Volume 37, Issue 4, April 2010
- medical physics letters
- radiation therapy physics
- radiation imaging physics
- radiation measurement physics
- magnetic resonance physics
- nuclear medicine physics
- optical physics
- ultrasound physics
- tissue measurements
- anatomy and physiology
- radiation biology
- books and publications
Index of content:
37(2010); http://dx.doi.org/10.1118/1.3298378View Description Hide Description
- MEDICAL PHYSICS LETTERS
GPU-based fast cone beam CT reconstruction from undersampled and noisy projection data via total variation37(2010); http://dx.doi.org/10.1118/1.3371691View Description Hide DescriptionPurpose:
Cone-beam CT(CBCT) plays an important role in image guided radiation therapy(IGRT). However, the large radiationdose from serial CBCT scans in most IGRT procedures raises a clinical concern, especially for pediatric patients who are essentially excluded from receiving IGRT for this reason. The goal of this work is to develop a fast GPU-based algorithm to reconstructCBCT from undersampled and noisy projection data so as to lower the imagingdose.Methods:
The CBCT is reconstructed by minimizing an energy functional consisting of a data fidelity term and a total variation regularization term. The authors developed a GPU-friendly version of the forward-backward splitting algorithm to solve this model. A multigrid technique is also employed.Results:
It is found that 20–40 x-ray projections are sufficient to reconstructimages with satisfactory quality for IGRT. The reconstruction time ranges from 77 to 130 s on an NVIDIA Tesla C1060 (NVIDIA, Santa Clara, CA) GPU card, depending on the number of projections used, which is estimated about 100 times faster than similar iterative reconstruction approaches. Moreover, phantom studies indicate that the algorithm enables the CBCT to be reconstructed under a scanning protocol with as low as 0.1 mA s/projection. Comparing with currently widely used full-fan head and neck scanning protocol of projections with 0.4 mA s/projection, it is estimated that an overall 36–72 times dose reduction has been achieved in our fast CBCTreconstruction algorithm.Conclusions:
This work indicates that the developed GPU-based CBCTreconstruction algorithm is capable of lowering imagingdose considerably. The high computation efficiency in this algorithm makes the iterative CBCTreconstruction approach applicable in real clinical environments.
- RADIATION THERAPY PHYSICS
Establishment of technical prerequisites for cell irradiation experiments with laser-accelerated electrons37(2010); http://dx.doi.org/10.1118/1.3301598View Description Hide DescriptionPurpose:
In recent years, laser-based acceleration of charged particles has rapidly progressed and medical applications, e.g., in radiotherapy, might become feasible in the coming decade. Requirements are monoenergetic particle beams with long-term stable and reproducible properties as well as sufficient particle intensities and a controlled delivery of prescribed doses at the treatment site. Although conventional and laser-based particle accelerators will administer the same dose to the patient, their different time structures could result in different radiobiological properties. Therefore, the biological response to the ultrashort pulse durations and the resulting high peak dose rates of these particle beams have to be investigated. The technical prerequisites, i.e., a suitable cellirradiation setup and the precise dosimetric characterization of a laser-based particle accelerator, have to be realized in order to prepare systematic cellirradiation experiments.Methods:
The Jena titanium:sapphire laser system (JETI) was customized in preparation for cellirradiation experiments with laser-accelerated electrons. The delivered electron beam was optimized with regard to its spectrum, diameter, dose rate, and dose homogeneity. A custom-designed beam and dose monitoring system, consisting of a Roos ionization chamber, a Faraday cup, and EBT-1 dosimetry films, enables real-time monitoring of irradiation experiments and precise determination of the dose delivered to the cells. Finally, as proof-of-principle experiment cell samples were irradiated using this setup.Results:
Laser-accelerated electron beams, appropriate forin vitro radiobiological experiments, were generated with a laser shot frequency of 2.5 Hz and a pulse length of 80 fs. After laser acceleration in the helium gas jet, the electrons were filtered by a magnet, released from the vacuum target chamber, and propagated in air for a distance of 220 mm. Within this distance a lead collimator (aperture of 35 mm) was introduced, leading, along with the optimized setup, to a beam diameter of 35 mm, sufficient for the irradiation of common cell culture vessels. The corresponding maximum dose inhomogeneity over the beam spot was less than 10% for all irradiated samples. At cell position, the electrons posses a mean kinetic energy of 13.6 MeV, a bunch length of about 5 ps (FWHM), and a mean pulse dose of 1.6 mGy/bunch. Cross correlations show clear linear dependencies for the online recorded accumulated bunch charges, pulse doses, and pulse numbers on absolute doses determined with EBT-1 films. Hence, the established monitoring system is suitable for beam control and a dedicated dose delivery. Additionally, reasonable day-to-day stable and reproducible properties of the electron beam were achieved.Conclusions:
Basic technical prerequisites for future cellirradiation experiments with ultrashort pulsed laser-accelerated electrons were established at the JETI laser system. The implemented online control system is suitable to compensate beam intensity fluctuations and the achieved accuracy of dose delivery to the cells is sufficient for radiobiological cell experiments. Hence, systematicin vitrocellirradiation experiments can be performed, being the first step toward clinical application of laser-accelerated particles. Further steps, including the transfer of the established methods to experiments on higher biological systems or to other laser-based particle accelerators, will be prepared.
Comparison of Bayesian network and support vector machine models for two-year survival prediction in lung cancer patients treated with radiotherapy37(2010); http://dx.doi.org/10.1118/1.3352709View Description Hide DescriptionPurpose:
Classic statistical and machine learning models such as support vector machines (SVMs) can be used to predict cancer outcome, but often only perform well if all the input variables are known, which is unlikely in the medical domain. Bayesian network (BN) models have a natural ability to reason under uncertainty and might handle missing data better. In this study, the authors hypothesize that a BN model can predict two-year survival in non-small cell lungcancer(NSCLC) patients as accurately as SVM, but will predict survival more accurately when data are missing.Methods:
A BN and SVM model were trained on 322 inoperable NSCLC patients treated with radiotherapy from Maastricht and validated in three independent data sets of 35, 47, and 33 patients from Ghent, Leuven, and Toronto. Missing variables occurred in the data set with only 37, 28, and 24 patients having a complete data set.Results:
The BN model structure and parameter learning identified gross tumor volume size, performance status, and number of positive lymph nodes on a PET as prognostic factors for two-year survival. When validated in the full validation set of Ghent, Leuven, and Toronto, the BN model had an AUC of 0.77, 0.72, and 0.70, respectively. A SVM model based on the same variables had an overall worse performance (AUC 0.71, 0.68, and 0.69) especially in the Ghent set, which had the highest percentage of missing the important GTV size data. When only patients with complete data sets were considered, the BN and SVM model performed more alike.Conclusions:
Within the limitations of this study, the hypothesis is supported that BN models are better at handling missing data than SVM models and are therefore more suitable for the medical domain. Future works have to focus on improving the BN performance by including more patients, more variables, and more diversity.
Direct tumor in vivo dosimetry in highly-conformal radiotherapy: A feasibility study of implantable MOSFETs for hypofractionated extracranial treatments using the Cyberknife® system37(2010); http://dx.doi.org/10.1118/1.3315370View Description Hide DescriptionPurpose:
In highly-conformal radiotherapy, due to the complexity of both beam configurations and dose distributions, traditionalin vivodosimetry is unpractical or even impossible. The ideal dosimeter would be implanted inside the planning treatment volume so that it can directly measure the total delivered dose during each fraction with no additional uncertainty due to calculation models. The aim of this work is to verify if implantable metal oxide semiconductors field effect transistors(MOSFETs) can achieve a sufficient degree of dosimetric accuracy when used inside extracranial targets undergoing radiotherapytreatments using the Cyberknife system.Methods:
Based on the preliminary findings of this study, new prototypes for high dose fractionations were developed to reduce the time dependence for long treatment delivery times. These dosimeters were recently cleared and are marketed as DVS®-HFT. Multiple measurements were performed using both Virtual Water and water phantoms to characterize implantable MOSFETs under the Cyberknife beams, and included the reference-dosimetry consistency, the dependence of the response on the collimator size, on the daily delivered dose, and the time irradiation modality. Finally a Cyberknife prostate treatment simulation using a body phantom was conducted, and both MOSFET and ionization readings were compared to Monte Carlo calculations. The feasibility analysis was conducted based on the ratios of the absorbed dose divided by the dose reading, named as “further calibration factor” (FCF).Results:
The average FCFs resulted to be 0.98 for the collimator dependence test, and about 1.00 for the reference-dosimetry test, the dose-dependence test, and the time-dependence test. The average FCF of the prostate treatment simulation test was 0.99.Conclusions:
The obtained results are well within DVS specifications, that is, the factory calibration is still valid for such kind of treatments using the Cyberknife system, with no need of further calibration factors to be applied. The final accuracy of implantable MOSFETs when used for such kind of treatments was estimated to be within ±4%. Additional investigations using dose/fraction higher than 12 Gy, different beam configurations, and tracking systems could extend the present findings to other kind of treatments.MOSFET technology was proven to have high versatility in fast adaptation of existing detectors to new applications. It is plausible to expect a general feasibility of implantable MOSFET technology forin vivodosimetry of the extracranial-targets treatments using the Cyberknife, provided each particular application will be validated by suitable both physical and clinical studies.
37(2010); http://dx.doi.org/10.1118/1.3315390View Description Hide DescriptionPurpose:
This study presents a novel technique in which a uniform radiationdose to the whole body, soles, and scalp vertex can be achieved in one electron beamtreatment fraction.Methods:
The patient was treated at a machine with a home-made rotating board. The patients were treated in two groups in the prone and supine positions by leaning onto an inner rotational board in the prone and supine positions. Each group can further be separated into two subgroups using tilting and rotational positions for treatment.Results:
One of the beams was directed 15.5° upward and 15.5° downward from the horizontal axis to provide a field size of as large as 200 cm in height and 140 cm in width. An incline angle of 31.5° anteriorly (forward) or posteriorly (backward) of the outer frame at an angle rotated 60° clockwise or counterclockwise to the inner frame was found to be most appropriate. The output for the rotating board total skin electron therapy (RB-TSET) was 0.046 cGy/MU at ISD of 350 cm. The beam characteristics of the RB-TSET depth dose curves were, , , and .Conclusions:
The RB-TSET technique presented in this study is able to deliver a uniform radiationdose to the patient’s skin surface, the scalp vertex, and soles of the feet all at one time, eliminating the trouble of having to further irradiate these two regions separately when using the Stanford six field technique.
37(2010); http://dx.doi.org/10.1118/1.3355888View Description Hide DescriptionPurpose:
Dose volume histograms (DVHs) are used in radiation therapy plan optimization and evaluation. Irradiation strategies are decided at the planning step, and an assessment of the reliability of computed dose distributions and DVHs is needed to ensure that decisions are made based on reliable information. This work describes a method used to assign confidence intervals to DVHs, caused by the uncertainty associated with dose computation.Methods:
A simple mathematical model has been developed, based on several alternative models for point dose uncertainty (rectangular, triangular, and Gaussian probability distributions). The result is an, to be computed for two values (0.25/0.75 or 0.10/0.90) to obtain a confidence interval. This method has been applied to several DVH curves.Results:
are dependent on the point uncertainty probability distribution, on the standard uncertainty, and on the choice of confidence level. Confidence intervals can be assigned either to the volume encompassed by an isodose level or to the dose corresponding to a given volume.Conclusions:
provide bounds for DVH values while dose expected volume histograms can be used as central estimates for DVH when uncertainty is taken into account. They are particularly useful when an optimization approach is necessary.
Fast, accurate, and robust automatic marker detection for motion correction based on oblique kV or MV projection image pairs37(2010); http://dx.doi.org/10.1118/1.3355871View Description Hide DescriptionPurpose:
A robust and accurate method that allows the automatic detection of fiducial markers in MV and kV projection image pairs is proposed. The method allows to automatically correct for inter or intrafraction motion.Methods:
Intratreatment MV projection images are acquired during each of five treatment beams of prostate cancer patients with four implanted fiducial markers. The projection images are first preprocessed using a series of marker enhancing filters. 2D candidate marker locations are generated for each of the filtered projection images and 3D candidate marker locations are reconstructed by pairing candidates in subsequent projection images. The correct marker positions are retrieved in 3D by the minimization of a cost function that combines 2D image intensity and 3D geometric or shape information for the entire marker configuration simultaneously. This optimization problem is solved using dynamic programming such that the globally optimal configuration for all markers is always found. Translational interfraction and intrafraction prostate motion and the required patient repositioning is assessed from the position of the centroid of the detected markers in different MV image pairs. The method was validated on a phantom using CT as ground-truth and on clinical data sets of 16 patients using manual marker annotations as ground-truth.Results:
The entire setup was confirmed to be accurate to around 1 mm by the phantom measurements. The reproducibility of the manual marker selection was less than 3.5 pixels in the MV images. In patient images, markers were correctly identified in at least 99% of the cases for anterior projection images and 96% of the cases for oblique projection images. The average marker detection accuracy was in the projection images. The centroid of all four reconstructed marker positions in 3D was positioned within 2 mm of the ground-truth position in 99.73% of all cases. Detecting four markers in a pair of MV images takes a little less than a second where most time is spent on the image preprocessing.Conclusions:
The authors have developed a method to automatically detect multiple markers in a pair of projection images that is robust, accurate, and sufficiently fast for clinical use. It can be used for kV, MV, or mixed image pairs and can cope with limited motion between the projection images.
37(2010); http://dx.doi.org/10.1118/1.3359819View Description Hide DescriptionPurpose:
Monitor unit (MU) calculations for electron arc therapy were carried out using Monte Carlo simulations and verified by measurements. Variations in the dwell factor (DF), source-to-surface distance (SSD), and treatment arc angle were studied. Moreover, the possibility of measuring the DF, which requires gantry rotation, using a solid water rectangular, instead of cylindrical, phantom was investigated.Methods:
A phase space file based on the 9 MeV electron beam with rectangular cutout (physical) attached to the block tray holder of a Varian 21 EX linear accelerator(linac) was generated using the EGSnrc-based Monte Carlo code and verified by measurement. The relative output factor (ROF), SSD offset, and DF, needed in the MU calculation, were determined using measurements and Monte Carlo simulations. An ionization chamber, a radiographic film, a solid water rectangular phantom, and a cylindrical phantom made of polystyrene were used in dosimetry measurements.Results:
Percentage deviations of ROF, SSD offset, and DF between measured and Monte Carlo results were 1.2%, 0.18%, and 1.5%, respectively. It was found that the DF decreased with an increase in, and such a decrease in DF was more significant in the range of 0°–60° than 60°–120°. Moreover, for a fixed , the DF increased with an increase in SSD. Comparing the DF determined using the rectangular and cylindrical phantom through measurements and Monte Carlo simulations, it was found that the DF determined by the rectangular phantom agreed well with that by the cylindrical one within ±1.2%. It shows that a simple setup of a solid water rectangular phantom was sufficient to replace the cylindrical phantom using our specific cutout to determine the DF associated with the electron arc.Conclusions:
By verifying using dosimetry measurements, Monte Carlo simulations proved to be an alternative way to perform MU calculations effectively for electron arc therapy. Since Monte Carlo simulations can generate a precalculated database of ROF, SSD offset, and DF for the MU calculation, with a reduction in human effort and linac beam-on time, it is recommended that Monte Carlo simulations be partially or completely integrated into the commissioning of electron arc therapy.
37(2010); http://dx.doi.org/10.1118/1.3360441View Description Hide DescriptionPurpose:
High dose-rate (HDR) brachytherapy is currently performed with sources, and has returned recently into clinical use as a source for this kind of cancer treatment. Both radionuclides have mean photon energies high enough to require specific shielded treatment rooms. In recent years, has been explored as an alternative for HDR-brachytherapy implants. Although it has mean photon energy lower than , it still requires extensive shielding to deliver treatment. An alternative radionuclide for brachytherapy is because it has three physical properties adequate for clinical practice: (a) 128.6 day half-life, (b) high specific activity, and (c) mean photon energy of 66.39 keV. The main drawback of this radionuclide is the low photon yield (six photons per 100 electrons emitted). The purpose of this work is to study the dosimetric characteristics of this radionuclide for potential use in HDR-brachytherapy.Methods:
The authors have assumed a theoretical cylindrical source encapsulated with stainless steel and typical dimensions taken from the currently available HDR brachytherapy sources. The dose-rate distribution was calculated for this source using the GEANT4Monte Carlo(MC) code considering both photon and electron spectra. The AAPM TG-43 U1 brachytherapydosimetry parameters were derived. To study general properties of encapsulated sources, spherical sources encapsulated with stainless steel and platinum were also studied. Moreover, the influence of small variations in the active core and capsule dimensions on the dosimetric characteristics was assessed. Treatment times required for a source were compared to those for and for the same contained activity.Results:
Due to the energetic beta spectrum and the large electron yield, the bremsstrahlung contribution to the dose was of the same order of magnitude as from the emitted gammas and characteristic x rays. Moreover, the electronspectrum contribution to the dose was significant up to 4 mm from the source center compared to the photon contribution. The dose-rate constant of the cylindrical source was . The behavior of the radial dose function showed promise for applications in brachytherapy. Due to the electronspectrum, the anisotropy was large for . Variations in manufacturing tolerances did not significantly influence the final dosimetry data when expressed in . For typical capsule dimensions, maximum reference dose rates of about 0.2, 10, and would then be obtained for , , and , respectively, resulting in treatment times greater than those for HDR brachytherapy.Conclusions:
The dosimetric characteristics of source designs exploiting the low photon energy of were studied for potential application in HDR-brachytherapy. Dose-rate distributions were obtained for cylindrical and simplified spherical source designs (stainless steel and platinum capsule materials) using MC calculations. Despite the high activity of , calculated treatment times were much longer than for .
Mechanical characterization of the Varian Exact-arm and R-arm support systems for eight aS500 electronic portal imaging devices37(2010); http://dx.doi.org/10.1118/1.3368604View Description Hide DescriptionPurpose:
The aim of this study is to compare the positioning accuracy at different gantry angles of two electronic portal imaging devices(EPIDs) support arm systems by using EPID difference images as a measure for displacement. This work presents a comparison of the mechanical performance of eight Varian aS500 (Varian Medical Systems, Palo Alto, CA) EPIDs, mounted using either the Varian Exact-arm or R-arm.Methods:
The mechanical performance of the two arm systems was compared by investigating the variation in sensitivity with gantry angle, both before and after the EPID position was adjusted after gantry rotation. Positional errors were investigated by subtracting images from a reference image taken at gantry 0°, and the amplitude of the peaks and troughs at the field edges for longitudinal (radial) and lateral (transverse) profiles across the resulting image was related to the distance of displacement. Calibration curves based on a pixel-by-pixel shift were generated for each EPID and the Varian hand pendant accuracy was compared to the calibration data.Results:
The response of the EPIDs was found to change with gantry rotation, with the largest difference at 180°. The Exact-arm was found to correct well for any displacement, while the R-arm tended to overcorrect following repositioning using the hand pendant. The calibration curves were consistent within each set of matched linacs, and the hand pendant accuracy was similar for both arm systems, although generally in different directions. With respect to gantry rotation effects, the mechanical performance of the Exact-arm systems was found to be much better than that of the R-arm systems. At gantry positions 90°, 270°, and 180° the average misalignment in the longitudinal direction was, , and for the R-arms, and , , and for the Exact-arms. In the lateral direction the average positional errors were , , and for the R-arms, and , , and for the Exact-arms. The hand pendant correction had minimal impact in the lateral direction for both arm systems. However in the longitudinal direction the mean errors for the R-arms were , , and at gantry angles 90°, 270°, and 180°, and the equivalent Exact-arm errors were , , and , respectively.Conclusions:
The performance of the EPIDs demonstrate that the Exact-arm system provides a more reproducible position and better agreement with the EPID position as indicated on the EPID pendant at all gantry angles than the R-arm.
Comparison of patient megavoltage cone beam CT images acquired with an unflattened beam from a carbon target and a flattened treatment beama)37(2010); http://dx.doi.org/10.1118/1.3359822View Description Hide Description
Purpose: To use an imaging beam line (IBL) to obtain the first megavoltage cone-beam computed tomography (MV CBCT)images of patients with a low atomic number target, and to compare these images to those taken of the same patients with the 6 MV flattened beam from the treatment beam line (TBL).
Methods: The IBL, which produces a 4.2 MV unflattened beam from a carbon target, was installed on a linear accelerator in use for radiotherapy. Provision was made for switching between the IBL and TBL for imaging the same patient with beams from the low- and high- targets. Dose was quoted as monitor units times the dose per monitor unit for the standard calibration geometry. Images were acquired with institutional approval and patient consent with both the IBL and TBL on a series of 23 patients undergoing radiotherapy. Patients were imaged daily to weekly and aligned to the planning CT using the images.Doses were reduced over the course of treatment to determine the minimum doses required for alignment. Images were assessed offline.
Results: IBL MV CBCTimages of prostate, head and neck, lung, and abdomen showed improvement in soft tissue contrast for the same dose as the TBL images. Bony anatomy, air cavities, and fiducial markers were sharper. CBCT with a dose of 1 cGy was sufficient for alignment of prostate and head and neck patients based on bony anatomy or implanted gold seeds, 2–4 cGy for lung, abdomen, and pelvis. Photon scatter in the patient had minimal effect on image quality. The metallic hip prosthesis in one patient showed reduced artifacts compared to diagnostic CT.
Conclusions: The IBL has the advantage of improved image quality at the same dose, or reduced dose for the same image quality, over the TBL.
Target definition of moving lung tumors in positron emission tomography: Correlation of optimal activity concentration thresholds with object size, motion extent, and source-to-background ratio37(2010); http://dx.doi.org/10.1118/1.3315369View Description Hide DescriptionPurpose:
Hardware integration of fluorodeoxyglucose positron emission tomography(PET) with computed tomography(CT) in combined PET/CT scanners has provided radiation oncologists and physicists with new possibilities for 3-D treatment simulation. The use of PET/CT simulation for target delineation of lungcancer is becoming popular and many studies concerning automatic segmentation of PETimages have been performed. Several of these studies consider size and source-to-background (SBR) in their segmentation methods but neglect respiratory motion. The purpose of the current study was to develop a functional relationship between optimal activity concentration threshold, tumor volume, motion extent, and SBR using multiple regression techniques by performing an extensive series of phantom scans simulating tumors of varying sizes, SBR, and motion amplitudes. Segmented volumes on PET were compared with the “motion envelope” of the moving sphere defined on cine CT.Methods:
A NEMA IEC thorax phantom containing six spheres (inner diameters ranging from 10 to 37 mm) was placed on a motion platform and moved sinusoidally at 0–30 mm (at 5 mm intervals) and six different SBRs (ranging from 5:1 to 50:1), producing 252 combinations of experimental parameters. PETimages were acquired for 18 min and split into three 6 min acquisitions for reproducibility. The spheres (blurred on PETimages due to motion) were segmented at 1% of maximum activity concentration intervals. The optimal threshold was determined by comparing deviations between the threshold volume surfaces with a reference volume surface defined on cine CT. Optimal activity concentration thresholds were normalized to background and multiple regression was used to determine the relationship between optimal threshold, volume, motion, and SBR. Standardized regression coefficients were used to assess the relative influence of each variable. The segmentation model was applied to three lungcancer patients and segmented regions of interest were compared with those segmented on cine CT.Results:
The resulting model and coefficients provided a functional form that fit the phantom data with an adjusted. The most significant contributor to threshold level was SBR. Surfaces of PET-segmented volumes of three lungcancer patients were within 2 mm of the reference CT volumes on average.Conclusions:
The authors successfully developed an expression for optimal activity concentration threshold as a function of object volume, motion, and SBR.
Technical Note: Modeling a complex micro-multileaf collimator using the standard BEAMnrc distribution37(2010); http://dx.doi.org/10.1118/1.3355873View Description Hide DescriptionPurpose:
The component modules in the standard BEAMnrc distribution may appear to be insufficient to model micro-multileaf collimators that have trifaceted leaf ends and complex leaf profiles. This note indicates, however, that accurate Monte Carlo simulations of radiotherapy beams defined by a complex collimation device can be completed using BEAMnrc’s standard VARMLC component module.Methods:
That this simple collimator model can produce spatially and dosimetrically accurate microcollimated fields is illustrated using comparisons with ion chamber and film measurements of the dose deposited by square and irregular fields incident on planar, homogeneous water phantoms.Results:
Monte Carlodose calculations for on-axis and off-axis fields are shown to produce good agreement with experimental values, even on close examination of the penumbrae.Conclusions:
The use of a VARMLC model of the micro-multileaf collimator, along with a commissioned model of the associated linear accelerator, is therefore recommended as an alternative to the development or use of in-house or third-party component modules for simulating stereotactic radiotherapy and radiosurgery treatments. Simulation parameters for the VARMLC model are provided which should allow other researchers to adapt and use this model to study clinical stereotactic radiotherapy treatments.
Using Bayesian logistic regression to evaluate a new type of dosimetric constraint for prostate radiotherapy treatment planning37(2010); http://dx.doi.org/10.1118/1.3367013View Description Hide DescriptionPurpose:
Modern radiotherapy treatments can be optimized using dose-volume constraints which specify the volume of tumor and organs-at-risk receiving a given threshold dose. Careful derivation and evaluation of rectal constraints is essential to allow safe dose escalation in radiotherapy of prostate cancer. The authors present a new type of hybrid dosimetric constraint which comprises both volumetric and spatial factors of the dose-distribution. The authors also propose a framework to evaluate these constraints.Methods:
The authors used data from the RT01 prostate radiotherapy trial (ISRCTN 47772397) to derive this set of hybrid constraints for the rectum based on measures extracted from dose-surface maps. For comparison, the authors also derive a set of dose-volume constraints. In order to evaluate these dosimetric constraints, the authors propose a new framework for predicting radiation-induced toxicities using Bayesian logistic regression with high-order interactions. The predictive power of the new RT01-based constraints, as well as of two sets of rectal dose-volume constraints proposed in the recent literature—The constraints proposed by other researchers [C. Fiorino, G. Fellin, T. Rancati, V. Vavassori, C. Bianchi, V. C. Borca, G. Girelli, M. Mapelli, L. Menegotti, S. Nava, and R. Valdagni, “Clinical and dosimetric predictors of late rectal syndrome after 3D-CRT for localized prostate cancer: Preliminary results of a multicenter prospective study,” Int. J. Radiat. Oncol., Biol., Phys.70, 1130–1137 (2008)] and the constraints used in the conventional or hypofractionated high dose intensity modulated radiotherapy for prostate cancer (CHHiP) trial [C. P. South, V. S. Khoo, O. Naismith, A. Norman, and D. P. Dearnaley, “A comparison of treatment planning techniques used in two randomised UK external beam radiotherapy trials for localised prostate cancer,” Clin. Oncol. (R Coll. Radiol)20, 15–21 (2008)]—were evaluated using a tenfold cross-validation with follow-up data from the RT01 trial. The predictive power was quantified using receiver-operator characteristic (ROC) curves. Toxicities considered were rectal bleeding, loose stools, and a global toxicity score.Results:
Dose-volume constraints had less predictive power than the new type of hybrid constraints. A probabilistic model for predicting rectal bleeding based on the dose-volume constraints proposed by other researchers [C. Fiorino, G. Fellin, T. Rancati, V. Vavassori, C. Bianchi, V. C. Borca, G. Girelli, M. Mapelli, L. Menegotti, S. Nava, and R. Valdagni, “Clinical and dosimetric predictors of late rectal syndrome after 3D-CRT for localized prostate cancer: Preliminary results of a multicenter prospective study,” Int. J. Radiat. Oncol., Biol., Phys.70, 1130–1137 (2008)], the CHHiP dose-volume constraints, the RT01-based dose-volume constraints, and the hybrid constraints resulted in average areas under the ROC curves of 0.56, 0.58, 0.62, and 0.67, respectively. For predicting loose stools, the corresponding values were 0.57, 0.53, 0.66, and 0.71, respectively. The areas under the respective ROC curves for predicting the global toxicity score were 0.58, 0.55, 0.61, and 0.63.Conclusions:
Thus, imposing the new type of hybrid constraints when generating a treatment plan should result in a reduction in the incidence of radiation-induced late rectal toxicity.
Intensity modulated radiotherapy of non-small-cell lung cancer incorporating SPECT ventilation imaging37(2010); http://dx.doi.org/10.1118/1.3358128View Description Hide DescriptionPurpose:
The authors performed this retrospective study to investigate the impact of using ventilation scans obtained from single photon emission computed tomography(SPECT) in selecting beam directions in intensity modulated radiation therapy(IMRT) planning in lungcancerradiotherapy to spare dosimetrically well ventilated lung.Methods:
For ten consecutive stage III non-small-cell lungcancer patients, the authors obtained both ventilation/perfusion SPECT scans and four-dimensional CT scans for treatment planning purposes. Each ventilation scan was registered with the corresponding planning CT and ventilation volumes corresponding to either (vv50) or (vv70) of the maximum SPECT count were automatically segmented. For each patient, three IMRT plans were generated: One using nine equally spaced beams optimized according to nonfunctional lung based mean lungdose and lung v20; a second using nine equally spaced beams optimized to avoid vv50 and vv70; and a third plan using only three beams with gantry angles chosen based on minimum mean ventilated lungdose calculated for each conformal beam at every 10° gantry angle avoiding vv50 and vv70. Resultant dose volume histogram indices were calculated for each plan and were compared with respect to calculated SPECT-based ventilation parameters in order to quantify the potential utility of ventilation SPECT in this setting.Results:
Two patient groups were identified based on (i) the overlap volume between PTV and vv50 and (ii) the average angular mean ventilated lungdose (AAMvLD). The first parameter quantifies the proximity of the PTV to well ventilated lung and the second parameter quantifies the degree of ventilation that surrounds the PTV. For group 1 patients, of the vv50 overlapped with the PTV. For group 2 patients, of the vv50 overlapped the PTV. Group 1 was further classified into subgroups 1A and 1B: For subgroup 1A, AAMvLD is , implying that the functional lung surrounds the PTV; for subgroup 1B, AAMvLD is , implying that the well ventilated lung does not completely surround PTV. For subgroup 1A, the plans generated using ventilated lung avoidance reduced dose to vv50 and vv70, with below tolerance dose to normal lung and acceptable coverage of the PTV. For subgroup 1B, the dose to the total lung and well ventilated lung are reduced with the beam direction optimization for the three-beam plan. For group 2, there was no significant dosimetric advantage of using SPECT-based ventilation information in IMRT plan optimization.Conclusions:
In conclusion, it is feasible to use SPECT ventilation scans to optimize IMRT beam direction and, subsequently, to reduce dose to ventilated lung when overlap of the PTV and the ventilated lung is minimal and that the PTV is not surrounded by the ventilated lung. The potential benefit of ventilation SPECT scanning can be determined by preplanning assessment of overlap volumes and the AAMvLD.
- RADIATION IMAGING PHYSICS
Evaluation of an a-Si EPID in direct detection configuration as a water-equivalent dosimeter for transit dosimetry37(2010); http://dx.doi.org/10.1118/1.3327456View Description Hide DescriptionPurpose:
A major problem associated with amorphous silicon(a-Si)electronic portal imaging devices(EPIDs) for transit dosimetry is the presence of a phosphor layer, which can introduce large deviations from water-equivalent behavior due to energy-dependent response and visible light scattering. In this study, an amorphous siliconEPID was modified to a direct detection configuration by removing the phosphor layer, and the accuracy of using it for transit dosimetry measurements was investigated for 6 and 18 MV treatment beams by comparison to ion-chamber in water measurements.Methods:
Solid water and copper were both evaluated as buildup materials. Using the optimum buildup thickness in each case, effects of changes in radiation field size, source to detector distance, and patient/phantom thickness were investigated by comparison to reference measurements made by an ionization chamber on the central axis. The off-axis response of the imager was also investigated by comparison of EPIDimage profiles to dose profiles obtained by a scanning ionization chamber in a water tank with various thicknesses of slab phantoms, and an anthropomorphic phantom in the beam using Gamma evaluation (3%, 3 mm criteria). The imaging characteristics of the direct EPID were investigated by comparison to a commercial EPID using QC3V phantom, and by taking images of an anthropomorphic pelvic phantom containing fiducial gold markers.Results:
Either 30 mm of solid water or 3.3 mm of copper were found to be the most suitable buildup thicknesses with solid water providing more accurate results. Using solid water buildup, the EPID response compared to the reference dosimeter within 2% for all conditions except phantom thicknesses larger than 25 cm in 6 MV beams, which was up to 6.5%. Gamma evaluation results comparing EPID profiles and reference ionization chamber profiles showed that for 6 and 18 MV beams, at least 91.8% and 90.9% of points had a for all phantoms, respectively. But using copper buildup, the EPID response had more discrepancies from the ionization chamber reference measurements, including: More than 2% difference for small air gaps using 6 MV beams, up to 8% difference for phantom thicknesses larger than 25 cm in 6 MV beams, and large differences (up to 9.3%) for increasing phantom thicknesses in 18 MV beams. The percentage of points with with copper buildup were at least 96.6% and 99.8% in 6 and 18 MV beams, respectively.Conclusions:
The direct EPID performs as an ion-chamber detector for transit dosimetry applications in all geometries studied except for small discrepancies at 6 MV for thick phantoms. This can be ameliorated by the calibration of the EPID to dose at an intermediate phantom thickness. The major current limitation of the direct EPID is poor quality of images compared with the clinical configuration, which could be overcome by a method to interchange between imaging and dosimetry setups.
37(2010); http://dx.doi.org/10.1118/1.3302833View Description Hide DescriptionPurpose:
A large number of false positives (FPs) generated by computer-aided detection (CAD) schemes is likely to distract radiologists’ attention and decrease their interpretation efficiency. This study aims to develop projection-based features which characterize true and false positives to increase the specificity while maintaining high sensitivity in detecting colonic polyps.Methods:
In this study, two-dimensional projection images are obtained from each initial polyp candidate or volume of interest, and features are extracted from both the gray and color projection images to differentiate FPs from true positives. These projection features were tested to exclude different types of FPs, such as haustral folds, rectal tubes, and residue stool using a database of 325 patient studies (from two different institutions), which includes 556 scans at supine and/or prone positions with 347 polyps and masses sized from 5 to 60 mm. For comparison, several well-established features were used to generate a baseline reference. The experimental evaluation was conducted for large polyps and medium-sized polyps (5–9 mm) separately.Results:
For large polyps, the additional usage of the projection features reduces the FP rate from 5.31 to 1.92 per scan at the comparable by-polyp sensitivity level of 93.1%. For medium-sized polyps, the FP rate is reduced from 8.89 to 5.23 at the sensitivity level of 80.6%. The percentages of FP reduction are 63.9% and 41.2% for the large and medium-sized polyps, respectively, without sacrificing detection sensitivity.Conclusions:
The results have demonstrated that the new projection features can effectively reduce the FPs and increase the detection specificity without sacrificing the sensitivity. CAD of colonic polyps is supposed to help radiologists to improve their performance in interpreting computed tomographic colonography images.
Segmentation and quantification of pulmonary artery for noninvasive CT assessment of sickle cell secondary pulmonary hypertension37(2010); http://dx.doi.org/10.1118/1.3355892View Description Hide DescriptionPurpose:
Pulmonary arterial hypertension (PAH) is a progressive vascular disease that results in high mortality and morbidity in sickle cell disease (SCD) patients. PAHdiagnosis is invasive via right heart catheterization, but manual measurements of the main pulmonary artery (PA) diameters from computed tomography(CT) have shown promise as noninvasive surrogate marker of PAH. The authors propose a semiautomated computer-assisted diagnostic(CAD) tool to quantify the main PA size from pulmonary CT angiography (CTA).Methods:
A follow-up retrospective study investigated the potential of CT and image analysis to quantify the presence of PAH secondary to SCD based on PA size. The authors segmented the main pulmonary arteries using a combination of fast marching level sets and geodesic active contours from smoothed pulmonary CTA images of 20 SCD patients with proven PAH by right heart catheterization and 20 matched negative controls. From the PA segmentation, a Euclidean distance map was calculated and an algorithm based on fast marching methods was used to compute subvoxel precise centerlines of the PA trunk (PT) and main left/right PA (PM). Maximum distentions of PT and PM were automatically quantified using the centerline and validated with manual measurements from two observers.Results:
The pulmonary trunk and main were significantly larger in PAH/SCD patients ( for PT and for PM) than controls ( for PT and for PM). The discrepancy was qualitatively improved when vessels’ diameters were normalized by body surface area . The validation of the method showed high correlation (mean for PT and for PM) and Bland–Altman agreement ( for PT and for PM) between CAD and manual measurements. Quantification errors were comparable to intraobserver and interobserver variability. CAD measurements between two different users were robust and reproducible with correlations of for both PT and PM and Bland–Altman agreements of for PT and for PM.Conclusion:
Results suggest that the semiautomated quantification of pulmonary artery has sufficient accuracy and reproducibility for clinical use. CT with image processing and extraction of PA biomarkers show great potential as a surrogate indicator for diagnosis or quantification of PAH, and could be an important tool for drug discovery and noninvasive clinical surveillance.
37(2010); http://dx.doi.org/10.1118/1.3357288View Description Hide DescriptionPurpose:
Tomosynthesis is a promising modality for breast imaging. The appearance of the tomosynthesisreconstructed image is greatly affected by the choice of acquisition and reconstruction parameters. The purpose of this study was to investigate the limitations of tomosynthesis breast imaging due to scan parameters and quantum noise.Tomosynthesisimage quality was assessed based on performance of a mathematical observer model in a signal-known exactly (SKE) detection task.Methods:
SKE detectability was estimated using a prewhitening observer model. Structured breast background was simulated using filtered noise.Detectability was estimated for designer nodules ranging from 0.05 to 0.8 cm in diameter. Tomosynthesis slices were reconstructed using iterative maximum-likelihood expectation-maximization. The tomosynthesis scan angle was varied between 15° and 60°, the number of views between 11 and 41 and the total number of x-ray quanta was , , and . Detectability in tomosynthesis was compared to that in a single projection.Results:
For constant angular sampling distance, increasing the angular scan range increased detectability for all signal sizes. Large-scale signals were little affected by quantum noise or angular sampling. For small-scale signals, quantum noise and insufficient angular sampling degraded detectability. At high quantum noise levels, angular step size of 3° or below was sufficient to avoid image degradation. At lower quantum noise levels, increased angular sampling always resulted in increased detectability. The ratio of detectability in the tomosynthesis slice to that in a single projection exhibited a peak that shifted to larger signal sizes when the angular range increased. For a given angular range, the peak shifted toward smaller signals when the number of views was increased. The ratio was greater than unity for all conditions evaluated.Conclusion:
The effect of acquisition parameters on lesion detectability depends on signal size. Tomosynthesis scan angle had an effect on detectability for all signals sizes, while quantum noise and angular sampling only affected the detectability small-scale signals.