Volume 39, Issue 4, April 2012
- vision 20/20
- radiation therapy physics
- radiation imaging physics
- radiation measurement physics
- magnetic resonance physics
- nuclear medicine physics
- optical physics
- ultrasound physics
- thermotherapy physics
- tissue measurements
- books and publications
- task group report
Index of content:
Accurate cardiac deformation analysis for cardiac displacement and strain imaging over time requires Lagrangian description of deformation of myocardial tissue structures. Failure to couple the estimated displacement and strain information with the correct myocardial tissue structures will lead to erroneous result in the displacement and strain distribution over time.Methods:
Lagrangian based tracking in this paper divides the tissue structure into a fixed number of pixels whose deformation is tracked over the cardiac cycle. An algorithm that utilizes a polar-grid generated between the estimated endocardial and epicardial contours for cardiac short axis images is proposed to ensure Lagrangian description of the pixels. Displacement estimates from consecutive radiofrequency frames were then mapped onto the polar grid to obtain a distribution of the actual displacement that is mapped to the polar grid over time.Results:
A finite element based canine heart model coupled with an ultrasound simulation program was used to verify this approach. Segmental analysis of the accumulated displacement and strain over a cardiac cycle demonstrate excellent agreement between the ideal result obtained directly from the finite element model and our Lagrangian approach to strain estimation. Traditional Eulerian based estimation results, on the other hand, show significant deviation from the ideal result. Anin vivo comparison of the displacement and strain estimated using parasternal short axis views is also presented.Conclusions:
Lagrangian displacement tracking using a polar grid provides accurate tracking of myocardial deformation demonstrated using both finite element andin vivo radiofrequency data acquired on a volunteer. In addition to the cardiac application, this approach can also be utilized for transverse scans of arteries, where a polar grid can be generated between the contours delineating the outer and inner wall of the vessels from the blood flowing though the vessel.
39(2012); http://dx.doi.org/10.1118/1.3681013View Description Hide Description
- VISION 20/20
39(2012); http://dx.doi.org/10.1118/1.3691903View Description Hide Description
Radiation therapy using high-energy charged particles is generally acknowledged as a powerful new technique in cancertreatment. However, particle therapy in oncology is still controversial, specifically because it is unclear whether the putative clinical advantages justify the high additional costs. However, particle therapy can find important applications in the management of noncancer diseases, especially in radiosurgery. Extension to other diseases and targets (both cranial and extracranial) may widen the applications of the technique and decrease the cost/benefit ratio of the accelerator facilities. Future challenges in this field include the use of different particles and energies, motion management in particle body radiotherapy and extension to new targets currently treated by catheter ablation (atrial fibrillation and renal denervation) or stereotactic radiation therapy (trigeminal neuralgia, epilepsy, and macular degeneration). Particle body radiosurgery could be a future key application of accelerator-based particle therapy facilities in 10 years from today.
- RADIATION THERAPY PHYSICS
RapidArc patient specific mechanical delivery accuracy under extreme mechanical limits using linac log files39(2012); http://dx.doi.org/10.1118/1.3690464View Description Hide DescriptionPurpose:
To assess the accuracy of RapidArc (RA) delivery for treatment machine operation near allowable mechanical limits in dynamic multileaf collimator (DMLC) leaf velocities, gantry speeds, and dose rates.Methods:
Thirty RA patient plans were created for treatment of lung, gastrointestinal, and head and neck cancers on a Trilogy unit. For each patient, three RA plans were generated; one with medium MLC velocities, highest gantry speeds, and dose rates (case A); one with maximal allowable MLC leaf velocities (case B); and one with lowest gantry speeds (case C). Combinations of dose rates (140–600 MU/min), gantry speeds (2–5.4°/s), and DMLC leaf velocities (1.3–2.4 cm/s) were utilized to test the RapidArc delivery accuracy. Linacdelivery log files were acquired after delivery of each plan. In-house developed software was used to read in the original RapidArc DICOM plan and update the plan to reflect the delivered plan by using the leaf position (L), gantry position (G), and MU dose values (D) extracted from the linac log files. This modified DICOM RT plan was imported back to ECLIPSE and the delivered 3D dose map recomputed. Finally, the planned and delivered 3D isodose maps were compared under three criteria to evaluate the dosimetric differences: maximum percentage dose difference, 3D gamma analysis criteria for 3%/3mm DTA, number of dose voxels having a dose difference that is greater than 1%, 2%, or 3% of the maximum dose, and their respective percentages.Results:
For the three cases indicated above, MLC leaf position discrepancies between planned and delivered values are 0.8 ± 0.2, 1.2 ± 0.2, and 0.8 ± 0.2 mm; the maximum gantry position discrepancies are 0.9° ± 0.2°, 0.9° ± 0.2°, and 0.6° ± 0.1°, and the maximum differences in delivered MU per control point are 0.2 ± 0.1, 0.2 ± 0.1, and 0.04 ± 0.01, respectively. Maximum percentage dose difference observed is 6.7%, for a case where 1 cm MLC leaves were used with high MLC leaf velocity. Maximum number (percentage) of dose voxels having a dose difference that is greater than 1%, 2%, and 3% of the maximum dose were 4761 (0.35%), 897 (0.07%), and 188 (0.01%). This also corresponds to the plan utilizing the most number of 1 cm MLC leaves. The 3D Gamma factor acceptance rates are better than 99%.Conclusions:
This work shows that the accuracy of RapidArc delivery holds across the full range of gantry speeds, leaf velocities, and dose rates with small dosimetric uncertainties for 0.5 cm MLC leaves. However, caution should be exercised when using large MLC leaves in RapidArc. A novel technique to obtain the delivered 3D dose distributions using machine log files is also presented.
Characteristics of optically stimulated luminescence dosimeters in the spread-out Bragg peak region of clinical proton beams39(2012); http://dx.doi.org/10.1118/1.3693055View Description Hide DescriptionPurpose:
Optically stimulated luminescent detectors (OSLDs) have a number of advantages in radiationdosimetry making them excellent dosimeters for quality assurance and patient dose verification. Although the dosimeters have been investigated in several modalities, relatively little work has been done in examining the dosimeters for use in clinical proton beams. This study examined a number of characteristics of the response of the dosimeters in the spread-out Bragg peak (SOBP) region of clinical proton beams.Methods:
Optically stimulated luminescence(OSL)dosimeters from Landauer, Inc., specifically the nanoDotdosimeter, were investigated. These dosimeters were placed in a special phantom with a recess to fit the dosimeters without an air gap. Beams with nominal energies of 160, 200, and 250 MeV were used in the passively-scattered proton beam at the MD Anderson Cancer Center Proton Therapy Center. Dosimetric properties including linearity, field size dependence, energy dependence, residual signal as a function of cumulative dose, and postirradiation fading were investigated by taking measurements at the center of SOBPs.Results:
The dosimeters showed 1% supralinearity at 200 cGy and 5% supralinearity at 1000 cGy. No noticeable field size dependence of the detector was found for field sizes from 2 × 2 cm2 to 18 × 18 cm2. Residual signal as a function of cumulative dose showed a small increase for measurements up to 1000 cGy. Readout signal depletion of the dosimeters after consecutive readings showed a slightly larger depletion in protons for doses up to 500 cGy but not by a clinically significant amount. Within the center of various SOBP widths and proton energies the variation in response was less than 2%. An average beam quality factor of 1.089 with experimental standard deviation of 0.007 was determined and applied to the data such that the results were within 1.2% of ion chamber data.Conclusions:
The nanoDotOSLdosimeter characteristics were studied in the SOBP region of clinical proton beams. To achieve accurate dosimetric readings, corrections to the dosimeter response were applied. Corrections tended to be minimal or broadly consistent. The nanoDot OSLD was found to be an acceptable dosimeter for measurement in the SOBP region for a range of clinical proton beams.
39(2012); http://dx.doi.org/10.1118/1.3693057View Description Hide DescriptionPurpose
: The graphic processing unit (GPU) based TomoTherapy convolution/superposition(C/S) dose engine (GPU dose engine) achieves a dramatic performance improvement over the traditional CPU-cluster based TomoTherapy dose engine (CPU dose engine). Besides the architecture difference between the GPU and CPU, there are several algorithm changes from the CPU dose engine to the GPU dose engine. These changes made the GPU dose slightly different from the CPU-cluster dose. In order for the commercial release of the GPU dose engine, its accuracy has to be validated.Methods
: Thirty eight TomoTherapy phantom plans and 19 patient plans were calculated with both dose engines to evaluate the equivalency between the two dose engines. Gamma indices (Γ) were used for the equivalency evaluation. The GPU dose was further verified with the absolute point dose measurement with ion chamber and film measurements for phantom plans. Monte Carlo calculation was used as a reference for both dose engines in the accuracy evaluation in heterogeneous phantom and actual patients.Results
: The GPU dose engine showed excellent agreement with the current CPU dose engine. The majority of cases had over 99.99% of voxels with Γ(1%, 1 mm) < 1. The worst case observed in the phantom had 0.22% voxels violating the criterion. In patient cases, the worst percentage of voxels violating the criterion was 0.57%. For absolute point dose verification, all cases agreed with measurement to within ±3% with average error magnitude within 1%. All cases passed the acceptance criterion that more than 95% of the pixels have Γ(3%, 3 mm) < 1 in film measurement, and the average passing pixel percentage is 98.5%–99%. The GPU dose engine also showed similar degree of accuracy in heterogeneous media as the current TomoTherapy dose engine.Conclusions
: It is verified and validated that the ultrafast TomoTherapy GPU dose engine can safely replace the existing TomoTherapy cluster based dose engine without degradation in dose accuracy.
39(2012); http://dx.doi.org/10.1118/1.3694110View Description Hide DescriptionPurpose:
A new technique called “curvilinear approach” for prostate seed implantation has been proposed. The purpose of this study is to evaluate the dosimetric benefit of curvilinear distribution of seeds for low-dose-rate (LDR) prostate brachytherapy.Methods:
Twenty LDR prostate brachytherapy cases planned intraoperatively with VariSeed planning system and I-125 seeds were randomly selected as reference rectilinear cases. All the cases were replanned by using curved-needle approach keeping the same individual source strength and the volume receiving 100% of prescribed dose 145 Gy (V100). Parameters such as number of needles, seeds, and the dose coverage of the prostate (D90, V150, V200), urethra (D30, D10) and rectum (D5, V100) were compared for the rectilinear and the curvilinear methods. Statistical significance was assessed using two-tailed student’s t-test.Results:
Reduction of the required number of needles and seeds in curvilinear method were 30.5% (p < 0.001) and 11.8% (p < 0.49), respectively. Dose to the urethra was reduced significantly; D30 reduced by 10.1% (p < 0.01) and D10 reduced by 9.9% (p < 0.02). Reduction in rectum dose D5 was 18.5% (p < 0.03) and V100 was also reduced from 0.93 cc in rectilinear to 0.21 cc in curvilinear (p < 0.001). Also the V150 and V200 coverage of prostate reduced by 18.8% (p < 0.01) and 33.9% (p < 0.001), respectively.Conclusions:
Significant improvement in the relevant dosimetric parameters was observed in curvilinear needle approach. Prostate dose homogeneity (V150, V200) improved while urethral dose was reduced, which might potentially result in better treatment outcome. Reduction in rectal dose could potentially reduce rectal toxicity and complications. Reduction in number of needles would minimize edema and thereby could improve postimplant urinary incontinence. This study indicates that the curvilinear implantation approach is dosimetrically superior to conventional rectilinear implantation technique.
39(2012); http://dx.doi.org/10.1118/1.3692177View Description Hide DescriptionPurpose:
The purpose of this work is threefold: (1) to explore biological consequences of the multileaf collimator(MLC)calibration errors in intensity modulated radiotherapy(IMRT) of prostate and head and neck cancers, (2) to determine levels of planning target volume (PTV) and normal tissue under- or overdose flagged with clinically used QA action limits, and (3) to provide biologically based input for MLC QA and IMRT QA action limits.Methods:
Ten consecutive prostate IMRT cases and ten consecutive head and neck IMRT cases were used. Systematic MLC offsets (i.e., calibration error) were introduced for each control point of the plan separately for X1 and X2 leaf banks. Offsets were from − 2 to 2 mm with a 0.5 mm increment. The modified files were imported into the planning system for forward dose recalculation. The original plan served as the reference. The generalized equivalent uniform dose (gEUD) was used as the biological index for the targets, rectum, parotid glands, brainstem, and spinal cord. Each plan was recalculated on a CT scan of a 27 cm diameter cylindrical phantom with a contoured 0.6 cc ion chamber.Dose to ion chamber and 3D gamma analysis were compared to the reference plan. QA pass criteria: (1) at least 95% of voxels with a dose cutoff of 50% of maximum dose have to pass at 3 mm/3% and (2) dose to chamber within 2% of the reference dose.Results:
For prostate cases, differences in PTV and rectum gEUD greater than 2% were identified. However, a larger proportion of plans leading to greater than 2% difference in prostate PTV gEUD passed the ion chamber QA but not 3D gamma QA. A similar trend was found for the rectum gEUD. For head and neck IMRT, the QA pass criteria flagged plans leading to greater than 4% differences in PTV gEUD and greater than 5% differences in the maximum dose to brainstem. If pass criteria were relaxed to 90% for gamma and 3% for ion chamber QA, plans leading to a 5% difference in PTV gEUD and a 5%–8% difference in brainstem maximum dose would likely pass IMRT QA. A larger proportion of head and neck plans with greater than 2% PTV gEUD difference passed 3D gamma QA compared to ion chamber QA.Conclusions:
For low modulation plans, there is a better chance to catch MLCcalibration errors with 3D gamma QA rather than ion chamber QA. Conversely, for high modulation plans, there is a better chance to catch MLCcalibration errors with ion chamber QA rather than with 3D gamma QA. Ion chamber and 3D gamma analysisIMRT QA can detect greater than 2% change in gEUD for PTVs and critical structures for low modulation treatment plans. For high modulation treatment plans, ion chamber and 3D gamma analysis can detect greater than 2% change in gEUD for PTVs and a 5% change in critical structure gEUD since either QA methods passes the QA criteria. For gEUD changes less than those listed above, either QA method has the same proportion of passing rate.
Correction factors for A1SL ionization chamber dosimetry in TomoTherapy: Machine-specific, plan-class, and clinical fields39(2012); http://dx.doi.org/10.1118/1.3692181View Description Hide DescriptionPurpose:
Recently, an international working group on nonstandard fields presented a new formalism for ionization chamber reference dosimetry of small and nonstandard fields [Alfonsoet al., Med. Phys. 35, 5179–5186 (2008)] which has been adopted by AAPM TG-148. This work presents an experimental determination of the correction factors for reference dosimetry with an Exradin A1SL thimble ionization chamber in a TomoTherapy unit, focusing on: (i) machine-specific reference field, (ii) plan-class-specific reference field, and (iii) two clinical treatments.Methods:
Ionization chamber measurements were performed in the TomoTherapy unit for intermediate (machine-specific and plan-class-specific) calibration fields, based on the reference conditions defined by AAPM TG-148, and two clinical treatments (lung and head-and-neck). Alanine reference dosimetry was employed to determine absorbed dose to water at the point of interest for the fields under investigation. The corresponding chamber correction factors were calculated from alanine to ionization chamber measurements ratios.Results:
Two different methods of determining the beam quality correction factor for the A1SL ionization chamber in this TomoTherapy unit, where reference conditions for conventional beam quality determination cannot be met, result in consistent values. The observed values of overall correction factors obtained for intermediate and clinical fields are consistently around 0.98 with a typical expanded relative uncertainty of 2% (k = 2), which when considered make such correction factors compatible with unity. However, all of them are systematically lower than unity, which is shown to be significant when a hypothesis test assuming a t-student distribution is performed (). Correction factors and , which are needed for the computation of field factors for relative dosimetry of clinical beams, have been found to be very close to unity for two clinical treatments.Conclusions:
The results indicate that the helical field deliveries in this study (including two clinical fields) do not introduce changes on the ion chamber correction factors for dosimetry. For those two specific clinical cases, ratios of chamber readings accurately represent field output factors. The values observed here for intermediate calibration fields are in agreement with previously published data based on alanine dosimetry but differ from values recently reported obtained via radiochromic dosimetry.
Characteristics of miniature electronic brachytherapy x-ray sources based on TG-43U1 formalism using Monte Carlo simulation techniquesa)39(2012); http://dx.doi.org/10.1118/1.3693046View Description Hide DescriptionPurpose:
The goal of this study is to determine a method for Monte Carlo(MC) characterization of the miniature electronic brachytherapy x-ray sources (MEBXS) and to set dosimetric parameters according to TG-43U1 formalism. TG-43U1 parameters were used to get optimal designs of MEBXS. Parameters that affect the dose distribution such as anode shapes, target thickness, target angles, and electron beamsource characteristics were evaluated. Optimized MEBXS designs were obtained and used to determine radial dose functions and 2D anisotropy functions in the electron energy range of 25–80 keV.Methods:
Tungstenanode material was considered in two different geometries, hemispherical and conical-hemisphere. These configurations were analyzed by the 4C MC code with several different optimization techniques. The first optimization compared target thickness layers versus electron energy. These optimized thicknesses were compared with published results by Ihsanet al. [Nucl. Instrum. Methods Phys. Res. B 264, 371–377 (2007)]. The second optimization evaluated electron source characteristics by changing the cathode shapes and electron energies. Electron sources studied included; (1) point sources, (2) uniform cylinders, and (3) nonuniform cylindrical shell geometries. The third optimization was used to assess the apex angle of the conical-hemisphere target. The goal of these optimizations was to produce 2D-dose anisotropy functions closer to unity. An overall optimized MEBXS was developed from this analysis. The results obtained from this model were compared to known characteristics of HDR 125I, LDR 103Pd, and Xoft Axxent™ electronic brachytherapysource (XAEBS) [Med. Phys. 33, 4020–4032 (2006)].Results:
The optimized anode thicknesses as a function of electron energy is fitted by the linear equation Y (μm) = 0.0459X (keV)–0.7342. The optimized electron source geometry is obtained for a disk-shaped parallel beam (uniform cylinder) with 0.9 mm radius. The TG-43 distribution is less sensitive to the shape of the conical-hemisphere anode than the hemispherical anode. However, the optimized apex angle of conical-hemisphere anode was determined to be 60°. For the hemispherical targets, calculated radial dose function values at a distance of 5 cm were 0.137, 0.191, 0.247, and 0.331 for 40, 50, 60, and 80 keV electrons, respectively. These values for the conical-hemisphere targets are 0.165, 0.239, 0.305, and 0.412, respectively. Calculated 2D anisotropy functions values for the hemispherical target shape wereF(1 cm, 0°) = 1.438 and F(1 cm, 0°) = 1.465 for 30 and 80 keV electrons, respectively. The corresponding values for conical-hemisphere targets are 1.091 and 1.241, respectively.Conclusions:
A method for the characterizations of MEBXS using TG-43U1 dosimetric data using the MC MCNP4C has been presented. The effects of target geometry, thicknesses, and electron source geometry have been investigated. The final choices of MEBXS design are conical-hemisphere target shapes having an apex angle of 60°. Tungsten material having an optimized thickness versus electron energy and a 0.9 mm radius of uniform cylinder as a cathode produces optimal electron source characteristics.
Dosimetric and thermal properties of a newly developed thermobrachytherapy seed with ferromagnetic core for treatment of solid tumors39(2012); http://dx.doi.org/10.1118/1.3693048View Description Hide DescriptionPurpose:
Studies of the curative effects of hyperthermia and radiation therapy on treatment of cancer show a strong evidence of a synergistic enhancement when both radiation and hyperthermia modalities are applied simultaneously. Varieties of tissue heating approaches developed up to date still fail to overcome such essential limitations as an inadequate temperature control, temperature nonuniformity, and prolonged time delay between hyperthermia and radiation treatments. The authors propose a new self-regulating thermobrachytherapy seed, which serves as a source of both radiation and heat for concurrent administration of brachytherapy and hyperthermia.Methods:
The proposed seed is based on the BEST Medical, Inc., Seed Model 2301-I125, where tungsten marker core and the air gap are replaced with a ferromagnetic material. The ferromagnetic core produces heat when subjected to alternating electromagnetic (EM) field and effectively shuts off after reaching the Curie temperature (TC) of the ferromagnetic material thus realizing the temperature self-regulation. The authors present a Monte Carlo study of the dose rate constant and other TG-43 factors for the proposed seed. For the thermal characteristics, the authors studied a model consisting of 16 seeds placed in the central region of a cylindrical water phantom using a finite-element partial differential equation solver package “COMSOL Multiphysics.”Results:
The modification of the internal structure of the seed slightly changes dose rate and other TG-43 factors characterizing radiation distribution. The thermal modeling results show that the temperature of the thermoseed surface rises rapidly and stays constant around TC of the ferromagnetic material. The amount of heat produced by the ferromagnetic core is sufficient to raise the temperature of the surrounding phantom to the therapeutic range. The phantom volume reaching the therapeutic temperature range increases with increase in frequency or magnetic field strength.Conclusions:
An isothermal distribution matching with the radiation isodose distribution can be achieved within a target volume by tuning frequency and intensity of the alternating magnetic field. The proposed combination seed model has a potential for implementation of concurrent brachytherapy and hyperthermia.
39(2012); http://dx.doi.org/10.1118/1.3694097View Description Hide DescriptionPurpose:
The need for an accurate estimate of absorbed doses within and around irradiated thorax tissues necessitates the use of carefully selected materials from which phantoms are constructed. A lung substitute is more difficult to establish mostly due to its low physical density. Although many researchers have used cork as a lung substitute, very little research data address cork’s characteristics to determine which type of cork is optimal as a substitute for lungtissue.Methods:
Natural cork, composition cork, rubber cork, ATOM, RANDO, and a reference lungmaterial (ICRU-44 lungtissue) were investigated to establish comparisons of physical properties. Following the determination of the respective physical properties, the dose distributions from 6 MV photon beams in water/lung substitute/water phantoms were assessed using the Monte Carlo method. Physical and electron densities affecting the dose distributions through lungtissues in different field size conditions were investigatedResults:
The physical properties (physical density, electronic density, and effective atomic number) of the composition cork are the most similar to those of the ICRU-44 lung, and the CT number of the composition cork is very similar to that of humans aged 30–60. PDD of the composition cork and the RANDO phantom are the most comparable to that of ICRU-44 lung in 1 × 1 cm2field size due to the combined properties of physical density (PD) and electron density per gram (EDG) of the studied lungmaterials. PD and EDG affect the lungdose primarily in small field size. The effects of PD are minimal in large fields, having a more rapid lateral electron equilibrium. EDG dominates PDD pattern in lungmaterial when large fields are applied. Combined effects of PD and EDG are nonlinear for all field sizes.Conclusions:
The composition cork is the preferred lung substitute based on physical and dosimetricproperties.
Online monitoring and error detection of real-time tumor displacement prediction accuracy using control limits on respiratory surrogate statistics39(2012); http://dx.doi.org/10.1118/1.3676690View Description Hide DescriptionPurpose:
To evaluate Hotelling’sT 2 statistic and the input variable squared prediction error (Q (X)) for detecting large respiratory surrogate-based tumor displacement prediction errors without directly measuring the tumor’s position.Methods:
Tumor and external marker positions from a database of 188 Cyberknife Synchrony™ lung,liver, and pancreas treatment fractions were analyzed. The first ten measurements of tumor position in each fraction were used to create fraction-specific models of tumor displacement using external surrogates as input; the models were used to predict tumor position from subsequent external marker measurements. A partial least squares (PLS) model with four scores was developed for each fraction to determineT 2 and Q (X) confidence limits based on the first ten measurements in a fraction. The T 2 and Q (X) statistics were then calculated for every set of external marker measurements. Correlations between model error and both T 2 and Q (X) were determined. Receiver operating characteristic analysis was applied to evaluate sensitivities and specificities of T 2, Q (X), and T 2∪Q (X) for predicting real-time tumor localization errors >3 mm over a range of T 2 and Q (X) confidence limits.Results:
Sensitivity and specificity of detecting errors >3 mm varied with confidence limit selection. At 95% sensitivity,T 2∪Q (X) specificity was 15%, 2% higher than either T 2 or Q (X) alone. The mean time to alarm for T 2∪Q (X) at 95% sensitivity was 5.3 min but varied with a standard deviation of 8.2 min. Results did not differ significantly by tumor site.Conclusions:
The results of this study establish the feasibility of respiratory surrogate-based online monitoring of real-time respiration-induced tumor motion model accuracy for lung,liver, and pancreas tumors. TheT 2 and Q (X) statistics were able to indicate whether inferential model errors exceeded 3 mm with high sensitivity. Modest improvements in specificity were achieved by combining T 2 and Q (X) results.
CT, MR, and ultrasound image artifacts from prostate brachytherapy seed implants: The impact of seed size39(2012); http://dx.doi.org/10.1118/1.3694669View Description Hide DescriptionPurpose:
To investigate the effects of brachytherapy seed size on the quality of x-ray computed tomography(CT),ultrasound(US), and magnetic resonance (MR)images and seed localization through comparison of the 6711 and 9011125I sources.Methods:
For CTimages, an acrylic phantom mimicking a clinical implantation plan and embedded with low contrast regions of interest (ROIs) was designed for both the 0.774 mm diameter 6711 (standard) and the 0.508 mm diameter 9011 (thin) seed models (Oncura, Inc., and GE Healthcare, Arlington Heights, IL). Image quality metrics were assessed using the standard deviation of ROIs between the seeds and the contrast to noise ratio (CNR) within the low contrast ROIs. For USimages, water phantoms with both single and multiseed arrangements were constructed for both seed sizes. For MRimages, both seeds were implanted into a porcine gel and imaged with pelvic imaging protocols. The standard deviation of ROIs and CNR values were used as metrics of artifact quantification. Seed localization within the CTimages was assessed using the automated seed finder in a commercial brachytherapy treatment planning system. The number of erroneous seed placements and the average and maximum error in seed placements were recorded as metrics of the localization accuracy.Results:
With the thin seeds, CTimagenoise was reduced from 48.5 ± 0.2 to 32.0 ± 0.2 HU and CNR improved by a median value of 74% when compared with the standard seeds. Ultrasoundimagenoise was measured at 50.3 ± 17.1 dB for the thin seed images and 50.0 ± 19.8 dB for the standard seed images, and artifacts directly behind the seeds were smaller and less prominent with the thin seed model. For MRimages,CNR of the standard seeds reduced on average 17% when using the thin seeds for all different imaging sequences and seed orientations, but these differences are not appreciable. Automated seed localization required an average (±SD) of 7.0 ± 3.5 manual corrections in seed positions for the thin seed scans and 3.0 ± 1.2 manual corrections in seed positions for the standard seed scans. The average error in seed placement was 1.2 mm for both seed types and the maximum error in seed placement was 2.1 mm for the thin seed scans and 1.8 mm for the standard seed scans.Conclusions:
The 9011 thin seeds yielded significantly improved image quality for CT and USimages but no significant differences in MRimage quality.
Development of array-type prompt gamma measurement system for in vivo range verification in proton therapy39(2012); http://dx.doi.org/10.1118/1.3694098View Description Hide DescriptionPurpose:
In vivo range verification is one of the most important parts of proton therapy to fully utilize its benefits delivering high radiationdose to tumor, while sparing the normal tissue with the so-called Bragg peak. Currently, however, range verification method is not used in clinics. The purpose of the present study is to optimize and evaluate the configuration of an array-type prompt gamma measurement system on determining distal dose edge for in vivo range verification of proton therapy.Methods:
To effectively measure the prompt gammas against the background gammas, the Monte Carlo simulations with the MCNPX code were employed in optimizing the configuration of the measurement system, and the Monte Carlo method was also used to understand the effect of the background gammas, mainly neutron capture gammas, in the measured gamma distribution. To reduce the effect of the background gammas, the optimized energy window of 4–10 MeV in measuring the prompt gammas was employed. A parameterized source was used to maximize computation speed in the optimization study. A simplified test measurement system, using only one detector moving from one measurement location to the next, was constructed and applied to therapeuticproton beams of 80–220 MeV. For accurate determination of the distal dose edge, the sigmoidal curve-fitting method was applied to the measured distributions of the prompt gammas, and then, the location of the half-value between the maximum and minimum value in the curve-fitting was determined as the distal dose edge and compared with the beam range assessed by the protondose distribution.Results:
The parameterized source term employed in optimization process improved the calculation speed by up to ∼300 times. The optimization study indicates that an array-type measurement system with 3, 2, 2, and 150 mm for scintillator thickness, slit width, septal thickness, and slit length, respectively, can effectively measure the prompt gamma distributions minimizing the contribution of background gammas. The present results show that a few hundred counts of prompt gammas can be easily obtained by measuring 10 s at each measurement location for proton beams of ∼4 nA. The distal dose edges determined by the prompt gamma distribution are 5.45, 14.73, and 27.74 cm for proton beams of 5.17 (80 MeV), 14.99 (150 MeV), and 27.38 (220 MeV) cm, respectively.Conclusions:
The results show that the array-type measurement system can measure prompt gamma distributions from a therapeuticproton beam within a short measurement time, and that the distal dose edge can be determined within a few millimeters of error without using any sophisticated analysis.
A method to evaluate dose errors introduced by dose mapping processes for mass conserving deformations39(2012); http://dx.doi.org/10.1118/1.3684951View Description Hide DescriptionPurpose:
To present a method to evaluate the dose mapping error introduced by the dose mapping process. In addition, apply the method to evaluate the dose mapping error introduced by the 4D dose calculation process implemented in a research version of commercial treatment planning system for a patient case.Methods:
The average dose accumulated in a finite volume should be unchanged when the dose delivered to one anatomic instance of that volume is mapped to a different anatomic instance—provided that the tissue deformation between the anatomic instances is mass conserving. The average dose to a finite volume on imageS is defined as , where is the energy deposited in the mass contained in the volume. Since mass and energy should be conserved, when is mapped to an image, the mean dose mapping error is defined as , where the and are integral doses (energy deposited), and and are the masses within the region of interest (ROI) on imageR and the corresponding ROI on imageS, where R and S are the two anatomic instances from the same patient. Alternatively, application of simple differential propagation yields the differential dose mapping error, with . A 4D treatment plan on a ten-phase 4D-CT lung patient is used to demonstrate the dose mapping error evaluations for a patient case, in which the accumulated dose,, and associated error values ( and ) are calculated for a uniformly spaced set of ROIs.Results:
For the single sample patient dose distribution, the average accumulated differential dose mapping error is 4.3%, the average absolute differential dose mapping error is 10.8%, and the average accumulated mean dose mapping error is 5.0%. Accumulated differential dose mapping errors within the gross tumor volume (GTV) and planning target volume (PTV) are lower, 0.73% and 2.33%, respectively.Conclusions:
A method has been presented to evaluate the dose mapping error introduced by the dose mapping process. This method has been applied to evaluate the 4D dose calculation process implemented in a commercial treatment planning system. The method could potentially be developed as a fully-automatic QA method in image guided adaptive radiation therapy (IGART).
39(2012); http://dx.doi.org/10.1118/1.3694100View Description Hide DescriptionPurpose:
Quantitatively determine an optimum image analysis procedure to mitigate inhomogeneities within the EBT2 film and from scanning for accurate absolute dose measurement deposited by an external radiation therapy beam. Multichannel dosimetry procedures were conceived, described, and quantitatively tested against single and dual channel dosimetry.Methods:
A solid waterTM block was placed on CTimaging and treatment tables in a configuration that avoids bulky compressive devices. CT markers helped register the CT to the treatment plan and the radiationdose distribution from the radiochromic film. The CTimages were digitally rotated and resampled to match the spatial resolution of the scanned dosimetric distribution and treatment plan. The ECLIPSE treatment plan planes were digitally translated through digital triangulation of the treatment isocenter to the CT markers in the CTimage. A 6 MV photon beam, conforming to the treatment plan, irradiated the EBT2 film sandwiched between solid waterTM slabs. The exposed radiochromic film images were rotated and translated to the CTimages using coincident markers in the CTimage that are associated with “tattoos” marked on the radiochromic film. The exposed radiochromic film gray-levels from a flatbed scanner in reflection mode were converted to dose using calibration films. The test dose distribution was scanned and averaged six times to reduce temporal noise. This study generated dose distributions using the red channel alone, green channel alone, ratio of the red to blue channel, ratio of the green to blue channel, a hybrid approach combining the green to blue ratio for higher doses (>80 cGy) with the red to blue ratio (<80 cGy), multichannel averaging and optimized autonomous multichannel correction. Single channel, multichannel, and channel ratio methods for processing the exposed radiochromic film were compared to the treatment plan via gamma analysis. The ellipsoidal decision surface was defined by its axes of 3% of the maximum dose and 3 mm in the horizontal and vertical directions.Results:
The multichannel dosimetry procedures provided excellent agreement with calculation of the dose distribution as determined by the gamma analysis. The green channel mostly performed as well or better than the red channel. The green to blue channel ratio for doses when combined with red to blue ratio (“Hybrid”) achieved a high level performance. In addition, new registration procedures were developed and tested for aiding the comparison of calculated and experimentally determined dose distributions.Conclusions:
This study described, developed, and tested new processing methods for reducing inaccuracies in absolute dose determination due to inhomogeneities within the film and from scanning. This study found better performance using optimized multichannel following averaging of all color channels. Combining the channel ratios in a hybrid approach also achieved high performance. Averaging the test films reduced temporal noise that severely degraded the blue channel. This methodology avoided using cumbersome, registered correction matrices. Novel registration and digital rotation of CTimages enabled quantitative testing and helped improve contact between the radiochromic film and phantom.
Quantifying the gantry sag on linear accelerators and introducing an MLC-based compensation strategy39(2012); http://dx.doi.org/10.1118/1.3697528View Description Hide DescriptionPurpose:
Gantry sag is one of the well-known sources of mechanical imperfections that compromise the spatial accuracy of radiation dose delivery. The objectives of this study were to quantify the gantry sag on multiple linear accelerators(linacs), to investigate a multileaf collimator (MLC)-based strategy to compensate for gantry sag, and to verify the gantry sag and its compensation with film measurements.Methods:
The authors used the Winston–Lutz method to measure gantry sag on three Varian linacs. A ball bearing phantom was imaged with megavolt radiation fields at 10° gantry angle intervals. The images recorded with an electronic portal imaging device were analyzed to derive the radiation isocenter and the gantry sag, that is, the superior–inferior wobble of the radiation field center, as a function of the gantry angle. The authors then attempted to compensate for the gantry sag by applying a gantry angle-specific correction to the MLC leaf positions. The gantry sag and its compensation were independently verified using film measurements.Results:
Gantry sag was reproducible over a six-month measurement period. The maximum gantry sag was found to vary from 0.7 to 1.0 mm, depending on the linac and the collimator angle. The radiation field center moved inferiorly (i.e., away from the gantry) when the gantry was rotated from 0° to 180°. After the MLC leaf position compensation was applied at 90° collimator angle, the maximum gantry sag was reduced to <0.2 mm. The film measurements at gantry angles of 0° and 180° verified the inferior shift of the radiation fields and the effectiveness of MLC compensation.Conclusions:
The results indicate that gantry sag on a linac can be quantitatively measured using a simple phantom and an electronic portal imaging device. Reduction of gantry sag is feasible by applying a gantry angle-specific correction to MLC leaf positions at 90° collimator angle.
A stochastic approach to estimate the uncertainty of dose mapping caused by uncertainties in b-spline registration39(2012); http://dx.doi.org/10.1118/1.3697524View Description Hide DescriptionPurpose:
In fractionated radiation therapy,image guidance with daily tomographic imaging becomes more and more clinical routine. In principle, this allows for daily computation of the delivered dose and for accumulation of these daily dose distributions to determine the actually delivered total dose to the patient. However, uncertainties in the mapping of the images can translate into errors of the accumulated total dose, depending on the dose gradient. In this work, an approach to estimate the uncertainty of mapping between medical images is proposed that identifies areas bearing a significant risk of inaccurate dose accumulation.Methods:
This method accounts for the geometric uncertainty of image registration and the heterogeneity of the dose distribution, which is to be mapped. Its performance is demonstrated in context of dose mapping based on b-spline registration. It is based on evaluation of the sensitivity of dose mapping to variations of the b-spline coefficients combined with evaluation of the sensitivity of the registration metric with respect to the variations of the coefficients. It was evaluated based on patient data that was deformed based on a breathing model, where the ground truth of the deformation, and hence the actual true dose mapping error, is known.Results:
The proposed approach has the potential to distinguish areas of the image where dose mapping is likely to be accurate from other areas of the same image, where a larger uncertainty must be expected.Conclusions:
An approach to identify areas where dose mapping is likely to be inaccurate was developed and implemented. This method was tested for dose mapping, but it may be applied in context of other mapping tasks as well.
Experimental validation of deterministic Acuros XB algorithm for IMRT and VMAT dose calculations with the Radiological Physics Center’s head and neck phantom39(2012); http://dx.doi.org/10.1118/1.3692180View Description Hide DescriptionPurpose:
The purpose of this study was to verify the dosimetric performance of Acuros XB (AXB), a grid-based Boltzmann solver, in intensity-modulated radiation therapy(IMRT) and volumetric-modulated arc therapy (VMAT).Methods:
The Radiological Physics Center (RPC) head and neck (H&N) phantom was used for all calculations and measurements in this study. Clinically equivalent IMRT and VMAT plans were created on the RPC H&N phantom in the Eclipse treatment planning system (version 10.0) by using RPC dose prescription specifications. The dose distributions were calculated with two different algorithms, AXB 11.0.03 and anisotropic analytical algorithm (AAA) 10.0.24. Two dose report modes of AXB were recorded: dose-to-medium in medium (Dm,m) and dose-to-water in medium (Dw,m). Each treatment plan was delivered to the RPC phantom three times for reproducibility by using a Varian Clinac iX linear accelerator. Absolute point dose and planar dose were measured with thermoluminescent dosimeters(TLDs) and GafChromic® EBT2 film, respectively. Profile comparison and 2D gamma analysis were used to quantify the agreement between the film measurements and the calculated dose distributions from both AXB and AAA. The computation times for AAA and AXB were also evaluated.Results:
Good agreement was observed between measured doses and those calculated with AAA or AXB. Both AAA and AXB calculated doses within 5% of TLD measurements in both the IMRT and VMAT plans. Results of AXB_Dm,m (0.1% to 3.6%) were slightly better than AAA (0.2% to 4.6%) or AXB_Dw,m (0.3% to 5.1%). The gamma analysis for both AAA and AXB met the RPC 7%/4 mm criteria (over 90% passed), whereas AXB_Dm,m met 5%/3 mm criteria in most cases. AAA was 2 to 3 times faster than AXB for IMRT, whereas AXB was 4–6 times faster than AAA for VMAT.Conclusions:
AXB was found to be satisfactorily accurate when compared to measurements in the RPC H&N phantom. Compared with AAA, AXB results were equal to or better than those obtained with film measurements for IMRT and VMAT plans. The AXB_Dm,m reporting mode was found to be closer to TLD and film measurements than was the AXB_Dw,m mode. AXB calculation time was found to be significantly shorter (× 4) than AAA for VMAT.
Concurrent segmentation of the prostate on MRI and CT via linked statistical shape models for radiotherapy planning39(2012); http://dx.doi.org/10.1118/1.3696376View Description Hide DescriptionPurpose
: Prostate gland segmentation is a critical step in prostate radiotherapy planning, where dose plans are typically formulated on CT. Pretreatment MRI is now beginning to be acquired at several medical centers. Delineation of the prostate on MRI is acknowledged as being significantly simpler to perform, compared to delineation on CT. In this work, the authors present a novel framework for building a linked statistical shape model (LSSM), a statistical shape model (SSM) that links the shape variation of a structure of interest (SOI) across multiple imaging modalities. This framework is particularly relevant in scenarios where accurate boundary delineations of the SOI on one of the modalities may not be readily available, or difficult to obtain, for training a SSM. In this work the authors apply the LSSM in the context of multimodal prostate segmentation for radiotherapy planning, where the prostate is concurrently segmented on MRI and CT.Methods
: The framework comprises a number of logically connected steps. The first step utilizes multimodal registration of MRI and CT to map 2D boundary delineations of the prostate from MRI onto corresponding CTimages, for a set of training studies. Hence, the scheme obviates the need for expert delineations of the gland on CT for explicitly constructing a SSM for prostate segmentation on CT. The delineations of the prostate gland on MRI and CT allows for 3D reconstruction of the prostate shape which facilitates the building of the LSSM. In order to perform concurrent prostate MRI and CT segmentation using the LSSM, the authors employ a region-based level set approach where the authors deform the evolving prostate boundary to simultaneously fit to MRI and CTimages in which voxels are classified to be either part of the prostate or outside the prostate. The classification is facilitated by using a combination of MRI-CT probabilistic spatial atlases and a random forest classifier, driven by gradient and Haar features.Results
: The authors acquire a total of 20 MRI-CT patient studies and use the leave-one-out strategy to train and evaluate four different LSSMs. First, a fusion-based LSSM (fLSSM) is built using expert ground truth delineations of the prostate on MRI alone, where the ground truth for the gland on CT is obtained via coregistration of the corresponding MRI and CT slices. The authors compare the fLSSM against another LSSM (xLSSM), where expert delineations of the gland on both MRI and CT are employed in the model building; xLSSM representing the idealized LSSM. The authors also compare the fLSSM against an exclusive CT-based SSM (ctSSM), built from expert delineations of the gland on CT alone. In addition, two LSSMs trained using trainee delineations (tLSSM) on CT are compared with the fLSSM. The results indicate that the xLSSM, tLSSMs, and the fLSSM perform equivalently, all of them out-performing the ctSSM.Conclusions
: The fLSSM provides an accurate alternative to SSMs that require careful expert delineations of the SOI that may be difficult or laborious to obtain. Additionally, the fLSSM has the added benefit of providing concurrent segmentations of the SOI on multiple imaging modalities.