Index of content:
Volume 38, Issue 2, February 2011
It is STILL necessary to validate each individual IMRT treatment plan with dosimetric measurements before delivery38(2011); http://dx.doi.org/10.1118/1.3512801View Description Hide Description
- MEDICAL PHYSICS LETTERS
Scaling law for noise variance and spatial resolution in differential phase contrast computed tomography38(2011); http://dx.doi.org/10.1118/1.3533718View Description Hide DescriptionPurpose:
The noise variance versus spatial resolution relationship in differential phase contrast (DPC) projection imaging and computed tomography(CT) are derived and compared to conventional absorption-based x-ray projection imaging and CT.Methods:
The scaling law for DPC-CT is theoretically derived and subsequently validated with phantom results from an experimental Talbot–Lau interferometer system.Results:
For the DPC imaging method, the noise variance in the differential projection images follows the same inverse-square law with spatial resolution as in conventional absorption-based x-ray imaging projections. However, both in theory and experimental results, in DPC-CT the noise variance scales with spatial resolution following an inverse linear relationship with fixed slice thickness.Conclusions:
The scaling law in DPC-CT implies a lesser noise, and therefore dose, penalty for moving to higher spatial resolutions when compared to conventional absorption-based CT in order to maintain the same contrast-to-noise ratio.
- RADIATION THERAPY PHYSICS
38(2011); http://dx.doi.org/10.1118/1.3533668View Description Hide DescriptionPurpose:
The authors investigate the plan quality and treatment times that may be achieved with Co-60 tomotherapy delivery for clinical IMRT cases.Methods:
A research version ofPINNACLEtreatment planning system software (V9.1) enabled the authors to specify custom source profiles for modeling of cylindrical Co-60 sources. The calculated profiles were validated against measurements for simulated MLC leaf openings. The reduction in dose due to a partially obscured source was analyzed. The thread effect was investigated for a source of typical linac spot dimensions and 2.0 and 2.8 cm diameter cylindrical Co-60 sources. Co-60 tomotherapy plans for three clinical treatment sites—prostate, brain, and head-and-neck—were generated for the Co-60 sources and compared to linac-based segmental IMRT plans in terms of the DVHs produced. Treatment times were also determined.Results:
The custom source profile utility allowed the authors to obtain good agreement between calculated and measured profiles for simulated MLC leaf openings with the commercial Co-60 source. It was found that the thread effect is significantly reduced for Co-60 sources and is not a clinical concern even for the large slice width (4.8 cm) and pitch value (0.5) studied. Co-60 tomotherapy plans for three clinical treatment sites compared favorably to the original segmental IMRT plans in terms of the DVHs produced. Treatment times, comparable to the actual segmental IMRTtreatment times, may be achieved for a high activity Co-60 source and dual-slice delivery may reduce these times further.Conclusions:
It may be possible to achieve clinically viable treatment times with Co-60 tomotherapy delivery without unacceptable loss of plan quality in terms of the DVHs produced.
38(2011); http://dx.doi.org/10.1118/1.3521406View Description Hide Description
Patients undergoing radiation therapy (and their physicians alike) are concerned with the probability of cure (long-term recurrence-free survival, meaning the absence of a detectable or symptomatic tumor). This is not what current practice categorizes as “tumor control (TC);” instead, TC is taken to mean the extinction of clonogenic tumorcells at the end of treatment, a sufficient but not necessary condition for cure. In this review, we argue that TC thus defined has significant deficiencies. Most importantly, (1) it is an unobservable event and (2) elimination of all malignant clonogenic cells is, in some cases, unnecessary. In effect, within the existing biomedical paradigm, centered on the evolution of clonogenic malignant cells, full information about the long-term treatment outcome is contained in the distribution of the number of malignant cells m that remain clonogenic at the end of treatment and the birth and death rates of surviving tumorcells after treatment. Accordingly, plausible definitions of tumor control are invariably traceable to . Many primary cancers, such as breast and prostate cancer, are not lethal per se; they kill through metastases. Therefore, an object of tumor control in such cases should be the prevention of metastatic spread of the disease. Our claim, accordingly, is that improvements in radiation therapy outcomes require a twofold approach: (a) Establish a link between survival time, where the events of interest are local recurrence or distant (metastatic) failure (cancer-free survival) or death (cancer-specific survival), and the distribution and (b) link to treatment planning (modality, total dose, and schedule of radiation) and tumor-specific parameters (initial number of clonogens, birth and spontaneous death rates during the treatment period, and parameters of the dose-response function). The biomedical, mathematical, and practical aspects of implementing this program are discussed.
Quality assurance of volumetric modulated arc therapy: Evaluation and comparison of different dosimetric systems38(2011); http://dx.doi.org/10.1118/1.3533900View Description Hide DescriptionPurpose:
To compare and evaluate different dosimetric techniques and devices for the QA of VMAT plans created by two treatment planning systems (TPSs).Methods:
A total of 50 VMAT plans were optimized for treatment of anatomical sites of various complexities by two TPSs which use rather different approaches to VMAT optimization. Dosimetric plan verifications were performed both as part of commissioning and as patient specific QA of clinical treatments. Absolute point doses were measured for all plans by a micro ion chamber inserted in a dedicated water-filled cylindrical phantom. Delivered dose distributions were verified by four techniques based on different detectors: radiographic and gafchromic films, two systems based on 2D diode arrays and an ion chamber array. Gamma index analysis with various tolerance levels (3%, 3 mm and 3%, 2 mm) was used to analyze differences between calculated and delivered doses. Sensitivity to possible delivery errors was also evaluated for three of the considered devices introducing ±3 mm shifts along the three directions and a 3° gantry offset.Results:
Ion chamber measured point doses were within 3% of calculated ones for 48 out of 50 values. For delivered dose distribution, the average fraction of passed gamma values using 3% and 3 mm criteria was above 95% for both TPSs and all detectors except gafchromic film which yielded on average of 91.4%. For 49 out of 50 plans, a pass-rate above 94% was obtained by at least one of the four techniques. Shrinking the tolerance to 3% and 2 mm, the average pass-rate by all detectors (except film) was still above 95% for one of the two TPSs, but lower for the other one. The detector sensitivity to 3 mm shifts and to gantry angle offset was strongly plan and partially detector dependent: the obtained pass-rate reduction ranged from 2% to 30%.Conclusions:
The presented results for VMAT plans QA assess the reliability of the delivered doses for both TPSs. The slightly lower pass-rate obtained for one of the considered TPS can be attributed to a higher level of complexity of the optimized plans. The results by different dosimetric techniques are coherent, apart from a few measurements by gafchromic films. The detector sensitivity to delivery errors, being strongly plan dependent, is not easy to evaluate.
38(2011); http://dx.doi.org/10.1118/1.3496327View Description Hide DescriptionPurpose:
Proof of principle study of the use of a CMOS active pixel sensor (APS) in producing protonradiographicimages using the proton beam at the Massachusetts General Hospital (MGH).Methods:
A CMOS APS, previously tested for use in s-ray radiation therapy applications, was used for proton beam radiographicimaging at the MGH. Two different setups were used as a proof of principle that CMOS can be used as protonimaging device: (i) a pen with two metal screws to assess spatial resolution of the CMOS and (ii) a phantom with lung tissue, bone tissue, and water to assess tissue contrast of the CMOS. The sensor was then traversed by a double scattered monoenergetic proton beam at 117 MeV, and the energy deposition inside the detector was recorded to assess its energy response. Conventional x-ray images with similar setup at voltages of 70 kVp and protonimages using commercial Gafchromic EBT 2 and Kodak X-Omat V films were also taken for comparison purposes.Results:
Images were successfully acquired and compared to x-ray kVp and proton EBT2/X-Omat film images. The spatial resolution of the CMOSdetectorimage is subjectively comparable to the EBT2 and Kodak X-Omat V film images obtained at the same object-detector distance. X-rays have apparent higher spatial resolution than the CMOS. However, further studies with different commercial films using proton beam irradiation demonstrate that the distance of the detector to the object is important to the amount of protonscatter contributing to the protonimage.Protonimages obtained with films at different distances from the source indicate that protonscatter significantly affects the CMOSimage quality.Conclusion:
Protonradiographicimages were successfully acquired at MGH using a CMOS active pixel sensordetector. The CMOS demonstrated spatial resolution subjectively comparable to films at the same object-detector distance. Further work will be done in order to establish the spatial and energy resolution of the CMOSdetector for protons. The development and use of CMOS in protonradiography could allowin vivoproton range checks, patient setup QA, and real-time tumor tracking.
Irradiation of gold nanoparticles by x-rays: Monte Carlo simulation of dose enhancements and the spatial properties of the secondary electrons production38(2011); http://dx.doi.org/10.1118/1.3539623View Description Hide DescriptionPurpose:
The aim of this study is to understand the characteristics of secondary electronsgenerated from the interaction of goldnanoparticles (GNPs) with x-rays as a function of nanoparticle size and beam energy and thereby further the understanding of GNP-enhanced radiotherapy.Methods:
The effective range, deflection angle, dose deposition, energy, and interaction processes of electrons produced from the interaction of x-rays with a GNP were calculated by Monte Carlo simulations. TheGEANT4 code was used to simulate and track electronsgenerated from a 2, 50, and 100 nm diameter GNP when it is irradiated with a 50 kVp, 250 kVp, cobalt-60, and 6 MV photonbeam in water.Results:
When a GNP was present, depending on the beam energies used, secondary electron production was increased by 10- to 2000-fold compared to an absence of a GNP. Low-energy photonbeams were much more efficient at interacting with the GNP by two to three orders of magnitude compared to MV energies and increased the deflection angle. GNPs with larger diameters also contributed more dose. The majority of the energy deposition was outside the GNP, rather than self-absorbed by the nanoparticle. The mean effective range of electron tracks for the beams tested ranged from approximately to 1 mm.Conclusions:
These simulated results yield important insights concerning the spatial distributions and elevated dose in GNP-enhanced radiotherapy. The authors conclude that the irradiation of GNP at lower photon energies will be more efficient for cell killing. This conclusion is consistent with published studies.
38(2011); http://dx.doi.org/10.1118/1.3539648View Description Hide DescriptionPurpose:
To design a postprocessing 3D adaptive median filter that minimizes streak artifacts and improves soft-tissue contrast in postoperative CTimages of brachytherapy seed implantations.Methods:
The filter works by identifying voxels that are likely streaks and estimating more reflective voxel intensity by using voxel intensities in adjacent CT slices and applying a median filter over voxels not identified as seeds. Median values are computed over a region of interest (ROI) within the CT volume. An acrylic phantom simulating a clinical seed implant arrangement and containing nonradioactive seeds was created. Low contrast subvolumes of tissuelike material were also embedded in the phantom. Pre- and postprocessed image quality metrics were compared using the standard deviation of ROIs between the seeds, the CT numbers of low contrast ROIs embedded within the phantom, the signal to noise ratio (SNR), and the contrast to noise ratio (CNR) of the low contrast ROIs. The method was demonstrated with a clinical postimplant CT dataset.Results:
After the filter was applied, the standard deviation of CT values in streak artifact regions was significantly reduced from 76.5 to 7.2 HU. Within the observable low contrast plugs, the mean of all ROI standard deviations was significantly reduced from 60.5 to 3.9 HU, SNR significantly increased from 2.3 to 22.4, and CNR significantly increased from 0.2 to 4.1 (all). The mean CT in the low contrast plugs remained within 5 HU of the original values.Conclusion:
An efficient postprocessing filter that does not require access to projection data, which can be applied irrespective of CT scan parameters has been developed, provided the slice thickness and spacing is 3 mm or less.
38(2011); http://dx.doi.org/10.1118/1.3539749View Description Hide DescriptionPurpose:
To ensure plan quality for adaptive IMRT of the prostate, we developed a quantitative evaluation tool using a machine learning approach. This tool generates dose volume histograms (DVHs) of organs-at-risk (OARs) based on prior plans as a reference, to be compared with the adaptive plan derived from fluence map deformation.Methods:
Under the same configuration using seven-field 15 MV photon beams, DVHs of OARs (bladder and rectum) were estimated based on anatomical information of the patient and a model learned from a database of high quality prior plans. In this study, the anatomical information was characterized by the organ volumes and distance-to-target histogram (DTH). The database consists of 198 high quality prostate plans and was validated with 14 cases outside the training pool. Principal component analysis (PCA) was applied to DVHs and DTHs to quantify their salient features. Then, support vector regression (SVR) was implemented to establish the correlation between the features of the DVH and the anatomical information.Results:
DVH/DTH curves could be characterized sufficiently just using only two or three truncated principal components, thus, patient anatomical information was quantified with reduced numbers of variables. The evaluation of the model using the test data set demonstrated its accuracy in prediction and effectiveness in improving ART planning quality.Conclusions:
An adaptive IMRT plan quality evaluation tool based on machine learning has been developed, which estimates OAR sparing and provides reference in evaluating ART.
38(2011); http://dx.doi.org/10.1118/1.3539725View Description Hide DescriptionPurpose:
Monte Carlo methods are considered as the gold standard for dosimetric computations in radiotherapy. Their execution time is, however, still an obstacle to the routine use of Monte Carlo packages in a clinical setting. To address this problem, a completely new, and designed from the ground up for the GPU, Monte Carlodose calculation package for voxelized geometries is proposed:GPUMCD.Method:
GPUMCD implements a coupled photon-electron Monte Carlo simulation for energies in the range of 0.01–20 MeV. An analog simulation of photon interactions is used and a class II condensed history method has been implemented for the simulation of electrons. A new GPU random number generator, some divergence reduction methods, as well as other optimization strategies are also described. GPUMCD was run on a NVIDIA GTX480, while single threaded implementations of EGSnrc and DPM were run on an Intel Core i7 860.Results:
Dosimetric results obtained withGPUMCD were compared to EGSnrc. In all but one test case, 98% or more of all significant voxels passed the gamma criteria of 2%-2 mm. In terms of execution speed and efficiency, GPUMCD is more than 900 times faster than EGSnrc and more than 200 times faster than DPM, a Monte Carlo package aiming fast executions. Absolute execution times of less than 0.3 s are found for the simulation of 1M electrons and 4M photons in water for monoenergetic beams of 15 MeV, including GPU-CPU memory transfers.Conclusion:
GPUMCD, a new GPU-oriented Monte Carlodose calculation platform, has been compared to EGSnrc and DPM in terms of dosimetric results and execution speed. Its accuracy and speed make it an interesting solution for full Monte Carlodose calculation in radiation oncology.
38(2011); http://dx.doi.org/10.1118/1.3537206View Description Hide DescriptionPurpose:
Tumor motion due to patient breathing is a factor that limits the accuracy of dose distribution in radiotherapy. One of the methods to improve the accuracy is by applying respiratory gating or tumor tracking. Both techniques require a precise determination of the spatial location of the tumor. We present an experimental evaluation of the performance of PeTrack, a technique that can track internal fiducial markers in real-time for tumor trackingMethods:
PeTrack uses position sensitive detectors to record annihilation coincidence gamma rays from fiducial positron emission markers implanted in or around the tumor. It uses an expectation-maximization clustering algorithm to track the position of the markers. A normalized least mean square adaptive filter was used to predict the position of the markers 100 and 200 ms in the future. We evaluated the performance of the tracking and of the prediction by using a dynamic anthropomorphic thorax phantom to generate three-dimensional (3D) motion of three fiducial markers. The algorithm was run with four different data sets. In the first run, the motion of the markers was based on a sinusoidal model of respiratory motion. Three additional runs were done with motion based on patient breathing data.Results:
In the case of the sinusoidal model, the average 3D root mean square error for all markers was 0.44 mm. For the three runs based on patient breathing data, the precision of the 3D localization was 0.49 mm. At a latency of 100 ms, the average 3D prediction error was for the sinusoidal model and for the three patient breathing runs. At a latency of 200 ms, the average 3D prediction errors were for the sinusoidal model and for the breathing runs.Conclusions:
We conclude that PeTrack can track multiple fiducial markers in real-time with an accuracy and precision smaller than 2 mm. PeTrack can have a direct application in tumor tracking for radiation therapy.
38(2011); http://dx.doi.org/10.1118/1.3533947View Description Hide DescriptionPurpose:
To assess image quality and image-guidance capabilities of a cone-beam CT based small-animal image-guided irradiation unit (micro-IGRT).Methods:
A micro-IGRT system has been developed in collaboration with the authors’ laboratory as a means to study the radiobiological effects of conformal radiationdose distributions in small animals. The system, the X-Rad 225Cx, consists of a 225 kVp x-ray tube and a flat-panel amorphous silicon detector mounted on a rotational C-arm gantry and is capable of both fluoroscopic x-ray and cone-beam CTimaging, as well as image-guided placement of the radiation beams. Image quality (voxel noise, modulation transfer, CT number accuracy, and geometric accuracy characteristics) was assessed using water cylinder and micro-CT test phantoms. Image guidance was tested by analyzing the dose delivered to radiochromic films fixed to BB’s through the end-to-end process of imaging, targeting the center of the BB, and irradiation of the film/BB in order to compare the offset between the center of the field and the center of the BB. Image quality and geometric studies were repeated over a 5–7 month period to assess stability.Results:
CT numbers reported were found to be linear and the noise for images of homogeneous water phantom was 30 HU at imagingdoses of approximately 1 cGy (to water). The presampled MTF at 50% and 10% reached 0.64 and , respectively. Targeting accuracy by means of film irradiations was shown to have a mean displacement error of , with standard deviations of [0.02, 0.20, 0.17] mm. The system has proven to be stable over time, with both the image quality and image-guidance performance being reproducible for the duration of the studies.Conclusions:
The micro-IGRT unit provides soft-tissue imaging of small-animal anatomy at acceptable imagingdoses. The geometric accuracy and targeting systems permit dose placement with submillimeter accuracy and precision. The system has proven itself to be stable over 2 yr of routine laboratory use and provides a platform for the exploration of targeted radiation effects in small-animal models.
38(2011); http://dx.doi.org/10.1118/1.3531985View Description Hide DescriptionPurpose:
A novel technique is proposed to construct CTimage of a totally deflated lung from a free-breathing 4D-CT image sequence acquired preoperatively. Such a constructed CTimage is very useful in performing tumor ablative procedures such as lungbrachytherapy.Tumor ablative procedures are frequently performed while the lung is totally deflated. Deflating the lung during such procedures renders preoperative images ineffective for targeting the tumor. Furthermore, the problem cannot be solved using intraoperative ultrasound (U.S.) images because U.S. images are very sensitive to small residual amount of air remaining in the deflated lung. One possible solution to address these issues is to register high quality preoperative CTimages of the deflated lung with their corresponding low quality intraoperative U.S. images. However, given that such preoperative images correspond to an inflated lung, such CTimages need to be processed to construct CTimages pertaining to the lung’s deflated state.Methods:
To obtain the CTimages of deflated lung, we present a novel image construction technique using extrapolated deformable registration to predict the deformation the lung undergoes during full deflation. The proposed construction technique involves estimating the lung’s air volume in each preoperative image automatically in order to track the respiration phase of each 4D-CT image throughout a respiratory cycle; i.e., the technique does not need any external marker to form a respiratory signal in the process of curve fitting and extrapolation. The extrapolated deformation field is then applied on a preoperative reference image in order to construct the totally deflated lung’s CTimage. The technique was evaluated experimentally usingex vivo porcine lung.Results:
Theex vivolung experiments led to very encouraging results. In comparison with the CTimage of the deflated lung we acquired for the purpose of validation, the constructed CTimage was very similar. The intensity mean absolute difference between these two images was calculated to be at 1%. Tumor center as well as a number of anatomical fiducial markers were traced in different corresponding slices of the two images. The average misalignment obtained for the constructed CTimage was (0.64, 0.39, 0.11) mm, which indicates a very desirable accuracy for lungbrachytherapy applications.Conclusions:
The image construction accuracy obtained in this research is suitable for intraoperative tasks; e.g., tumor localization and fusing with real time navigation data in lungbrachytherapy. These applications involve image registration with intraoperative U.S. images in order to enhance their poor quality. The proposed technique is also useful for preoperative tasks such as planning of lungbrachytherapy treatment.
38(2011); http://dx.doi.org/10.1118/1.3532824View Description Hide DescriptionPurpose:
It is recommended to have a method for independently verifying planned doses in stereotactic radiosurgery. The problem is one of how to model the geometry of a skull sampled by a limited number of points and how to subsequently calculate numerous attenuation pathlengths through the modeled skull. While methods of verification have been previously published for model B and C Gamma Knife® units, the aims of the current work were to apply the principles of these previously published techniques for the verification of plans for Gamma Knife® PERFEXION™, to present a new method of verification, and to compare all methods in terms of their agreement with GammaPlan®.Methods:
Four algorithms were implemented: the previously published spherical approximation method (SAM) and bubble helmet skull (BHS), plus a modified BHS named interpolated BHS (IBHS) and a newly developed variable radius SAM (VRSAM). Reference point doses calculated by the four algorithms were compared to those reported by GammaPlan® for 54 simple test plans and for 35 targets in 20 recent patient plans.Results:
For test plans, the mean (standard deviation) discrepancies against GammaPlan®-reported doses were 0.3(1.3)%, 0.3(1.3)%, −1.6(3.4)%, and −0.4(1.0)% for SAM, VRSAM, BHS, and IBHS, respectively. For patient plans both the VRSAM and IBHS showed insignificant ( and ) discrepancies against GammaPlan® of 0.38(1.86)% and −0.11(1.86)%, respectively. More significant discrepancies against GammaPlan® of 2.64(2.98)% and −4.43(3.39)% were observed for the SAM and BHS.Conclusions:
The SAM can lead to large discrepancies against GammaPlan® when a sphere is a poor approximation of the true skull surface, and in peripheral locations can lead to nonreal solutions to the attenuation pathlength calculations. While the BHS does not suffer the same geometric assumptions of the SAM, it can underestimate dose for peripherally located shots. The IBHS exhibits better agreement with GammaPlan® than does the BHS, but requires two-dimensional interpolation that was found to be impractical to implement in the Excel-based software used in the current work. Combining aspects of both the previously published SAM and BHS algorithms, the newly presented VRSAM exhibits comparable results to the IBHS but without the need for interpolation and is therefore considered the preferred technique of the four implemented.
38(2011); http://dx.doi.org/10.1118/1.3534196View Description Hide DescriptionPurpose:
Total body irradiation (TBI) techniques aim to deliver a uniform radiationdose to a patient with an irregular body contour and a heterogeneous density distribution to within ±10% of the prescribed dose. In the current article, the authors present a novel, aperture modulated, translating bed TBI (AMTBI) technique that produces a high degree of dose uniformity throughout the entire patient.Methods:
The radiation beam is dynamically shaped in two dimensions using a multileaf collimator(MLC). The irregular surface compensation algorithm in the Eclipse™ treatment planning system is used for fluence optimization, which is performed based on penetration depth and internal inhomogeneities. Two optimal fluence maps (AP and PA) are generated and beam apertures are created to deliver these optimal fluences. During treatment, the patient/phantom is translated on a motorized bed close to the floor (source to bed distance: 204.5 cm) under a stationary radiation beam with 0° gantry angle. The bed motion and dynamic beam apertures are synchronized.Results:
The AMTBI technique produces a more homogeneous dose distribution than fixed open beam translating bed TBI. In phantom studies, the dose deviation along the midline is reduced from 10% to less than 5% of the prescribed dose in the longitudinal direction. Dose to the lung is reduced by more than 15% compared to the unshielded fixed open beam technique. At the lateral body edges, the dose received from the open beam technique was 20% higher than that prescribed at umbilicus midplane. With AMTBI the dose deviation in this same region is reduced to less than 3% of the prescribed dose. Validation of the technique was performed using thermoluminescent dosimeters in a Rando® phantom. Agreement between calculation and measurement was better than 3% in all cases.Conclusions:
A novel, translating bed, aperture modulated TBI technique that employs dynamically shaped MLC defined beams is shown to improve dose uniformity in three dimensions. In comparison with the fixed open beam TBI technique, homogeneity of dose distribution is greatly improved.
Position-probability-sampled Monte Carlo calculation of VMAT, 3DCRT, step-shoot IMRT, and helical tomotherapy dose distributions using BEAMnrc/DOSXYZnrc38(2011); http://dx.doi.org/10.1118/1.3538922View Description Hide DescriptionPurpose:
The commercial release of volumetric modulated arc therapy techniques using a conventional linear accelerator and the growing number of helical tomotherapy users have triggered renewed interest in dose verification methods, and also in tools for exploring the impact of machine tolerance and patient motion on dose distributions without the need to approximate time-varying parameters such as gantry position, MLC leaf motion, or patient motion. To this end we have developed a Monte Carlo-based calculation method capable of simulating a wide variety of treatment techniques without the need to resort to discretization approximations.Methods:
The ability to perform complete position-probability-sampled Monte Carlodose calculations was implemented in the BEAMnrc/DOSXZYnrc user codes of EGSnrc. The method includes full accelerator head simulations of our tomotherapy and Elekta linacs, and a realistic representation of continous motion via the sampling of a time variable. The functionality of this algorithm was tested via comparisons with both measurements and treatment planningdose distributions for four types of treatment techniques: 3D conformal, step-shoot intensity modulated radiation therapy, helical tomotherapy, and volumetric modulated arc therapy.Results:
For static fields, the absolute dose agreement between the EGSnrc Monte Carlo calculations and measurements is within 2%/1 mm. Absolute dose agreement between Monte Carlo calculations and treatment planning system for the four different treatment techniques is within 3%/3 mm. Discrepancies with the tomotherapy TPS on the order of 10%/5 mm were observed for the extreme example of a small target located 15 cm off-axis and planned with a low modulation factor. The increase in simulation time associated with using position-probability sampling, as opposed to the discretization approach, was less than 2% in most cases.Conclusions:
A single Monte Carlo simulation method can be used to calculate patient dose distribution for various types of treatment techniques delivered with either tomotherapy or a conventional linac. The method simplifies the simulation process, improves dose calculation accuracy, and involves an acceptably small change in computation time.
38(2011); http://dx.doi.org/10.1118/1.3547714View Description Hide DescriptionPurpose:
Electronic portal imaging devices(EPIDs) are increasingly used for IMRTdose verification, both pretreatment andin vivo. In this study, an earlier developed backprojection model has been modified to avoid the need for patient-specific transmission measurements and, consequently, leads to a faster procedure.Methods:
Currently, the transmission, an essential ingredient of the backprojection model, is estimated from the ratio of EPIDmeasurements with and without a phantom/patient in the beam. Thus, an additional irradiation to obtain “open images” under the same conditions as the actual phantom/patient irradiation is required. However, by calculating the transmission of the phantom/patient in the direction of the beam instead of using open images, this extra measurement can be avoided. This was achieved by using a model that includes the effect of beam hardening and off-axis dependence of the EPID response on photon beam spectral changes. The parameters in the model were empirically obtained by performing EPIDmeasurements using polystyrene slab phantoms of different thickness in 6, 10, and 18 MV photon beams. A theoretical analysis to verify the sensitivity of the model with patient thickness changes was performed. The new model was finally applied for the analysis of EPIDdose verification measurements of step-and-shoot IMRT treatments of head and neck, lung, breast, cervix, prostate, and rectum patients. All measurements were carried out using Elekta SL20i linear accelerators equipped with a hydrogenated amorphous silicon EPID, and the IMRT plans were made usingPINNACLE software (Philips Medical Systems).Results:
The results showed generally good agreement with the dose determined using the old model applying the measured transmission. The average differences between EPID-basedin vivodose at the isocenter determined using either the new model for transmission and its measured value were , , and for 47 patients treated with 6, 10, and 18 MV IMRT beams, respectively. For the same group of patients, the differences in mean analysis (3% maximum dose, 3 mm) were , , and , respectively. For a subgroup of 11 patients, pretreatment verification was also performed, showing similar dose differences at the isocenter: , , and , with somewhat lower mean difference values: , , and , respectively. Clinical implementation of the new model would save 450 h/yr spent in measurement of open images.Conclusions:
It can be concluded that calculating instead of measuring the transmission leads to differences in the isocenter dose generally smaller than 2% (2.6% for 6 MV photon beams forin vivodose) and yielded only slightly higher -evaluation parameter values in planes through the isocenter. Hence, the new model is suitable for clinical implementation and measurement of open images can be omitted.
Sensitivity of postplanning target and OAR coverage estimates to dosimetric margin distribution sampling parameters38(2011); http://dx.doi.org/10.1118/1.3544364View Description Hide DescriptionPurpose:
A dosimetric margin (DM) is the margin in a specified direction between a structure and a specified isodose surface, corresponding to a prescription or tolerance dose. The dosimetric margin distribution (DMD) is the distribution of DMs over all directions. Given a geometric uncertainty model, representing inter- or intrafraction setup uncertainties or internal organ motion, the DMD can be used to calculate coverage, which is the probability that a realized target or organ-at-risk (OAR) dose metric exceeds the corresponding prescription or tolerance dose. Postplanning coverage evaluation quantifies the percentage of uncertainties for which target and OAR structures meet their intended dose constraints. The goal of the present work is to evaluate coverage probabilities for 28 prostate treatment plans to determine DMD sampling parameters that ensure adequate accuracy for postplanning coverage estimates.Methods:
Normally distributed interfraction setup uncertainties were applied to 28 plans for localized prostate cancer, with prescribed dose of 79.2 Gy and 10 mm clinical target volume to planning target volume (CTV-to-PTV) margins. Using angular or isotropic sampling techniques, dosimetric margins were determined for the CTV, bladder and rectum, assuming shift invariance of the dose distribution. For angular sampling, DMDs were sampled at fixed angular intervals (e.g., ). Isotropic samples were uniformly distributed on the unit sphere resulting in variable angular increments, but were calculated for the same number of sampling directions as angular DMDs, and accordingly characterized by the effective angular increment . In each direction, the DM was calculated by moving the structure in radial steps of size until the specified isodose was crossed. Coverage estimation accuracy was quantified as a function of the sampling parameters or and .Results:
The accuracy of coverage estimates depends on angular and radial DMD sampling parameters or and , as well as the employed sampling technique. Target and OAR can be achieved with sampling parameters or , . Better accuracy (target and OAR ) can be achieved with or , . As the number of sampling points decreases, the isotropic sampling method maintains better accuracy than fixed angular sampling.Conclusions:
Coverage estimates for post-planning evaluation are essential since coverage values of targets and OARs often differ from the values implied by the static margin-based plans. Finer sampling of the DMD enables more accurate assessment of the effect of geometric uncertainties on coverage estimates prior to treatment. DMD sampling with or and should be adequate for planning purposes.
38(2011); http://dx.doi.org/10.1118/1.3544660View Description Hide DescriptionPurpose
: In recent years, several Monte Carlo studies have been published concerning the perturbation corrections of a parallel-plate chamber in clinical electron beams. In these studies, a strong depth dependence of the relevant correction factors ( and ) for depth beyond the reference depth is recognized and it has been shown that the variation with depth is sensitive to the choice of the chamber’s effective point of measurement. Recommendations concerning the positioning of parallel-plate ionization chambers in clinical electron beams are not the same for all current dosimetry protocols. The IAEA TRS-398 as well as the IPEM protocol and the German protocol DIN 6800-2 interpret the depth of measurement within the phantom as the water equivalent depth, i.e., the nonwater equivalence of the entrance window has to be accounted for by shifting the chamber by an amount . This positioning should ensure that the primary electrons traveling from the surface of the water phantom through the entrance window to the chamber’s reference point sustain the same energy loss as the primary electrons in the undisturbed phantom. The objective of the present study is the determination of the shift for a NACP-02 chamber and the calculation of the resulting wall perturbation correction as a function of depth. Moreover, the contributions of the different chamber walls to the wall perturbation correction are identified.Methods:
The dose and fluence within the NACP-02 chamber and a wall-less air cavity is calculated using the Monte Carlo code EGSnrc in a water phantom at different depths for different clinical electron beams. In order to determine the necessary shift to account for the nonwater equivalence of the entrance window, the chamber is shifted in steps around the depth of measurement. The optimal shift is determined from a comparison of the spectral fluence within the chamber and the bare cavity. The wall perturbation correction is calculated as the ratio between doses for the complete chamber and a wall-less air cavity.Results:
The high energy part of the fluence spectra within the chamber strongly varies even with small chamber shifts, allowing the determination of within micrometers. For the NACP-02 chamber a shift results. This value is independent of the energy of the primary electrons as well as of the depth within the phantom and it is in good agreement with the value recommended in the German dosimetry protocol. Applying this shift, the calculated wall perturbation correction as a function of depth is varying less than 1% from zero up to the half value depth for electron energies in the range of 6–21 MeV. The remaining depth dependence can mainly be attributed to the scatter properties of the entrance window. When neglecting the nonwater equivalence of the entrance window, the variation of with depth is up to 10% and more, especially for low electron energies.Conclusions:
The variation of the wall perturbation correction for the NACP-02 chamber in clinical electron beams strongly depends on the positioning of the chamber. Applying a shift toward the focus ensures that the primary electron spectrum within the chamber bears the largest resemblance to the fluence of a wall-less cavity. Hence, the influence of the chamber walls on the perturbation correction can be separated out and the residual variation of with depth is minimized.
Localizing intracavitary brachytherapy applicators from cone-beam CT x-ray projections via a novel iterative forward projection matching algorithm38(2011); http://dx.doi.org/10.1118/1.3544661View Description Hide DescriptionPurpose:
To present a novel method for reconstructing the 3D pose (position and orientation) of radio-opaque applicators of known but arbitrary shape from a small set of 2D x-ray projections in support of intraoperative brachytherapy planning.Methods:
The generalized iterative forward projection matching (gIFPM) algorithm finds the six degree-of-freedom pose of an arbitrary rigid object by minimizing the sum-of-squared-intensity differences (SSQD) between the computed and experimentally acquired autosegmented projection of the objects. Starting with an initial estimate of the object’s pose, gIFPM iteratively refines the pose parameters (3D position and three Euler angles) until the SSQD converges. The object, here specialized to a Fletcher–Weeks intracavitary brachytherapy (ICB) applicator, is represented by a fine mesh of discrete points derived from complex combinatorial geometric models of the actual applicators. Three pairs of computed and measured projection images with known imaging geometry are used. Projection images of an intrauterine tandem and colpostats were acquired from an ACUITY cone-beam CT digital simulator. An image postprocessing step was performed to create blurred binary applicators only images. To quantify gIFPM accuracy, the reconstructed 3D pose of the applicator model was forward projected and overlaid with the measured images and empirically calculated the nearest-neighbor applicator positional difference for each image pair.Results:
In the numerical simulations, the tandem and colpostats positions and orientations were estimated with accuracies of 0.6 mm and 2°, respectively. For experimentally acquired images of actual applicators, the residual 2D registration error was less than 1.8 mm for each image pair, corresponding to about 1 mm positioning accuracy at isocenter, with a total computation time of less than 1.5 min on a 1 GHz processor.Conclusions:
This work describes a novel, accurate, fast, and completely automatic method to localize radio-opaque applicators of arbitrary shape from measured 2D x-ray projections. The results demonstrate accuracy while compared against the measured applicator projections. No lateral film is needed. By localizing the applicator internal structure as well as radioactive sources, the effect of intra-applicator and interapplicator attenuation can be included in the resultant dose calculations. Further validation tests using clinically acquired tandem and colpostats images will be performed for the accurate and robust applicator/sources localization in ICB patients.