Volume 37, Issue 9, September 2010
- medical physics letters
- task group report
- radiation therapy physics
- radiation imaging physics
- radiation measurement physics
- magnetic resonance physics
- nuclear medicine physics
- optical physics
- ultrasound physics
- thermotherapy physics
- tissue measurements
- anatomy and physiology
- radiation biology
- books and publications
Index of content:
Ultrasonography is soon likely to become a viable alternative to x-ray mammography for breast cancer screening37(2010); http://dx.doi.org/10.1118/1.3459019View Description Hide Description
- MEDICAL PHYSICS LETTERS
37(2010); http://dx.doi.org/10.1118/1.3483096View Description Hide DescriptionPurpose:
The feasibility of a practical solid-state technology for low photon flux imaging applications was investigated. The technology is based on an amorphous selenium photoreceptor with a voltage-controlled avalanche multiplication gain. If this photoreceptor can provide sufficient internal gain, it will be useful for an extensive range of diagnostic imaging systems.Methods:
The avalanchephotoreceptor under investigation is referred to as HARP-DRL. This is a novel concept in which a high-gain avalanche rushing photoconductor (HARP) is integrated with a distributed resistance layer (DRL) and sandwiched between two electrodes. The avalanche gain and leakage current characteristics of this photoreceptor were measured.Results:
HARP-DRL has been found to sustain very high electric field strengths without electrical breakdown. It has shown avalanche multiplication gains as high as and a very low leakage current .Conclusions:
This is the first experimental demonstration of a solid-stateamorphousphotoreceptor which provides sufficient internal avalanche gain for photon counting and photon starved imaging applications.
- TASK GROUP REPORT
37(2010); http://dx.doi.org/10.1118/1.3462971View Description Hide Description
Helical tomotherapy is a relatively new modality with integrated treatment planning and delivery hardware for radiation therapy treatments. In view of the uniqueness of the hardware design of the helical tomotherapy unit and its implications in routine quality assurance, the Therapy Physics Committee of the American Association of Physicists in Medicine commissioned Task Group 148 to review this modality and make recommendations for quality assurance related methodologies. The specific objectives of this Task Group are: (a) To discuss quality assurance techniques, frequencies, and tolerances and (b) discuss dosimetric verification techniques applicable to this unit. This report summarizes the findings of the Task Group and aims to provide the practicing clinical medical physicist with the insight into the technology that is necessary to establish an independent and comprehensive quality assurance program for a helical tomotherapy unit. The emphasis of the report is to describe the rationale for the proposed QA program and to provide example tests that can be performed, drawing from the collective experience of the task group members and the published literature. It is expected that as technology continues to evolve, so will the test procedures that may be used in the future to perform comprehensive quality assurance for helical tomotherapy units.
- RADIATION THERAPY PHYSICS
Automated registration of diagnostic to prediagnostic x-ray mammograms: Evaluation and comparison to radiologists’ accuracy37(2010); http://dx.doi.org/10.1118/1.3457470View Description Hide DescriptionPurpose:
To compare and evaluate intensity-based registration methods for computation of serial x-raymammogram correspondence.Methods:
X-raymammograms were simulated from MRIs of 20 women using finite element methods for modeling breast compressions and employing a MRI/x-ray appearance change model. The parameter configurations of three registration methods, affine, fluid, and free-form deformation (FFD), were optimized for registering x-raymammograms on these simulated images. Five mammography film readers independently identified landmarks (tumor, nipple, and usually two other normal features) on pairs of diagnostic and corresponding prediagnostic digitized images from 52 breast cancer cases. Landmarks were independently reidentified by each reader. Target registration errors were calculated to compare the three registration methods using the reader landmarks as a gold standard. Data were analyzed using multilevel methods.Results:
Between-reader variability varied with landmark and screen , with between-reader mean distance (mm) in point location on the diagnostic/prediagnostic images of 2.50 (95% CI 1.95, 3.15)/2.84 (2.24, 3.55) for nipples and 4.26 (3.43, 5.24)/4.76 (3.85, 5.84) for tumors. Registration accuracy was sensitive to the type of landmark and the amount of breast density. For dense breasts , the affine and fluid methods outperformed FFD. For breasts with lower density, the affine registration surpassed both fluid and FFD. Mean accuracy (mm) of the affine registration varied between 3.16 (95% CI 2.56, 3.90) for nipple points in breasts with density 20%–39% and 5.73 (4.80, 6.84) for tumor points in breasts with density .Conclusions:
Affine registration accuracy was comparable to that between independent film readers. More advanced two-dimensional nonrigid registration algorithms were incapable of increasing the accuracy of image alignment when compared to affine registration.
37(2010); http://dx.doi.org/10.1118/1.3463609View Description Hide DescriptionPurpose:
To develop a new metric for image registration that incorporates the (sub)pixelwise differential importance along spatial location and to demonstrate its application for image guided radiation therapy(IGRT).Methods:
It is well known that rigid-body image registration with mutual information is dependent on the size and location of the image subset on which the alignment analysis is based [the designated region of interest (ROI)]. Therefore, careful review and manual adjustments of the resulting registration are frequently necessary. Although there were some investigations of weighted mutual information (WMI), these efforts could not apply the differential importance to a particular spatial location since WMI only applies the weight to the joint histogram space. The authors developed the spatially weighted mutual information (SWMI) metric by incorporating an adaptable weight function with spatial localization into mutual information. SWMI enables the user to apply the selected transform to medically “important” areas such as tumors and critical structures, so SWMI is neither dominated by, nor neglects the neighboring structures. Since SWMI can be utilized with any weight function form, the authors presented two examples of weight functions for IGRT application: A Gaussian-shaped weight function (GW) applied to a user-defined location and a structures-of-interest (SOI) based weight function. An image registration example using a synthesized 2D image is presented to illustrate the efficacy of SWMI. The convergence and feasibility of the registration method as applied to clinical imaging is illustrated by fusing a prostate treatment planning CT with a clinical cone beam CT(CBCT)image set acquired for patient alignment. Forty-one trials are run to test the speed of convergence. The authors also applied SWMI registration using two types of weight functions to two head and neck cases and a prostate case with clinically acquired CBCT/MVCT image sets. The SWMI registration with a Gaussian weight function (SWMI-GW) was tested between two different imaging modalities: CT and MRIimage sets.Results:
SWMI-GW converges 10% faster than registration using mutual information with an ROI. SWMI-GW as well as SWMI with SOI-based weight function (SWMI-SOI) shows better compensation of the target organ’s deformation and neighboring critical organs’ deformation. SWMI-GW was also used to successfully fuse MRI and CTimages.Conclusions:
Rigid-body image registration using our SWMI-GW and SWMI-SOI as cost functions can achieve better registration results in (a) designated image region(s) as well as faster convergence. With the theoretical foundation established, we believe SWMI could be extended to larger clinical testing.
Special report: Workshop on 4D-treatment planning in actively scanned particle therapy—Recommendations, technical challenges, and future research directions37(2010); http://dx.doi.org/10.1118/1.3475944View Description Hide Description
This article reports on a 4D-treatment planning workshop (4DTPW), held on 7–8 December 2009 at the Paul Scherrer Institut (PSI) in Villigen, Switzerland. The participants were all members of institutions actively involved in particle therapy delivery and research. The purpose of the 4DTPW was to discuss current approaches, challenges, and future research directions in 4D-treatment planning in the context of actively scanned particle radiotherapy. Key aspects were addressed in plenary sessions, in which leaders of the field summarized the state-of-the-art. Each plenary session was followed by an extensive discussion. As a result, this article presents a summary of recommendations for the treatment of mobile targets (intrafractional changes) with actively scanned particles and a list of requirements to elaborate and apply these guidelines clinically.
Estimation of organ doses from kilovoltage cone-beam CT imaging used during radiotherapy patient position verification37(2010); http://dx.doi.org/10.1118/1.3476459View Description Hide DescriptionPurpose:
The purpose of this study was to develop a practical method for estimating organdoses from kilovoltage cone-beam CT(CBCT) that can be performed with readily available phantoms and dosimeters. The accuracy of organdose estimates made using the ImPACT patient dose calculator was also evaluated.Methods:
A 100 mm pencil chamber and standard CTdose index (CTDI) phantoms were used to measure the cone-beam dose index (CBDI). A weighted CBDI was then calculated from these measurements to represent the average volumetric dose in the CTDI phantom. By comparing to the previously published organdoses,organdose conversion coefficients were developed. The measured CBDI values were also used as inputs for the ImPACT calculator to estimate organdoses. All CBDI dosemeasurements were performed on both the Elekta XVI and Varian OBI at three clinically relevant locations: Head, chest, and pelvis.Results:
The head, chest, and pelvis protocols yielded values of 0.98, 16.62, and 24.13 mGy for the XVI system and 5.17, 6.14, and 21.57 mGy for the OBI system, respectively. Organdoses estimated with the ImPACTCTdose calculator showed a large range of variation from the previously measuredorgandoses, demonstrating its limitations for use with CBCT.Conclusions:
The organdose conversion coefficients developed in this work relate values to organdoses previously measured using the same clinical protocols. Ultimately, these coefficients will allow for the quick estimation of organdoses from routine measurements performed using standard CTDI phantoms and pencil chambers.
37(2010); http://dx.doi.org/10.1118/1.3476467View Description Hide DescriptionPurpose:
This article presents an improved pencil-beam dose calculation formalism based on an experimental kernel obtained by deconvolution. The new algorithm makes it possible to calculate the absorbed dose for all field sizes.Methods:
The authors have enhanced their previous work [J. D. Azcona and J. Burguete, Med. Phys.35, 248–259 (2008)] by correcting the kernel tail representing the contribution to the absorbed dose far from the photon interaction point. The correction was performed by comparing the calculated and measured output factors. Dose distributions and absolute dose values calculated using the new formalism have been compared to measurements. The agreement between calculated and measured dose distributions was evaluated according to the -index criteria. In addition, 35 individual intensity-modulated radiation therapy(IMRT) fields were calculated and measured in polystyrene using an ionization chamber. Furthermore, a series of 541 IMRT fields was calculated using the algorithm proposed here and using a commercial IMRT optimization and calculation software package. Comparisons were made between the calculations at single points located at the isocenter for all the beams, as well as between beams grouped by anatomic location.Results:
The percentage of points passing the-index criteria (3%, 3 mm) when comparing calculated and measured dose distributions is generally greater than 99% for the cases studied. The agreement between the calculations and the experimental measurements generally lies in the ±2% interval for single points, with a mean value of 0.2%. The agreement between calculations using the proposed algorithm and using a commercial treatment planning system is also between ±5%.Conclusions:
An improved algorithm based on an experimental pencil-beam kernel obtained by deconvolution has been developed. It has been validated clinically and promises to be a valuable tool for IMRT quality assurance as an independent calculation system for monitor units and dose distributions. An important point is that the algorithm presented here uses an experimental kernel, which is therefore independent of Monte-Carlo-calculated kernels.
Real-time monitoring and control on deep inspiration breath-hold for lung cancer radiotherapy—Combination of ABC and external marker tracking37(2010); http://dx.doi.org/10.1118/1.3476463View Description Hide DescriptionPurpose:
In this article, the breath-hold and gating concepts were combined for application of lungcancerradiation treatment. The tumor movement was immobilized based on deep inspiration breath hold (DIBH), in which the breath-hold consistency and stability were monitored by infrared (IR) tracking and controlled by gating with a predefined threshold. The authors’ goal is to derive the benefits from both techniques, namely, the minimized treatment margin and the known advantages of deep inspiration. The efficacy of the technique in terms of tumor immobility and treatment setup accuracy was evaluated in the study.Methods:
Fourteen patients who were diagnosed with non small cell lungcancer were included in this study. The control of tumor immobility was investigated interfractionally and intrafractionally. The intrabreath-hold tumor motion was devised based on the external marker movement, in which the tumor-marker correlation was studied. The margin of the planning target volume (PTV) was evaluated based on two factors: (1) The treatment setup error accounts for the patient setup and interbreath-hold variations and (2) the intrabreath-hold tumor motion in which the residual tumor motion during irradiation was studied.Results:
As the result of the study, the group systematic error and group random error of treatment setup measured at the isocenter were, , and in the left-right (LR), anterior-posterior (AP), and caudal-cranial (CC) directions, respectively. The Pearson correlation coefficient were 0.81 (LR), 0.76 (AP), and 0.85 (CC) mm and suggest tendency in linear correlation of tumor and marker movement. The intrabreath-hold tumor was small in all directions. The group PTV margins of 3.8 (LR), 4.6 (AP), and 4.8 (CC) mm were evaluated to account for both setup errors and residual tumor motion during irradiation.Conclusions:
The study applies the DIBH technique in conjunction with IR positional tracking for tumor immobilization and treatment setup localization. The technique not only proved to be reliable in terms of good tumor immobility and accurate treatment positioning but also to be potentially useful for dose escalation treatment as regarding of the substantially reduced PTV margin and minimizing radiation toxicity from the fully expanded lung volume.
Dosimetric characterization of a multileaf collimator for a new four-dimensional image-guided radiotherapy system with a gimbaled x-ray head, MHI-TM2000a)37(2010); http://dx.doi.org/10.1118/1.3480510View Description Hide DescriptionPurpose:
To present the dosimetric characterization of a multileaf collimator(MLC) for a new four-dimensional image-guidedradiotherapy system with a gimbaled x-ray head, MHI-TM2000.Methods:
MHI-TM2000 has an x-ray head composed of an ultrasmall linear accelerator guide and a system-specific MLC. The x-ray head can rotate along the two orthogonal gimbals (pan and tilt rotations) up to ±2.5°, which swings the beam up to ±41.9 mm in each direction from the isocenter on the isocenter plane perpendicular to the beam. The MLC design is a single-focus type, has 30 pairs of 5 mm thick leaves at the isocenter, and produces a maximum field size of. Leaf height and length are 110 and 260 mm, respectively. Each leaf end is circular, with a radius of curvature of 370 mm. The distance that each leaf passes over the isocenter is 77.5 mm. Radiation leakage between adjacent leaves is minimized by an interlocking tongue-and-groove (T&G) arrangement with the height of the groove part 55 mm. The dosimetric characterizations including field characteristics, leaf position accuracy, leakage, and T&G effect were evaluated using a well-commissioned 6 MV photon beam, EDR2 films (Kodak, Rochester, NY), and water-equivalent phantoms. Furthermore, the field characteristics and leaf position accuracy were evaluated under conditions of pan or tilt rotation.Results:
The differences between nominal and measured field sizes were within ±0.5 mm. Although the penumbra widths were greater with wider field size, the maximum width was even for the fully opened field. Compared to the results of field characteristics without pan or tilt rotation, the variation in field size, penumbra width, flatness, and symmetry was within ±1 mm/1% at the maximum pan or tilt rotational angle. The leaf position accuracy was , ranging from −0.3 to 0.2 mm at four gantry angles of 0°, 90°, 180°, and 270° with and without pan or tilt rotation. The interleaf leakage was up to 0.21%, whereas the intraleaf leakage was . T&G decreased the doses by 10.7%, on average.Conclusions:
This study demonstrated that MHI-TM2000 has the capability for high leaf position accuracy and low leakage, leading to highly accurate intensity-modulated radiotherapy delivery. Furthermore, substantial changes in the dosimetric data on field characteristics and leaf position accuracy were not observed even at the maximum pan or tilt rotation.
37(2010); http://dx.doi.org/10.1118/1.3475942View Description Hide DescriptionPurpose
: There is interest in developing linac-MR systems for MRI-guided radiation therapy. To date, the designs for such linac-MR devices have been restricted to a transverse geometry where the static magnetic field is oriented perpendicular to the direction of the incident photon beam. This work extends possibilities in this field by proposing and examining by Monte Carlo simulations, a probable longitudinal configuration where the magnetic field is oriented in the same direction as the photon beam.Methods
: The EGSnrc Monte Carlo(MC)radiation transport codes with algorithms implemented to account for the magnetic field deflection of charged particles were used to compare dose distributions for linac-MR systems in transverse and longitudinal geometries. Specifically, the responses to a 6 MV pencil photon beam incident on water and lung slabs were investigated for 1.5 and 3.0 T magnetic fields. Further a five field lung plan was simulated in the longitudinal and transverse geometries across a range of magnetic field strengths from 0.2 through 3.0 T.Results
: In a longitudinal geometry, the magnetic field is shown to restrict the radial spread of secondary electrons to a small degree in water, but significantly in low density tissues such as lung in contrast to the lateral shift in dose distribution seen in the transverse geometry. These effects extend to the patient case, where the longitudinal configuration demonstrated dose distributions more tightly confined to the primary photon fields, which increased dose to the planning target volume (PTV), bettered dose homogeneity within a heterogeneous (in density) PTV, and reduced the tissue interface effects associated with the transverse geometry.Conclusions
: Dosimetry issues observed in a transverse linac-MR geometry such as changes to the depth dose distribution and tissue interface effects were significantly reduced or eliminated in a longitudinal geometry on a representative lung plan. Further, an increase in dose to the PTV, resulting from the magnetic field confining electrons to the forward direction, shows potential for a reduction in dose to the surrounding tissues.
Dosimetric impact of an brachytherapy source cable length modeled using a grid-based Boltzmann transport equation solver37(2010); http://dx.doi.org/10.1118/1.3478278View Description Hide DescriptionPurpose:
To evaluate the dose distributions of an source (model VS2000) in homogeneous water geometry calculated using a deterministic grid-based Boltzmann transport equation solver (GBBS) in the commercial treatment planning system (TPS) (BRACHYVISION-ACUROS v8.8).Methods:
Using percent dose differences, the GBBS (BV-ACUROS) was compared to the (1) published TG-43 data, (2) MCNPXMonte Carlo(MC) simulations of the source centered in a 15 cm radius water sphere, and (3) TG-43 output from the TPS using vendor supplied (BV-TG43-vendor) and user extended (BV-TG43-extended) 2D anisotropy functions . BV-ACUROS assumes 1 mm of NiTi cable, while the TPS TG-43 algorithm uses data based on a 15 cm cable. MC models of various cable lengths were simulated.Results:
The MC simulations resulted indose deviations along the cable for 1, 2, and 3 mm cable lengths relative to 15 cm. BV-ACUROS comparisons with BV-TG43-vendor and BV-TG43-extended yielded magnitude of differences, consistent with those seen in MC simulations. However, differences extended further when using the vendor supplied anisotropy function . These differences were also seen in comparisons of derived from the TPS output.Conclusions:
The results suggest that near the cable region is larger than previously estimated. The spatial distribution of the dose deviation is highly dependent on the reference TG-43 data used to compare to GBBS. The differences observed, while important to realize, should not have an impact on clinical dosimetry in homogeneous water.
37(2010); http://dx.doi.org/10.1118/1.3480481View Description Hide DescriptionPurpose:
Due to the close proximity of the linear accelerator(linac) to the magnetic resonance(MR)imager in linac-MR systems, it will be subjected to magnet fringe fields larger than the Earth’s magnetic field of. Even with passive or active shielding designed to reduce these fields, some magnitude of the magnetic field is still expected to intersect the linac, causing electron deflection and beam loss. This beam loss, resulting from magnetic fields that cannot be eliminated with shielding, can cause a detuning of the waveguide due to excessive heating. The detuning, if significant, could lead to an even further decrease in output above what would be expected strictly from electron deflections caused by an external magnetic field. Thus an investigation of detuning was performed through various simulations.Methods:
According to the Lorentz force, the electrons will be deflected away from their straight course to the target, depositing energy as they impact the linaccopper waveguide. The deposited energy would lead to a heating and deformation of the copper structure resulting in resonant frequency changes.PARMELA was used to determine the mean energy and fraction of total beam lost in each linaccavity. The energy deposited into the copper waveguide from the beam losses caused by transverse magnetic fields was calculated using the Monte Carlo program DOSRZnrc. From the total energy deposited, the rise in temperature and ultimately the deformation of the structure was estimated. The deformed structure was modeled using the finite element method program COMSOL MULTIPHYSICS to determine the change in cavityresonant frequency.Results:
The largest changes in resonant frequency were found in the first two accelerating cavities for each field strength investigated. This was caused by a high electron fluence impacting the waveguide inner structures coupled with their low kinetic energies. At each field strength investigated, the total change in accelerator frequency was less than a manufacturing tolerance of 10 kHz and is thus not expected to have a noticeable effect on accelerator performance.Conclusions:
The amount of beam loss caused by magnetic fringe fields for a linac in a linac-MR system depends on the effectiveness of its magnetic shielding. Despite the best efforts to shield the linac from the magnetic fringe fields, some persistent magnetic field is expected which would result in electron beam loss. This investigation showed that the detuning of the waveguide caused by additional electron beam loss in persistent magnetic fields is not a concern.
37(2010); http://dx.doi.org/10.1118/1.3480482View Description Hide DescriptionPurpose:
The integration of a low field biplanar magnetic resonance(MR)imager and linear accelerator(linac) causes magnetic interference at the linac due to the MR fringe fields. In order to eliminate this interference, passive and active magnetic shielding designs are investigated.Methods:
The optimized design of passive magnetic shielding was performed using the finite element method. The design was required to achieve no greater than a 20% electron beam loss within the linac waveguide and electron gun, no greater than 0.06 T at the multileaf collimator(MLC) motors, and generate a distortion of the main MRimaging volume of no greater than 300 ppm. Through the superposition of the analytical solution for a single current carrying wire loop, active shielding designs in the form of three and four sets of coil pairs surrounding the linac waveguide and electron gun were also investigated. The optimized current and coil center locations that yielded the best cancellation of the MR fringe fields at the linac were determined using sequential quadratic programming.Results:
Optimized passive shielding in the form of two steel cylinders was designed to meet the required constraints. When shielding the MLC motors along with the waveguide and electron gun, the thickness of the cylinders was less than 1 mm. If magnetically insensitive MLC motors are used, no MLC shielding would be required and the waveguide shield (shielding the waveguide and electron gun) became 1.58 mm thick. In addition, the optimized current and coil spacing for active shielding was determined for both three and four coil pair configurations. The results of the active shielding optimization produced no beam loss within the waveguide and electron gun and a maximum MR field distortion of 91 ppm over a 30 cm diameter spherical volume.Conclusions:
Very simple passive and active shielding designs have been shown to magnetically decouple the linac from the MRimager in a low field biplanar linac-MR system. The MLC passive shielding produced the largest distortion of the MR field over the imaging volume. With the use of magnetically insensitive motors, the MR field distortion drops substantially since no MLC shield is required. The active shielding designs yielded no electron beam loss within the linac.
37(2010); http://dx.doi.org/10.1118/1.3476413View Description Hide DescriptionPurpose:
Intensity modulated radiation therapy(IMRT)treatmentdelivery requires higher precision than conventional 3D treatmentdelivery because of the sensitivity of the resulting dose to small geometric misalignment of the modulated beamlets. The chosen treatmentdelivery technique will affect the treatment precision in different ways, based on the characteristics of the delivery method. Delivery using a multileaf collimator(MLC) can reduce treatment time and therapist workload, but typically requires a greater number of monitor units and the fields are prone to both systematic and random leaf positioning errors. An alternative to MLC-based fields, patient specific brass compensators, do not suffer from these leaf positioning errors. In our study, we set out to investigate which delivery method will provide the highest levels of dosimetric reproducibility and the minimum amount of interfraction variability.Methods:
In our study, a seven field IMRT plan for a head and neck treatment was created using the Pinnacle3treatment planning system and the intensity maps for each field were obtained. The intensity maps of the fields were delivered with a Varian 2100C/D linear accelerator, using solid compensators and sliding window (SW) and step-and-shoot (SS) MLC segments. Three fields were selected from the seven-beam IMRT plan for comparison. Analysis was carried out using the MatriXX ion chamber array, radiochromic film, and Varian dynalog files.Results:
Our results show that the error in MLC leaf positioning has no gantry angle dependence. The compensator and SW deliveries showed excellent agreement, even when stricter than usual gamma criteria were applied. However, we noted that under these strict conditions, the SS fields had at least ten times more pixels out of range than did the compensators. When using step-and-shoot MLC fields, it was observed that the increase in dose rate or the increase of MU/segment degrades the quality of the plan. Analysis of the dynalog files showed that while each individual field had its own propensity for error, all fields showed the same trend: a greater percentage of time the leaves are out of position as dose rate increases, MUs decrease, or both.Conclusions:
The compensator-based field and both types of MLC-based fields have MatriXX results that are within the clinically acceptable tolerance of 3% dose difference and 2 mm DTA. However, when the criteria are tightened, it becomes evident that the compensators have a definite advantage over their comparable MLC-based competitors in terms of interfraction reproducibility. Fewer monitor units are required to deliver each portal, potentially improving patient outcomes and reducing unwanted side effects to both patients and therapists. In centers without MLC, compensators represent a simple and cost effective way to offer patients state of the art treatment. Based on the results of this study, compensator-based IMRT is a reliable, viable option for use in clinics both with and without MLC-equipped linacs.
37(2010); http://dx.doi.org/10.1118/1.3480964View Description Hide DescriptionPurpose:
The authors propose an algorithm based on the k-d tree for nearest neighbor searching to improve the calculation time for 2D and 3D dose distributions.Methods:
The calculation method has been widely used for comparisons of dose distributions in clinical treatment plans and quality assurances. By specifying the acceptable dose and distance-to-agreement criteria, the method provides quantitative measurement of the agreement between the reference and evaluation dose distributions. The value indicates the acceptability. In regions where , the predefined criterion is satisfied and thus the agreement is acceptable; otherwise, the agreement fails. Although the concept of the method is not complicated and a quick naïve implementation is straightforward, an efficient and robust implementation is not trivial. Recent algorithms based on exhaustive searching within a maximum radius, the geometric Euclidean distance, and the table lookup method have been proposed to improve the computational time for multidimensional dose distributions. Motivated by the fact that the least searching time for finding a nearest neighbor can be an operation with a k-d tree, where is the total number of the dose points, the authors propose an algorithm based on the k-d tree for the evaluation in this work.Results:
In the experiment, the authors found that the average k-d tree construction time per reference point is, while the nearest neighbor searching time per evaluation point is proportional to , where is between 2 and 3 for two-dimensional and three-dimensional dose distributions, respectively.Conclusions:
Comparing with other algorithms such as exhaustive search and sorted list, the k-d tree algorithm for evaluation is much more efficient.
37(2010); http://dx.doi.org/10.1118/1.3481512View Description Hide DescriptionPurpose:
In moving target irradiation with pencil beam scanning, the interplay effect between the target motion and the scanned beam is a problem because this effect causes over or under dosage in the target volume. To overcome this, we have studied rescanning using a gating technique.Methods:
A simulation and experimental study was carried out. In the experiment, we used the fast scanning system developed at the HIMAC to verify the validity of phase controlled rescanning method, in which the time for rescanning irradiation of each slice is matched to the gating duration.Results:
Simulation and experimental results showed that controlling the scan speed to match the respiration cycle with rescans can deliver the blurred dose distribution. In the comparison between the static measurements and the moving measurements with the phase controlled rescanning method, the dose difference was less than 2% for pinpoint chambers in the target volume.Conclusions:
The simulation and experimental results demonstrated that the phase controlled rescanning method makes it possible to deliver the dose distribution close to the expected one. As an experimental result for 3D irradiation, it was estimated that blurring by the probability density function was not only for a lateral distribution, but also for a distal distribution, even in the lateral rescanning.
37(2010); http://dx.doi.org/10.1118/1.3481515View Description Hide DescriptionPurpose:
Single photon emission computed tomography(SPECT) is being investigated for imaging inside radiation therapy treatment rooms to localize biological targets. Here, computer simulations were used to analyze locational and directional dependencies in localization errors and to assess the effects of spatial resolution modeling and observer normalization on localization performance.Methods:
SPECTimages of the XCAT phantom, containing 12 hot tumors, were reconstructed with detector response function compensation (DRC) and without DRC (nDRC). Numerical observers were forced to select the most suspicious tumor location, using normalized cross correlation (NXC) or un-normalized cross correlation (XC), from 3 cm diameter search volumes that each contained only one tumor. For each tumor site, localization was optimized as a function of the iteration number and postreconstruction smoothing. Localization error, the distance between true and estimated tumor positions, was calculated across the ensembles of 80 images. Direction-dependent localization bias and precision were estimated from the image ensemble.Results:
For the six superficial tumors in close proximity to the detector trajectory, mean localization errors were and were lowest or comparable using DRC-NXC, though differences from DRC-XC and nDRC-NXC were not statistically significant. DRC-NXC did provide statistically significantly better localization than nDRC-XC for five of these six tumors. At the other six sites where attenuation was more severe and the distance was generally greater between the tumor and detector, DRC typically did not show better localization than nDRC. Observer normalization improved the localization substantially for a tumor near the hotter heart. Localization errors were anisotropic and dependent on tumor location relative to the detector trajectory.Conclusions:
This computer-simulation study compared localization performance for normalized and un-normalized numerical observers, which were used to estimate tumor positions in SPECTimages,reconstructed with and without DRC. For tumors localized to on average, which are good candidates for SPECT-guided radiation therapy, localization performance typically improved by compensating for the detector response function and by using a normalized observer. The observed direction-dependent localization errors have important implications for radiation therapy and are relevant to SPECTimaging in general.
37(2010); http://dx.doi.org/10.1118/1.3481513View Description Hide DescriptionPurpose:
Linac-magnetic resonance (MR) systems have been proposed in order to achieve real-time image guided radiotherapy. The design of a new linac-MR system with the in-line 6 MV linac generating x-rays along the symmetry axis of an open MRimager is outlined. This new design allows for a greater MR field strength to achieve better quality images while reducing hot and cold spots in treatment planning. An investigation of linac’s performance in the longitudinal fringe magnetic fields of the MRimager is given.Methods:
The open MRimager fringe magnetic field was modeled using the analytic solution of the magnetic field generated from current carrying loops. The derived solution was matched to the magnetic fringe field isolines provided for a 0.5 T open MRimager through Monte Carlo optimization. The optimized field solution was then added to the previously validated 6 MV linac simulation to quantify linac’s performance in the fringe magnetic field of a 0.5 T MRimager. To further the investigation, linac’s performance in large fringe fields expected from other imagers was investigated through the addition of homogeneous longitudinal fields.Results:
The Monte Carlo optimization of the analytic current loop solution provided good agreement with the magnetic fringe field isolines supplied by the manufacturer. The range of magnetic fields the linac is expected to experience when coupled to the 0.5 T MRimager was determined to be from 0.0022 to 0.011 T (as calculated at the electron gun cathode). The effect of the longitudinal magnetic field on the electron beam was observed to be only in the electron gun. The longitudinal field changed the electron gun optics, affecting beam characteristics, such as a slight increase in the injection current and beam diameter, and an increasingly nonlaminar transverse phase space. Although the target phase space showed little change in its energy spectrum from the altered injection phase space, a reduction in the target current and spatial distribution peak intensity was observed. Despite these changes, the target phase space had little effect on the depth dose curves or dose profiles calculated for a field at 1.5 cm depth. At longitudinal fields larger than 0.012 T, a drastic reduction in the injection current from the electron gun was observed due to a large fraction of electrons striking the anode. This further reduced the target current, which reached a minimum of at 0.06 T. A slow increase in the injection and target currents was observed at fields larger than 0.06 T due to greater beam collimation in the anode beam tube.Conclusions:
In an effort to achieve higher quality images and a reduction in hot and cold spots in the treatment plan, a parallel configuration linac-MR system is presented. The longitudinal magnetic fields of the MRimager caused large beam losses within the electron gun. These losses may be eliminated through a redesign of the electron gun optics incorporating a longitudinal magnetic field, or through magnetic shielding, which has already been proven successful for the transverse configuration.