Volume 37, Issue 8, August 2010
- radiation therapy physics
- radiation imaging physics
- radiation measurement physics
- magnetic resonance physics
- ultrasound physics
- infrared and microwave imaging
- tissue measurements
- radiation protection physics
- radiation biology
- books and publications
- task group report
Index of content:
Authorization to practice as a medical physicist is sometimes better achieved by registration rather than licensure37(2010); http://dx.doi.org/10.1118/1.3456440View Description Hide Description
- RADIATION THERAPY PHYSICS
Effects of breathing variation on gating window internal target volume in respiratory gated radiation therapya)37(2010); http://dx.doi.org/10.1118/1.3457329View Description Hide DescriptionPurpose:
To investigate the effects of breathing variation on gating window internal target volume in respiratory gated radiation therapy.Method and Materials:
Two-dimensional dynamic MRI (dMRI) of lung motion was acquired in ten volunteers and eight lungcancer patients. Resorted dMRI using 4DCT acquisition method (RedCAM) was generated for selected subjects by simulating the image rebinning process. A dynamic software generated phantom (dSGP) was created by moving a solid circle (to mimic the “tumor”) with dMRI-determined motion trajectories. The gating window internal target area (, 2D counterpart of ) was determined from both RedCAM and dSGP/dMRI. Its area , major axis , minor axis , and similarity were calculated and compared.Results:
In the phantom study of 3 cm tumor, measurements of the from dSGP (, , and ) are significantly greater than those from RedCAM (, , and ). Similarly, the differences are significantly greater for the 1 cm tumor (, , and in dSGP; , , and in RedCAM). In patient studies, measurements of the from dMRI (, , and ) are also significantly greater than those from RedCAM (, , and ). Similarities were , , and in the 3 cm tumor phantom, 1 cm tumor phantom, and patient studies, respectively.Conclusion:
can be underestimated by 4DCT due to breathing variations. An additional margin may be needed to account for this potential error in generating a . Cautions need to be taken when generating from 4DCT in respiratory gated radiation therapy, especially for small tumors with a large motion range .
Enhanced epidermal dose caused by localized electron contamination from lead cutouts used in kilovoltage radiotherapy37(2010); http://dx.doi.org/10.1118/1.3458722View Description Hide DescriptionPurpose:
To investigate and quantify electron contamination from the lead cutouts used in kilovoltage x-ray radiotherapy.Methods:
The lead cutouts were modeled with the Monte Carlo EGSnrc user codes DOSXYZnrc and DOSRZnrc for x-ray beams ranging from 50 to. The results from the model were confirmed with Gafchromic film measurements. The model and measurements investigated the dose distribution with and without gladwrap™ shielding under the lead, and dose distributions with round, square, and serrated edge cutouts.Results:
Large dose enhancement near the edges of the lead was observed due to electron contamination. At the epidermal/dermal border, there is double the dose at the edge of the lead compared to the central dose due to electron contamination for a beam and three times the dose for a beam. gladwrap™ shielding effectively removes the contaminantdose enhancement using ten and four layers for 300 and beams, respectively.Conclusions:
The contaminantdose enhancement is undesirable as it could cause unnecessary erythema and hyperpigmentation at the border of the treated and untreated skin and lead to a poorer cosmetic outcome. The contamination is easily removed by gladwrap™ shielding placed under or around the lead cutout.
37(2010); http://dx.doi.org/10.1118/1.3457333View Description Hide DescriptionPurpose:
Organ movement is still the biggest challenge in cancer treatment despite advances in online imaging. Due to the resulting geometric uncertainties, the delivered dose cannot be predicted precisely at treatment planning time. Consequently, all associated dose metrics (e.g., EUD and maxDose) are random variables with a patient-specific probability distribution. The method that the authors propose makes these distributions the basis of the optimization and evaluation process.Methods:
The authors start from a model of motion derived from patient-specific imaging. On a multitude of geometry instances sampled from this model, a dose metric is evaluated. The resulting pdf of this dose metric is termed outcome distribution. The approach optimizes the shape of the outcome distribution based on its mean and variance. This is in contrast to the conventional optimization of a nominal value (e.g., PTV EUD) computed on a single geometry instance. The mean and variance allow for an estimate of the expected treatment outcome along with the residual uncertainty. Besides being applicable to the target, the proposed method also seamlessly includes the organs at risk (OARs).Results:
The likelihood that a given value of a metric is reached in the treatment is predicted quantitatively. This information reveals potential hazards that may occur during the course of the treatment, thus helping the expert to find the right balance between the risk of insufficient normal tissue sparing and the risk of insufficient tumor control. By feeding this information to the optimizer, outcome distributions can be obtained where the probability of exceeding a given OAR maximum and that of falling short of a given target goal can be minimized simultaneously.Conclusions:
The method is applicable to any source of residual motion uncertainty in treatment delivery. Any model that quantifies organ movement and deformation in terms of probability distributions can be used as basis for the algorithm. Thus, it can generate dose distributions that are robust against interfraction and intrafraction motion alike, effectively removing the need for indiscriminate safety margins.
Effects of breast-air and breast-lung interfaces on the dose rate at the planning target volume of a MammoSite® catheter for Yb-169 and Ir-192 HDR sources37(2010); http://dx.doi.org/10.1118/1.3458720View Description Hide DescriptionPurpose:
To study the effects of the breast-air and breast-lung interfaces on the absorbed dose within the planning target volume (PTV) of a MammoSite® balloon dose delivery system as well as the effect of contrastmaterial on the dose rate in the PTV.Methods:
The Monte CarloMCNP5 code was used to simulate dose rate in the PTV of a 2 cm radius MammoSite® balloon dose delivery system. The simulations were carried out using an average female chest phantom (AFCP) and a semi-infinite water phantom for both Yb-169 and Ir-192 high dose rate sources for brachytherapy application. Gastrografin was introduced at varying concentrations to study the effect of contrastmaterial on the dose rate in the PTV.Results:
The effect of the density of the materials surrounding the MammoSite® balloon containing 0% contrastmaterial on the calculated dose rate at different radial distances in the PTV was demonstrated. Within the PTV, the ratio of the calculated dose rate for the AFCP and the semi-infinite water phantom for the point closest to the breast-air interface (90°) is less than that for the point closest to the breast-lung interface (270°) by 11.4% and 4% for the HDR sources of Yb-169 and Ir-192, respectively. When contrastmaterial was introduced into the 2 cm radius MammoSite® balloon at varying concentrations, (5%, 10%, 15%, and 20%), the dose rate in the AFCP at 3.0 cm radial distance at 90° was decreased by as much as 14.8% and 6.2% for Yb-169 and Ir-192, respectively, when compared to that of the semi-infinite water phantom with contrast concentrations of 5%, 10%, 15%, and 20%, respectively.Conclusions:
Commercially available software used to calculate dose rate in the PTV of a MammoSite® balloon needs to account for patient anatomy and density of surrounding materials in the dosimetryanalyses in order to avoid patient underdose.
Fundamental properties of the delivery of volumetric modulated arc therapy (VMAT) to static patient anatomy37(2010); http://dx.doi.org/10.1118/1.3453575View Description Hide DescriptionPurpose:
The primary goal of this article is to formulate volumetric modulated arc therapy (VMAT) delivery problem and study interdependence between several parameters (beam dose rate, gantry angular speed, and MLC leaf speed) in the delivery of VMAT treatment plan. The secondary aim is to provide deliverysolution and prove optimality (minimal beam on time) of the solution. An additional goal of this study is to investigate alternative delivery approaches to VMAT (like constant beam dose rate and constant gantry angular speed delivery).Method:
The problem of the VMAT delivery is formulated as a control problem with machine constraints. The relationships between parameters of arc therapy delivery are derived under the constraint of treatment plan invariance and limitations on delivery parameters. The nonuniqueness of arc therapy deliverysolutions is revealed from these relations. The most efficient delivery of arc therapy is then formulated as optimal control problem and solved by geometrical methods. A computer program is developed to find numerical solutions for deliveries of specific VMAT plan.Results:
Explicit examples of VMAT plan deliveries are computed and illustrated with graphical representations of the variability of delivery parameters. Comparison of delivery parameters with that of Varian’s delivery are shown and discussed. Alternative delivery strategies such as constant gantry angular speed delivery and constant beam dose rate delivery are formulated and solutions are provided. The treatment times for all the deliverysolutions are provided.Conclusion:
The investigations derive and prove time optimal VMAT deliveries. The relationships between delivery parameters are determined. The optimal alternative delivery strategies are discussed.
37(2010); http://dx.doi.org/10.1118/1.3453577View Description Hide Description
Purpose: To investigate a protocol which efficiently localizes TomoTherapy patients with a scout imaging (topogram) mode that can be used with or instead of 3D megavoltage computed tomography (MVCT) imaging.
Methods: The process presented here is twofold: (a) The acquisition of the topogram using the TomoTherapy MV imaging system and (b) the generation of a digitally reconstructed topogram (DRT) derived from a standard kV CT simulation data set. The unique geometric characteristics of the current TomoTherapy imaging system were explored both theoretically and by acquiring topograms of anthropomorphic phantoms and comparing these images to DRT images. The performance of the MV topogram imaging system in terms of image quality, dose incurred to the patient, and acquisition time was investigated using ionization chamber and radiographic film measurements.
Results: The time required to acquire a clinically usable topogram, limited by the maximum couch speed of , was 12.5 s for a 50 cm long field. The patient dose was less than 1% of that delivered by a helical MVCT scan. Further refinements within the current TomoTherapy system, most notably decreasing the imaging beam repetition rate during MV topogram acquisition, would further reduce the topogram dose to less than per scan without compromising image quality.
Conclusions: Topogram localization on TomoTherapy is a fast and low-dose alternative to 3D MVCT localization. A protocol designed that exclusively utilized MV topograms would result in a 30-fold reduction in imaging time and a 100-fold reduction in dose from localization scans using the current TomoTherapy workflow.
37(2010); http://dx.doi.org/10.1118/1.3464799View Description Hide DescriptionPurpose:
In the segmentation of sequential treatment-timeCT prostate images acquired in image-guidedradiotherapy, accurately capturing the intrapatient variation of the patient under therapy is more important than capturing interpatient variation. However, using the traditional deformable-model-based segmentation methods, it is difficult to capture intrapatient variation when the number of samples from the same patient is limited. This article presents a new deformable model, designed specifically for segmenting sequential CTimages of the prostate, which leverages both population and patient-specific statistics to accurately capture the intrapatient variation of the patient under therapy.Methods:
The novelty of the proposed method is twofold:First, a weighted combination of gradient and probability distribution function (PDF) features is used to build the appearance model to guide model deformation. The strengths of each feature type are emphasized by dynamically adjusting the weight between the profile-based gradient features and the local-region-based PDF features during the optimization process. An additional novel aspect of the gradient-based features is that, to alleviate the effect of feature inconsistency in the regions of gas and bone adjacent to the prostate, the optimal profile length at each landmark is calculated by statistically investigating the intensity profile in the training set. The resulting gradient-PDF combined feature produces more accurate and robust segmentations than general gradient features. Second, an online learning mechanism is used to build shape and appearance statistics for accurately capturing intrapatient variation.Results:
The performance of the proposed method was evaluated on 306 images of the 24 patients. Compared to traditional gradient features, the proposed gradient-PDF combination features brought 5.2% increment in the success ratio of segmentation (from 94.1% to 99.3%). To evaluate the effectiveness of online learning mechanism, the authors carried out a comparison between partial online update strategy and full online update strategy. Using the full online update strategy, the mean DSC was improved from 86.6% to 89.3% with 2.8% gain. On the basis of full online update strategy, the manual modification before online update strategy was introduced and tested, the best performance was obtained; here, the mean DSC and the mean ASD achieved 92.4% and 1.47 mm, respectively.Conclusions:
The proposed prostate segmentation method provided accurate and robust segmentation results for CTimages even under the situation where the samples of patient under radiotherapy were limited. A conclusion that the proposed method is suitable for clinical application can be drawn.
Collimator angle influence on dose distribution optimization for vertebral metastases using volumetric modulated arc therapya)37(2010); http://dx.doi.org/10.1118/1.3462560View Description Hide DescriptionPurpose:
The cylindrical symmetry of vertebrae favors the use of volumetric modulated arc therapy in generating a dose “hole” on the center of the vertebrae limiting the dose to the spinal cord. The authors have evaluated if collimator angle is a significant parameter for dose distribution optimization in vertebral metastases.Methods:
Three patients with one-three vertebrae involved were considered. Twenty-one differently optimized plans (nine single-arc and 12 double-arc plans) were performed, testing various collimator angle positions. Clinical target volume was defined as the whole vertebrae, excluding the spinal cord canal. The planning target volume (PTV) was defined as. Dose prescription was with normalization to PTV mean dose. The dose at of spinal cord was limited to 11.5Gy.Results:
The best plans in terms of target coverage and spinal cord sparing were achieved by two arcs and Arc1–80° and Arc2–280° collimator angles for all the cases considered (i.e., leaf travel parallel to the spinal cord primary orientation). If one arc is used, only 80° reached the objectives.Conclusions:
This study demonstrated the role of collimation rotation for the vertebrae metastasis irradiation, with the leaf travel parallel to the spinal cord primary orientation to be better than other solutions. Thus, optimal choice of collimator angle increases the optimization freedom to shape a desired dose distribution.
A method for optimizing LINAC treatment geometry for volumetric modulated arc therapy of multiple brain metastases37(2010); http://dx.doi.org/10.1118/1.3455286View Description Hide DescriptionPurpose:
Volumetric modulated arc therapy (VMAT) is a rotational delivery technique in which MLC shapes, dose rate, and gantry rotation speed are optimized to produce conformal dose distributions. The aim of this work is to develop a beam projection method for deriving the optimal table and collimator angles for multilesion treatment planning.Methods:
The method consists of four steps. The first step is to define the vector of beam-eye-view (BEV)--axis in the treatment planning CT coordinates. The second step is to project each target onto the BEV--axis vector. In the third step, the best table and collimator angle are found with a brute-force optimization technique that minimizes MLC leaf sharing between lesions. The fourth step is to generate an optimized VMAT plan with appropriate table/collimator angles and evaluate the plan quality.Results:
The authors tested the method on three example cases with targets of various locations in the brain and sizes ranging from 1.18 to. Applying the optimized geometric parameter to generate VMAT plan, a reduction of the 12 Gy volume was more than 6.1% for all cases; the plan homogeneity was improved from to vs a VMAT plan with the manufacturer recommended table and collimator angles.Conclusions:
The authors conclude that the use of the projection method minimizes the sharing of MLC leaves between lesions and improves the plan quality for multilesion VMAT delivery.
37(2010); http://dx.doi.org/10.1118/1.3460341View Description Hide DescriptionPurpose:
Digital tomosynthesis (DTS) recently gained extensive research interests in both diagnostic and radiation therapy fields. Conventional DTS images are generated by scanning an x-ray source and flat-panel detector pair on opposite sides of an object, with the scanning trajectory on a one-dimensional curve. A novel tomosynthesis method named solid-angle tomosynthesis (SAT) is proposed, where the x-ray source scans on an arbitrary shaped two-dimensional surface.Methods:
An iterative algorithm in the form of total variation regulated expectation maximization is developed for SAT image reconstruction. The feasibility and effectiveness of SAT is corroborated by computer simulation studies using three-dimensional (3D) numerical phantoms including a 3D Shepp–Logan phantom and a volumetric CT image set of a human breast.Results:
SAT is able to cover more space in Fourier domain more uniformly than conventional DTS. Greater coverage and more isotropy in the frequency domain translate to fewer artifacts and more accurately restored features in the in-plane reconstruction.Conclusions:
Comparing with conventional DTS, SAT allows cone-shaped x-ray beams to project from more solid angles, thus provides more coverage in the spatial-frequency domain, resulting in better quality of reconstructed image.
37(2010); http://dx.doi.org/10.1118/1.3467753View Description Hide DescriptionPurpose:
Entrance dose (or skindose) is an important part of patient quality assurance in external beam radiation therapy. However, entrance dose verification in proton beam is not routinely performed. In this study, the OneDose single use MOSFETdetector system forin vivodosimetry measurement in proton therapy is investigated.Methods:
Using a solid water phantom, several fundamental dosimetric characteristics of the OneDose system are studied with a proton beam: The reproducibility (consistency) of the dosimeter, the linearity with dose and dose rate, energy dependence, directional dependence, LET dependence, and fading (delay readout with time) is studied.Results:
OneDose detectors show dose and dose rate linearity but exhibit pronounced energy dependence at depth and a large variation in dose response with LET. On the other hand, the detector response remain relatively constant (within 3%) at surface over a wide range of energies. There is also a slight angular dependence (about 2%) up to 60° angle of incidence. However, detector orientation such that incidence along the long axis of the detector should be avoided as the proton beam will have to traverse a large amount of the copper backing. Since mostin vivodosimetry involves entrance dose measurement, the OneDose at surface appears to be well suited for such application. OneDose exhibits small intrabatch variation ( at one SD) indicating that it is only necessary to calibration a few detectors from each batch. The interbatch variation is generally within 3%.Conclusions:
The small detector size and its relatively flexible design of OneDose allow dose measurement to be performed on a curved surface or in small cavities that is otherwise difficult with the conventional diode detectors. The slight drawback in its angular dependence can be easily handled by angular dependence table. However, since OneDose is a single use detector, the intrabatch consistency must be verified before the remaining detectors from the same batch could be used forin vivodosimetry. It is advisable that the detectors from the same batch be taken for the same application to reduce the dosimetric uncertainty. For detectors from different batches, interbatch consistency should also be verified to obtain reliable results. OneDose provides an opportunity to measure in vivodose with proton beam within acceptable clinical criterion of ±(5.0%–6.5%).
37(2010); http://dx.doi.org/10.1118/1.3462558View Description Hide DescriptionPurpose:
To compare and evaluate the dosimetric water equivalence of several commonly used solid phantoms for low energyphotonbeams.Methods:
A total of ten different solid phantom materials was used in the study. ThePENELOPEMonte Carlo code was used to calculate depth doses and beam profiles in all the phantom materials as well as the dose to a small water voxel at the surface of the solid phantom. These doses were compared to the corresponding doses calculated in a water phantom. The primary photonbeams used ranged in energy from 50 to 280 kVp.Results:
A number of phantom materials had excellent agreement in dose compared to water for all the x-ray beamenergies studied. RMI457 Solid Water, Virtual Water, PAGAT, A150, and Plastic Water DT all had depth doses that agreed with those in water to within 2%. For these same phantom materials, the dose changes in the water voxel at the surface of the solid phantom were within 2%, except for A150, which agreed to within 2.7%. By comparison, the largest differences in depth doses occurred for Plastic Water (−21.7%) and polystyrene (17.6%) for the 50 kVp energyphotonbeam and 8 cm diameter field size. Plastic Water gave the largest difference in the normalized beam profiles with differences of up to 3.5% as compared to water. Surface dose changes, due to the presence of the solid phantom acting as the backscattermaterial, were found to be up to 9.1% for polystyrene with significant differences also found for Plastic Water, PMMA, and RW3 phantoms.Conclusions:
The following solid phantoms can be considered water equivalent and are recommended for relative dosimetry of low energyphotonbeams: A150, PAGAT, Plastic Water DT, RMI457 Solid Water, and Virtual Water. However, the following solid phantoms give significant differences, compared to water, in depth doses, profiles, and/or in surface doses due to backscatter changes: Plastic Water, PMMA, polystyrene, PRESAGE, and RW3.
Predictive modeling of lung motion over the entire respiratory cycle using measured pressure-volume data, 4DCT images, and finite-element analysis37(2010); http://dx.doi.org/10.1118/1.3455276View Description Hide DescriptionPurpose:
Predicting complex patterns of respiration can benefit the management of the respiratory motion for radiation therapy of lungcancer. The purpose of the present work was to develop a patient-specific, physiologically relevant respiratory motion model which is capable of predicting lungtumor motion over a complete normal breathing cycle.Methods:
Currently employed techniques for generating the lung geometry from four-dimensional computed tomography data tend to lose details of mesh topology due to excessive surface smoothening. Some of the existing models apply displacement boundary conditions instead of the intrapleural pressure as the actual motive force for respiration, while others ignore the nonlinearity of lungtissues or the mechanics of pleural sliding. An intermediate nonuniform rational basis spline surface representation is used to avoid multiple geometric smoothing procedures used in the computational mesh preparation. Measured chest pressure-volume relationships are used to simulate pressure loading on the surface of the model for a given lung volume, as in actual breathing. A hyperelastic model, developed from experimental observations, has been used to model the lungtissue material. Pleural sliding on the inside of the ribcage has also been considered.Results:
The finite-element model has been validated using landmarks from four patient CT data sets over 34 breathing phases. The average differences of end-inspiration in position between the landmarks and those predicted by the model are observed to be for Patient P1, for Patient P2, for Patient P3, and for Patient P4 in the magnitude of error vector, respectively. The average errors of prediction at landmarks over multiple breathing phases in superior-inferior direction are less than 3 mm.Conclusions:
The prediction capability of pressure-volume curve driven nonlinear finite-element model is consistent over the entire breathing cycle. The biomechanical parameters in the model are physiologically measurable, so that the results can be extended to other patients and additional neighboring organs affected by respiratory motion.
37(2010); http://dx.doi.org/10.1118/1.3460795View Description Hide DescriptionPurpose:
In 2008, a national intensity modulated radiation therapy(IMRT)dosimetry intercomparison was carried out for all 23 radiation oncology institutions in Switzerland. It was the aim to check the treatment chain focused on the planning, dose calculation, and irradiation process.Methods:
A thorax phantom with inhomogeneities was used, in which thermoluminescence dosimeter(TLD) and ionization chamber measurements were performed. Additionally, absolute dosimetry of the applied beams has been checked. Altogether, 30 plan-measurement combinations have been used in the comparison study. The results have been grouped according to dose calculation algorithms, classified as “type a” or “type b,” as proposed byKnöös et al. [“Comparison of dose calculation algorithms for treatment planning in external photon beam therapy for clinical situations,” Phys. Med. Biol.51, 5785–5807 (2006)].Results:
Absolute dosimetry check under standard conditions: The mean ratio between the dose derived from the single field measurement and the stated dose, calculated with the treatment planning system, was for the ionization chamber and ( deviation) for the TLD measurements. IMRT Plan Check: In the lung tissue of the planning target volume, a significantly better agreement between measurements (TLD,ionization chamber) and calculations is shown for type b algorithms than for type a . In regions outside the lungs, the absolute differences between TLD measured and stated dose values, relative to the prescribed dose,, are and , respectively. These data show the same degree of accuracy between the two algorithm types if low-density medium is not present.Conclusions:
The results demonstrate that the performed intercomparison is feasible and confirm the calculation accuracies of type a and type b algorithms in a water equivalent and low-density environment. It is now planned to offer the intercomparison on a regular basis to all Swiss institutions using IMRT techniques.
Measurement and verification of positron emitter nuclei generated at each treatment site by target nuclear fragment reactions in proton therapy37(2010); http://dx.doi.org/10.1118/1.3462559View Description Hide DescriptionPurpose:
The purpose of this study is to verify the characteristics of the positron emitter nuclei generated at each treatment site by proton irradiation.Methods:
Proton therapy using a beam on-line PET system mounted on a rotating gantry port (BOLPs-RGp), which the authors developed, is provided at the National Cancer Center Kashiwa, Japan. BOLPs-RGp is a monitoring system that can confirm the activity distribution of the proton irradiated volume by detection of a pair of annihilation gamma rays coincidentally from positron emitter nuclei generated by the target nuclear fragment reactions between irradiated proton nuclei and nuclei in the human body. Activity is measured from a start of proton irradiation to a period of 200 s after the end of the irradiation. The characteristics of the positron emitter nuclei generated in a patient’s body were verified by the measurement of the activity distribution at each treatment site using BOLPs-RGp.Results:
The decay curves for measured activity were able to be approximated using two or three half-life values regardless of the treatment site. The activity of half-life value of about 2 min was important for a confirmation of the proton irradiated volume.Conclusions:
In each proton treatment site, verification of the characteristics of the generated positron emitter nuclei was performed by using BOLPs-RGp. For the monitoring of the proton irradiated volume, the detection of generated in a human body was important.
Dosimetric verification of the anisotropic analytical algorithm in lung equivalent heterogeneities with and without bone equivalent heterogeneities37(2010); http://dx.doi.org/10.1118/1.3464748View Description Hide Description
Purpose: In this study, the authors evaluated the accuracy of dose calculations performed by the convolution/superposition based anisotropic analytical algorithm (AAA) in lung equivalent heterogeneities with and without bone equivalent heterogeneities.
Methods: Calculations of PDDs using the AAA and Monte Carlo simulations (MCNP4C) were compared to ionization chamber measurements with a heterogeneous phantom consisting of lung equivalent and bone equivalent materials. Both 6 and 10 MV photon beams of and field sizes were used for the simulations. Furthermore, changes of energy spectrum with depth for the heterogeneous phantom using MCNP were calculated.
Results: The ionization chamber measurements and MCNP calculations in a lung equivalent phantom were in good agreement, having an average deviation of only . For both 6 and 10 MV beams, the average deviation was less than 2% for the and fields in the water-lung equivalent phantom and the field in the water-lung-bone equivalent phantom. Maximum deviations for the field in the lung equivalent phantom before and after the bone slab were 5.0% and 4.1%, respectively. The Monte Carlo simulation demonstrated an increase of the low-energy photon component in these regions, more for the field compared to the field.
Conclusions: The low-energy photon by Monte Carlo simulation component increases sharply in larger fields when there is a significant presence of bone equivalent heterogeneities. This leads to great changes in the build-up and build-down at the interfaces of different density materials. The AAA calculation modeling of the effect is not deemed to be sufficiently accurate.
- RADIATION IMAGING PHYSICS
An analytical model of the effects of pulse pileup on the energy spectrum recorded by energy resolved photon counting x-ray detectors37(2010); http://dx.doi.org/10.1118/1.3429056View Description Hide DescriptionPurpose:
Recently, novel CdTe photon counting x-ray detectors (PCXDs) with energy discrimination capabilities have been developed. When such detectors are operated under a high x-ray flux, however, coincident pulses distort the recorded energy spectrum. These distortions are called pulse pileup effects. It is essential to compensate for these effects on the recorded energy spectrum in order to take full advantage of spectral information PCXDs provide. Such compensation can be achieved by incorporating a pileup model into the image reconstruction process for computed tomography, that is, as a part of the forward imaging process, and iteratively estimating either the imaged object or the line integrals using, e.g., a maximum likelihood approach. The aim of this study was to develop a new analytical pulse pileup model for both peak and tail pileup effects for nonparalyzable detectors.Methods:
The model takes into account the following factors: The bipolar shape of the pulse, the distribution function of time intervals between random events, and the input probability density function of photon energies. The authors used Monte Carlo simulations to evaluate the model.Results:
The recorded spectra estimated by the model were in an excellent agreement with those obtained by Monte Carlo simulations for various levels of pulse pileup effects. The coefficients of variation (i.e., the root mean square difference divided by the mean of measurements) were 5.3%–10.0% for deadtime losses of 1%–50% with a polychromatic incident x-rayspectrum.Conclusions:
The proposed pulse pileup model can predict recorded spectrum with relatively good accuracy.
Design and optimization of large area thin-film CdTe detector for radiation therapy imaging applications37(2010); http://dx.doi.org/10.1118/1.3438082View Description Hide DescriptionPurpose:
The authors investigate performance of thin-film cadmium telluride (CdTe) in detecting high-energy (6 MV) x rays. The utilization of this material has become technologically feasible only in recent years due to significant development in large area photovoltaic applications.Methods:
The CdTe film is combined with a metal plate, facilitating conversion of incoming photons into secondary electrons. The system modeling is based on the Monte Carlo simulations performed to determine the optimized CdTe layer thickness in combination with various converter materials.Results:
The authors establish a range of optimal parameters producing the highest DQE due to energy absorption, as well as signal and noise spatial spreading. The authors also analyze the influence of the patient scatter on image formation for a set of detector configurations. The results of absorbed energy simulation are used in device operation modeling to predict the detector output signal. Finally, the authors verify modeling results experimentally for the lowest considered device thickness.Conclusions:
The proposed CdTe-based large area thin-film detector has a potential of becoming an efficient low-cost electronic portal imaging device for radiation therapy applications.