion two-stage Marx surge generator are charged from −50 to −70 kV by a power supply, and the electric charges in the condensers are discharged to an x-ray tube after closing gap switches in the surge generator with a trigger device. The x-ray tube is a demountable diode, and the turbo molecular pump evacuates air from the tube with a pressure of approximately 1 mPa. Clean molybdenum lines are produced using a 20 μm-thick zirconium filter, since the tube utilizes a disk cathode and a rod target, and bremsstrahlung rays are not emitted in the opposite direction to that of electron acceleration. At a charging voltage of −70 kV, the instantaneous tube voltage and current were 120 kV and 1.0 kA, respectively. The x-ray pulse widths were approximately 70 ns, and the generator produced instantaneous number of photons was approximately per pulse at 0.5 m from the source of 3.0 mm in diameter.
32(2005); http://dx.doi.org/10.1118/1.1827751View Description Hide Description
X-rayimage intensifier (XRII) geometric distortion reduces the accuracy of image-guided procedures and quantitative image reconstructions. Due to the dependence of this error on the earth’s magnetic field, the required correction is angle dependent, and calibration data should ideally be acquired simultaneously with clinical image data, at a specific orientation. We describe a technique to correct XRII geometric image distortion at any angular position during a stereotactic procedure. This approach uses a machined plastic grid, which contains channels that can be filled with iodinated contrast agent and subsequently flushed with water, providing contrast and mask images, respectively, of a geometric calibration grid. The standard image subtraction capabilities of conventional digital subtraction angiography devices can then be used to create a subtraction image of the iodine-filled channels, without any confounding anatomical structure. Grid-line intersection points are used to determine the control points that are required for a global polynomial correction algorithm, creating a correction map that is specific to the current angular position and XRII field of view (FOV). Tests with a clinical -arm based XRII show that control points can be obtained with a precision of ±0.053 mm, resulting in geometric correction accuracy of ±0.152 mm, at a nominal FOV of 40 cm. While the precision and accuracy are both poorer than that achieved with a high-contrast steel-bead grid, the fact that the liquid grid can remain rigidly attached to the XRII during an entire procedure results in the establishment of an absolute detector coordinate system (referenced to the liquid-filled correction grid). The design of the liquid-filled channels allows the required control points to be introduced into the image or removed in about 30 s, avoiding the appearance of obscuring or confounding markers during clinical image acquisition, with a concurrent increase in patient dose of about 8% in the current design. Applications for this technique include stereotactic surgery, radiosurgery, x-ray stereogrammetry, and other image-guided procedures.