Index of content:
Volume 38, Issue 3, March 2011
PDT is better than alternative therapies such as brachytherapy, electron beams, or low-energy x rays for the treatment of skin cancers38(2011); http://dx.doi.org/10.1118/1.3512802View Description Hide Description
- MEDICAL PHYSICS LETTERS
38(2011); http://dx.doi.org/10.1118/1.3549762View Description Hide DescriptionPurpose:
The binding of nanoparticles toin vivo targets impacts their use for medical imaging, therapy, and the study of diseases and disease biomarkers. Though an array of techniques can detect binding in vitro, the search for a robust in vivo method continues. The spectral response of magnetic nanoparticles can be influenced by a variety of changes in their physical environment including viscosity and binding. Here, the authors show that nanoparticles in these different environmental states produce spectral responses, which are sufficiently unique to allow for simultaneous quantification of the proportion of nanoparticles within each state.Methods:
The authors measured the response to restricted Brownian motion using an array of magnetic nanoparticle designs. With a chosen optimal particle type, the authors prepared particle samples in three distinct environmental states. Various combinations of particles within these three states were measured concurrently and the authors attempted to solve for the quantity of particles within each physical state.Results:
The authors found the spectral response of the nanoparticles to be sufficiently unique to allow for accurate quantification of up to three bound states with errors on the order of 1.5%. Furthermore, the authors discuss numerous paths for translating these measurements toin vivo applications.Conclusions:
Multiple nanoparticle environmental states can be concurrently quantified using the spectral response of the particles. Such an ability, if translated to thein vivo realm, could provide valuable information about the fate of nanoparticlesin vivo or improve the efficacy of nanoparticle based treatments.
Full-field 3D photoacoustic imaging based on plane transducer array and spatial phase-controlled algorithm38(2011); http://dx.doi.org/10.1118/1.3555036View Description Hide DescriptionPurpose:
Photoacoustic imaging(PAI) used for noninvasive imaging of biological tissue has been reported in many literature. However, there are still some disadvantages in the novel technique, such as the poor efficiency of imaging. In the current PAI, multiple excitation of laser and multiple acquisition of signal are necessary for image reconstruction. In this case, laser pulses may injure biological tissue due to energy accumulation. To popularize PAI in clinical applications, it is necessary to develop a new imaging approach to increase the efficiency of PAI.Methods:
A spatial phase-controlled algorithm is presented for full-field three-dimensional (3D) image reconstruction. By using the algorithm, photoabsorption sources at different depths can be reconstructed using just one set of data acquired in single laser shot. Unfocused plane transducer array and parallel data-acquisition (PDA) equipment are used for real-time photoacoustic (PA) signal detection and acquisition.Results:
The spatial resolution of the 3D PAI system was analyzed. Two graphite rods at various positions in a simulation model and two bifurcate vessels in the ear of rabbit were imaged. In addition, the motion trace of one particle flowing at constant velocity was captured dynamically. Experimental results showed that spatial phase-controlled algorithm based on plane transducer array and PDA system was capable of static and dynamic 3D PAI.Conclusions:
Spatial phase-controlled algorithm is introduced for 3D image reconstruction. The PA signals are collected by plane transducer array and PDA system in single pulse excitation. The acquired volumetric data are sufficient for 3D image reconstruction. Therefore, tissue can avoid the long-term exposure to light source and it is safer than the current PAI forin vivoimaging. With an increase in the repetition rate of laser pulse and speed of image display, the imaging method will realize real-time 3D imaging, which will be significant in clinical detection and medical diagnosis.
- RADIATION THERAPY PHYSICS
38(2011); http://dx.doi.org/10.1118/1.3551996View Description Hide DescriptionPurpose:
Collapsed-cone convolution/superposition (CCCS) dose calculation is the workhorse for IMRTdose calculation. The authors present a novel algorithm for computing CCCS dose on the modern graphic processing unit (GPU).Methods:
The GPU algorithm includes a novel TERMA calculation that has no write-conflicts and has linear computation complexity. The CCCS algorithm uses either tabulated or exponential cumulative-cumulative kernels (CCKs) as reported in literature. The authors have demonstrated that the use of exponential kernels can reduce the computation complexity by order of a dimension and achieve excellent accuracy. Special attentions are paid to the unique architecture of GPU, especially the memory accessing pattern, which increases performance by more than tenfold.Results:
As a result, the tabulated kernel implementation in GPU is two to three times faster than other GPU implementations reported in literature. The implementation of CCCS showed significant speedup on GPU over single core CPU. On tabulated CCK, speedups as high as 70 are observed; on exponential CCK, speedups as high as 90 are observed.Conclusions:
Overall, the GPU algorithm using exponential CCK is 1000–3000 times faster over a highly optimized single-threaded CPU implementation using tabulated CCK, while the dose differences are within 0.5% and 0.5 mm. This ultrafast CCCS algorithm will allow many time-sensitive applications to use accurate dose calculation.
The use of a silicon strip detector dose magnifying glass in stereotactic radiotherapy QA and dosimetry38(2011); http://dx.doi.org/10.1118/1.3549759View Description Hide DescriptionPurpose:
Stereotactic radiosurgery/therapy (SRS/SRT) is the use of radiation ablation in place of conventional surgical excision to remove or create fibrous tissue in small target volumes. The target of the SRT/SRS treatment is often located in close proximity to critical organs, hence the requirement of high geometric precision including a tight margin on the planning target volume and a sharp dose fall off. One of the major problems with quality assurance (QA) of SRT/SRS is the availability of suitable detectors with the required spatial resolution. The authors present a novel detector that they refer to as the dose magnifying glass (DMG), which has a high spatial resolution (0.2 mm) and is capable of meeting the stringent requirements of QA and dosimetry in SRS/SRT therapy.Methods:
The DMG is an array of 128 phosphor implanted strips on a -type Si wafer. The sensitive area defined by a single strip is . The Si wafer is thick. It is mounted on a 0.12 mm thick Kapton substrate. The authors studied the dose per pulse (dpp) and angular response of the detector in a custom-made SRS phantom. The DMG was used to determine the centers of rotation and positioning errors for the linear accelerator’s gantry, couch, and collimator rotations. They also used the DMG to measure the profiles and the total scatter factor of the SRS cones. Comparisons were made with the EBT2 film and standard values. The DMG was also used for dosimetric verification of a typical SRStreatment with various noncoplanar fields and arc treatments when applied to the phantom.Results:
The dose per pulse dependency of the DMG was found to be for a dpp change of 7.5 times. The angular response of the detector was investigated in the azimuthal and polar directions. The maximum polar angular response was 13.8% at the gantry angle of 320°, which may be partly due to the phantom geometry. The maximum azimuthal angular response was 15.3% at gantry angles of 90° and 270°. The angular response at the gantry angle of 180° was 6.3%. A correction function was derived to correct for the angular dependence of the detector, which takes into account the contribution of the azimuthal and polar angular response at different treatment couch positions. The maximum positioning errors due to collimator, gantry, and couch rotation were , , and , respectively. The SRS cone agrees very well with the standard data with an average difference of . Comparison of the relative intensity profiles of the DMG and EBT2 measurements for a simulated SRStreatment shows a maximum difference of 2.5%.Conclusions:
The DMG was investigated for dose per pulse and angular dependency. Its application to SRS/SRT delivery verification was demonstrated. The DMG with its high spatial resolution and real time capability allows measurement of dose profiles for cone applicators down to 5 mm in diameter, both accurately and rapidly as required in typical SRS/SRT deliveries.
38(2011); http://dx.doi.org/10.1118/1.3547722View Description Hide DescriptionPurpose:
Traditionally, the tongue-and-groove effect due to the multileaf collimator architecture in intensity-modulated radiation therapy(IMRT) has typically been deferred to the leaf sequencing stage. The authors propose a new direct aperture optimization method for IMRTtreatment planning that explicitly incorporates dose calculation inaccuracies due to the tongue-and-groove effect into the treatment plan optimization stage.Methods:
The authors avoid having to accurately estimate the dosimetric effects of the tongue-and-groove architecture by using lower and upper bounds on the dose distribution delivered to the patient. They then develop a model that yields a treatment plan that is robust with respect to the corresponding dose calculation inaccuracies.Results:
Tests on a set of ten clinical head-and-neck cancer cases demonstrate the effectiveness of the new method in developing robust treatment plans with tight dose distributions in targets and critical structures. This is contrasted with the very loose bounds on the dose distribution that are obtained by solving a traditional treatment plan optimizationmodel that ignores tongue-and-groove effects in the treatment planning stage.Conclusions:
A robust direct aperture optimization approach is proposed to account for the dosimetric inaccuracies caused by the tongue-and-groove effect. The experiments validate the ability of the proposed approach in designing robust treatment plans regardless of the exact consequences of the tongue-and-groove architecture.
38(2011); http://dx.doi.org/10.1118/1.3552925View Description Hide DescriptionPurpose:
A patient-specific quality assurance (QA) method was developed to verify gantry-specific individual multileaf collimator(MLC) apertures (control points) in volumetric modulated arc therapy (VMAT) plans using an electronic portal imaging device(EPID).Methods:
VMAT treatment plans were generated in an Eclipse treatment planning system (TPS). DICOM images from a Varian EPID (aS1000) acquired in continuous acquisition mode were used for pretreatment QA. Each cine image file contains the grayscale image of the MLC aperture related to its specific control point and the corresponding gantry angle information. The TPS MLC file of this RapidArc plan contains the leaf positions for all 177 control points (gantry angles). In-house software was developed that interpolates the measured images based on the gantry angle and overlays them with the MLC pattern for all control points. The 38% isointensity line was used to define the edge of the MLC leaves on the portal images. The software generates graphs and tables that provide analysis for the number of mismatched leaf positions for a chosen distance to agreement at each control point and the frequency in which each particular leaf mismatches for the entire arc.Results:
Seven patients plans were analyzed using this method. The leaves with the highest mismatched rate were found to be treatment plan dependent.Conclusions:
This in-house software can be used to automatically verify the MLC leaf positions for all control points of VMAT plans using cine images acquired by an EPID.
Dosimetric verification of surface and superficial doses for head and neck IMRT with different PTV shrinkage margins38(2011); http://dx.doi.org/10.1118/1.3553406View Description Hide DescriptionPurpose:
Dosimetric uncertainty in the surface and superficial regions is still a major concern for radiation therapy and becomes more important when using the inverse planning algorithm for IMRT. The purpose of this study was to measure dose distributions and to evaluate the calculation accuracy in the superficial region for different planning target volume (PTV) shrinkage methods for head and neck IMRT plans.Methods:
A spherical polystyrene phantom 160 mm in diameter (ball phantom) was used to simulate the shape of the head. Strips of superflab bolus with thicknesses of 3.5 and 7.0 mm were spread on the surface of the ball phantom. Three sets of CTimages were acquired for the ball phantom without and with the bolus. The hypothetical clinical target volume (CTV) and critical structures (spinal cord and parotid glands) were outlined on each set of CTimages. The PTVs were initially created by expanding an isotropic 3 mm margin from the CTV and then margins of 0, 3, and 5 mm were shrunk from the phantom surface for dosimetric analysis. Seven-field IMRT plans with a prescribed dose of 180 cGy and same dose constraints were designed using an Eclipse treatment planning system. Superficial doses at depths of 0, 3.5, and 7.0 mm and at seven beam axis positions (gantry angles of 0°, 30°, 60°, 80°, 330°, 300°, and 280°) were measured for each PTV shrinkage margin using 0.1 mm ultrathin thermoluminescent dosimeters. For each plan, the measured doses were compared to the calculated doses.Results:
The PTV without shrinkage had the highest intensity and the steepest dose gradient in the superficial region. The mean measured doses for different positions at depths of 0, 3.5, and 7.0 mm were, , and , respectively. For a PTV with 3 mm shrinkage, the mean measured doses were , , and . For a PTV with 5 mm shrinkage, the mean measured doses were , , and . The comparisons indicated that more than 73.3% of the calculated points are with doses lower than the measured points and the difference of the dose becomes more significant in the shallower region. At 7.0 mm depth, the average difference between calculations and measurements was 2.5% (maximum 5.5%).Conclusions:
Application of the PTV shrinkage method should take into account the calculation inaccuracy, tumor coverage, and possible skin reaction. When the tumor does not invade the superficial region, an adequate shrinkage margin from the surface is helpful for reducing the skin reaction. As the tumor invades the superficial region, adding a bolus is a method better than only contouring PTV with skin inclusion.
Real-time verification of multileaf collimator-driven radiotherapy using a novel optical attenuation-based fluence monitor38(2011); http://dx.doi.org/10.1118/1.3549766View Description Hide DescriptionPurpose:
Multileaf collimator (MLC)-driven conformal radiotherapy modalities [e.g., such as intensity-modulated radiotherapy(IMRT), intensity-modulated arc therapy, and stereotactic body radiotherapy] are more subject to delivery errors and dose calculation inaccuracies than standard modalities. Fluence monitoring during treatment delivery could reduce such errors by allowing an independent interface to quantify and assess measured difference between the delivered and planned treatment administration. We developed an optical attenuation-baseddetector to monitor fluence for the on-line quality control of radiotherapy delivery. The purpose of the current study was to develop the theoretical background of the invention and to evaluate the detector’s performance both statistically and in clinical situations.Methods:
We aligned 60 27-cm scintillating fibers coupled to a photodetector via clear optical fibers in the direction of motion of each of the 60 leaf pairs of a 120 leaves Millenium MLC on a Varian Clinac iX. We developed a theoretical model to predict the intensity of light collected on each side of the scintillating fibers when placed under radiation fields of varying sizes, intensities, and positions. The model showed that both the central position of the radiation field on the fiber and the integral fluence passing through the fiber could be assessed independently in a single measurement. We evaluated the performance of the prototype by (1) measuring the intrinsic variation of the measured values of and , (2) measuring the impact on the measured values of and of random leaf positioning errors introduced into IMRT fields, and (3) comparing the predicted values of and calculated with the treatment planning software to the measured values of and in order to assess the predictive effectiveness of the developed theoretical model.Results:
We observed a very low intrinsic dispersion, dominated by Poisson statistics, for both (standard deviations of less than 1 mm) and (standard deviations of less than 0.20%). When confronted with random leaf positioning errors from IMRT segments, was highly sensitive to single leaf positioning errors as small as 1 mm at isocenter, while was sensitive to leaf pair translation errors of at least 2 mm at isocenter. Owing to the uncertainties in the doses calculated in regions of high perpendicular dose gradients, the measured values of and deviated from the predicted values of and by a mean of 1.3 mm and 2.6%, respectively.Conclusion:
Our study showed that an optical attenuation-baseddetector can be used to effectively monitor integral fluence during radiotherapy delivery. The performance of such a system would enable real-time quality control of the incident fluence in current MLC-driven treatments such as IMRT and future adaptive radiotherapy procedures where new treatment plans will have to be delivered without passing thru the current standard quality control chain.
Treatment planning of a skin-sparing conical breast brachytherapy applicator using conventional brachytherapy software38(2011); http://dx.doi.org/10.1118/1.3552921View Description Hide DescriptionPurpose:
AccuBoost is a noninvasive image-guided technique for the delivery of partial breast irradiation to the tumor bed and currently serves as an alternate to conventional electron beam boost. To irradiate the target volume while providing dose sparing to the skin, the round applicator design was augmented through the addition of an internally truncated conical shield and the reduction of the source to skin distance.Methods:
Brachytherapydose distributions for two types of conical applicators were simulated and estimated using Monte Carlo(MC) methods for radiation transport and a conventional treatment planning system (TPS). MC-derived and TPS-generated dose volume histograms (DVHs) and dose distribution data were compared for both the conical and round applicators for benchmarking purposes.Results:
Agreement using the gamma-index test was ⩾99.95% for distance to agreement and dose accuracy criteria of 2 mm and 2%, respectively. After observing good agreement, TPS DVHs and dose distributions for the conical and round applicators were obtained and compared. Brachytherapydose distributions generated using Pinnacle3 for ten CTdata sets showed that the parallel-opposed beams of the conical applicators provided similar PTV coverage to the round applicators and reduced the maximum dose to skin, chest wall, and lung by up to 27%, 42%, and 43%, respectively.Conclusions:
Brachytherapydose distributions for the conical applicators have been generated using MC methods and entered into the Pinnacle3 TPS via the Tufts technique. Treatment planning metrics for the conical AccuBoost applicators were significantly improved in comparison to those for conventional electron beam breast boost.
The difference of scoring dose to water or tissues in Monte Carlo dose calculations for low energy brachytherapy photon sources38(2011); http://dx.doi.org/10.1118/1.3549760View Description Hide DescriptionPurpose:
The goal of this work is to compare(radiation transported in medium; dose scored in medium) and (radiation transported in medium; dose scored in water) obtained from Monte Carlo(MC) simulations for a subset of human tissues of interest in low energy photonbrachytherapy. Using low dose rate seeds and an electronic brachytherapy source (EBS), the authors quantify the large cavity theory conversion factors required. The authors also assess whether applying large cavity theory utilizing the sources’ initial photonspectra and average photon energy induces errors related to spatial spectral variations. First, ideal spherical geometries were investigated, followed by clinical brachytherapy LDR seed implants for breast and prostate cancer patients.Methods:
Two types of dose calculations are performed with theGEANT4MC code. (1) For several human tissues,dose profiles are obtained in spherical geometries centered on four types of low energy brachytherapy sources: , , and seeds, as well as an EBS operating at 50 kV. Ratios of over are evaluated in the 0–6 cm range. In addition to mean tissue composition, compositions corresponding to one standard deviation from the mean are also studied. (2) Four clinical breast (using ) and prostate (using ) brachytherapy seed implants are considered. MCdose calculations are performed based on postimplant CT scans using prostate and breast tissue compositions. PTV values are compared for and .Results:
(1) Differences of −3% to 70% are observed for the investigated tissues. For a given tissue, is similar for all sources within 4% and does not vary more than 2% with distance due to very moderate spectral shifts. Variations of tissue composition about the assumed mean composition influence the conversion factors up to 38%. (2) The ratio of over for clinical implants matches at 1 cm from the single point sources.Conclusions:
Given the small variation with distance, using conversion factors based on the emitted photonspectrum (or its mean energy) of a given source introduces minimal error. The large differences observed between scoring schemes underline the need for guidelines on choice of media for dose reporting. Providing such guidelines is beyond the scope of this work.
On the relationships between electron spot size, focal spot size, and virtual source position in Monte Carlo simulations38(2011); http://dx.doi.org/10.1118/1.3556560View Description Hide DescriptionPurpose:
Every year, new radiotherapy techniques including stereotactic radiosurgery using linear accelerators give rise to new applications of Monte Carlo(MC) modeling. Accurate modeling requires knowing the size of the electron spot, one of the few parameters to tune in MC models. The resolution of integrated megavoltage imaging systems, such as the tomotherapy system, strongly depends on the photon spot size which is closely related to the electron spot. The aim of this article is to clarify the relationship between the electron spot size and the photon spot size (i.e., thefocal spot size) for typical incident electron beam energies and target thicknesses.Methods:
Three electron energies (3, 5.5, and 18 MeV), four electron spot sizes (, 0.5, 1, and 1.5 mm), and two tungsten target thicknesses (0.15 and 1 cm) were considered. The formation of the photon beam within the target was analyzed through electron energy deposition with depth, as well as photon production at several phase-space planes placed perpendicular to the beam axis, where only photons recorded for the first time were accounted for. Photon production was considered for “newborn” photons intersecting a plane at the isocenter (85 cm from source). Finally, virtual source position and “effective” focal spot size were computed by backprojecting all the photons from the bottom of the target intersecting a plane. The virtual source position and focal spot size were estimated at the plane position where the latter is minimal.Results:
In the relevant case of considering only photons intersecting the plane, the results unambiguously showed that the effective photon spot is created within the first 0.25 mm of the target and that electron and focal spots may be assumed to be equal within 3–4%.Conclusions:
In a good approximation photon spot size equals electron spot size for high energy X-ray treatments delivered by linear accelerators.
A novel lateral disequilibrium inclusive (LDI) pencil-beam based dose calculation algorithm: Evaluation in inhomogeneous phantoms and comparison with Monte Carlo calculations38(2011); http://dx.doi.org/10.1118/1.3557952View Description Hide DescriptionPurpose:
Pencil-beam (PB) based dose calculation for treatment planning is limited by inaccuracies in regions of tissue inhomogeneities, particularly in situations with lateral electron disequilibrium as is present at tissue/lung interfaces. To overcome these limitations, a new “lateral disequilibrium inclusive” (LDI) PB based calculation algorithm was introduced. In this study, the authors evaluated the accuracy of the new model by film and ionization chamber measurements and Monte Carlo simulations.Methods:
To validate the performance of the newLDI algorithm implemented in Corvus 09®, eight test plans were generated on inhomogeneous thorax and pelvis phantoms. In addition, three plans were calculated with a simple effective path length (EPL) algorithm on the inhomogeneous thorax phantom. To simulate homogeneous tissues, four test plans were evaluated in homogeneous phantoms (homogeneous dose calculation).Results:
The mean pixel pass rates and standard deviations of the gamma 4%/4 mm test for the film measurements were for the plans calculated with LDI, for the plans calculated with EPL, and for the homogeneous plans. Ionization chamber measurements and Monte Carlo simulations confirmed the high accuracy of the new algorithm (dose deviations ; gamma 3%/3 mm )Conclusions:
LDI represents an accurate and fast dose calculation algorithm for treatment planning.
38(2011); http://dx.doi.org/10.1118/1.3553404View Description Hide DescriptionPurpose:
To identify the most informative methods for reporting results of treatment planning comparisons.Methods:
Seven articles from the past year ofInternational Journal of RadiationOncology Biology Physics reported on comparisons of treatment plans for IMRT and IMAT. The articles were reviewed to identify methods of comparisons. Decision theoretical concepts were used to evaluate the study methods and highlight those that provide the most information.Results:
None of the studies examined the correlation between objectives. Statistical comparisons provided some information but not enough to provide support for a robust decision analysis.Conclusions:
The increased use of treatment planning studies to evaluate different methods in radiation therapy requires improved standards for designing the studies and reporting the results.
38(2011); http://dx.doi.org/10.1118/1.3555298View Description Hide DescriptionPurpose:
To evaluate the robustness of TG119-based quality assurance metrics for an IMRT system.Methods:
Four planners constructed treatment plans for the five IMRT test cases described in TG119. All plans were delivered to a solid water phantom in one treatment session in order to minimize session-dependent variation from phantom setup, film quality, machine performance, etc. Composite measurements utilized film and an ionization chamber. Per-field measurements were collected using a diode array device at an effective depth of 5 cm. All data collected were analyzed using the TG119 specifications to determine the confidence limit values for each planner separately and then compared.Results:
The mean variance of ion chamber measurements for each planner was within 1.7% of the planned dose. The resulting confidence limits were 3.13%, 1.98%, 3.65%, and 4.39%. Confidence limit values determined by composite film analysis were 8.06%, 13.4%, 9.30%, and 16.5%. Confidence limits from per-field measurements were 1.55%, 0.00%, 0.00%, and 2.89%.Conclusions:
For a single IMRT system, the accuracy assessment provided by TG119-based quality assurance metrics showed significant variations in the confidence limits between planners across all composite and per-field evaluations. This observed variation is likely due to the different levels of modulation between each planner’s set of plans. Performing the TG119 evaluation using plans produced by a single planner may not provide an adequate estimation of IMRT system accuracy.
38(2011); http://dx.doi.org/10.1118/1.3556559View Description Hide DescriptionPurpose:
Intensity modulated proton therapy (IMPT) is sensitive to errors, mainly due to high stopping power dependency and steep beam dose gradients. Conventional margins are often insufficient to ensure robustness of treatment plans. In this article, a method is developed that takes the uncertainties into account during the plan optimization.Methods:
Dose contributions for a number of range and setup errors are calculated and a minimax optimization is performed. The minimax optimization aims at minimizing the penalty of the worst case scenario. Any optimization function from conventional treatment planning can be utilized by the method. By considering only scenarios that are physically realizable, the unnecessary conservativeness of other robust optimization methods is avoided. Minimax optimization is related to stochastic programming by the more general minimax stochastic programming formulation, which enables accounting for uncertainties in the probability distributions of the errors.Results:
The minimax optimization method is applied to a lung case, a paraspinal case with titanium implants, and a prostate case. It is compared to conventional methods that use margins, single field uniform dose (SFUD), and material override (MO) to handle the uncertainties. For the lung case, the minimax method and the SFUD with MO method yield robust target coverage. The minimax method yields better sparing of the lung than the other methods. For the paraspinal case, the minimax method yields more robust target coverage and better sparing of the spinal cord than the other methods. For the prostate case, the minimax method and the SFUD method yield robust target coverage and the minimax method yields better sparing of the rectum than the other methods.Conclusions:
Minimax optimization provides robust target coverage without sacrificing the sparing of healthy tissues, even in the presence of low density lungtissue and high density titanium implants. Conventional methods using margins, SFUD, and MO do not utilize the full potential of IMPT and deliver unnecessarily high doses to healthy tissues.
38(2011); http://dx.doi.org/10.1118/1.3557884View Description Hide DescriptionPurpose:
To calibrate a Gamma Knife (GK) Perfexion using TG-21 with updated chamber-dependent values for modern microionization chambers in the new solid water Leksell dosimetry phantom. This work illustrates a calibration method using commercially available equipment, instruments, and an established dosimetry protocol that may be adopted at any GK center, thus reducing the interinstitutional variation in GK calibration. The calibration was verified by three third-party dosimetry checks. In addition, measurements of the relative output factors are presented and compared to available data and the new manufacturer-provided relative output factors yet to be released.Methods:
An absolute dosecalibration based on the TG-21 formalism, utilizing recently reported phantom material and chamber-dependent factors, was performed using a microionization chamber in a spherical solid water phantom. The result was compared to other calibration protocols based on TG-51. Independent verification of the machine output was conducted through M.D. Anderson Dosimetry Services (MDADS), using thermoluminescent dosimeters(TLDs) in an anthropomorphic head phantom; the Radiological Physics Center (RPC), using TLDs in the standard Elekta ABS plastic calibration phantom (gray phantom), included with the GK; and through a collaborative international calibration survey by the University of Pittsburgh Medical Center (UPMC) using alanine dosimeters, also in the gray phantom. The alanine dosimeters were read by the National Institute of Standards and Technology. Finally, Gafchromic EBT film was used to measure relative output factors and these factors were compared to values reported in the literature as well as new values announced for release by Elekta. The films were exposed in the solid water phantom using an included film insert accessory.Results:
Compared to the TG-21 protocol in the solid water phantom, the modified and unmodified TG-51 calibrations resulted in dose rates which were 1.8% and 1.3% lower, respectively. Ratios of the doses measured by third parties to the dose reported showed excellent agreement. MDADS returned ratios of 1.00 and 0.98 for the two TLDs irradiated. The RPC returned a mean ratio of 0.98 of the dose reported and the UPMC alanine study returned a mean ratio of 1.008. Relative output factors were found to be and for the 4 and 8 mm collimators, respectively, which are in excellent agreement with revised Monte Carlo-derived relative output factors Elekta is expected to recommend with the next version of the GK treatment planning software (GAMMAPLAN version 10).Conclusions:
The TG-21 dosimetry protocol, performed in a solid water phantom in conjunction with modern dosimeters and phantom material and chamber-dependent factors, can yield an accurate dose measurement in the unique GK treatment geometry. The technique described here can be easily adopted by institutions worldwide since all equipment and instruments used are commercially available, thus reducing the existing interinstitutional variation in GK calibration techniques. Relative output factor measurements made in this same solid water phantom were used to verify the relative output factors provided by Elekta and agreed excellently with output factors expected to be released in conjunction withGAMMAPLAN version 10.
Dosimetry protocol for the forthcoming clinical trials in synchrotron stereotactic radiation therapy (SSRT)38(2011); http://dx.doi.org/10.1118/1.3556561View Description Hide DescriptionPurpose:
An adequate dosimetry protocol for synchrotron radiation and the specific features of the ID17 Biomedical Beamline at the European Synchrotron Radiation Facility are essential for the preparation of the forthcoming clinical trials in the synchrotron stereotactic radiation therapy (SSRT). The main aim of this work is the definition of a suitable protocol based on standards of dose absorbed to water. It must allow measuring the absolute dose with an uncertainty within the recommended limits for patient treatment of 2%–5%.Methods:
Absolute dosimetry is performed with a thimble ionization chamber (PTW semiflex 31002) whose center is positioned at equivalent depth in water. Since the available synchrotron beam at the ESRF Biomedical Beamline has a maximum height of 3 mm, a scanning method was employed to mimic a uniform exposition of the ionization chamber. The scanning method has been shown to be equivalent to a broad beam irradiation. Different correction factors have been assessed by using Monte Carlo simulations.Results:
The absolute dose absorbed to water at 80 keV was measured in reference conditions with a 2% global uncertainty, within the recommended limits. The dose rate was determined to be in the range between 14 and 18 Gy/min, that is to say, a factor two to three times higher than the 6 Gy/min achievable in RapidArc or VMAT machines. The dose absorbed to water was also measured in a RW3 solid water phantom. This phantom is suitable for quality assurance purposes since less than 2% average difference with respect to the water phantom measurements was found. In addition, output factors were assessed for different field sizes.Conclusions:
A dosimetry protocol adequate for the specific features of the SSRT technique has been developed. This protocol allows measuring the absolute dose absorbed to water with an accuracy of 2%. It is therefore satisfactory for patient treatment.
- RADIATION IMAGING PHYSICS
38(2011); http://dx.doi.org/10.1118/1.3551999View Description Hide DescriptionPurpose:
To objectively characterize the performance of the gemstone spectral imaging (GSI) mode of GE CT750 HD scanner from a user’s perspective.Methods:
A regular scan protocol that approximates the adult abdomen scan protocol frequently used in the authors’ institute was selected as the baseline, and a GSI protocol (preset 11) that is similar to the regular protocol and has a moderate dose level was compared to the baseline protocol. The resolving power of both protocols was characterized in terms of modulation transfer functions and high contrast resolution bar readings. Their noise characteristics were studied through noise power spectra, and their low contrast detectability was compared via contrast-to-noise ratio. Material decomposition capability of GSI was evaluated by scanning iodine solutions of 9–24 mg/ml iodine concentration in a Gammex CT phantom and by examining the estimated iodine concentration. In addition, a formula describing the dependency of HU in iodine enhanced area on GSI monochromatic energies and iodine concentrations was provided and the theoretical values were compared with the measured results.Results:
The resolutions levels of 50%, 10%, and 5% MTF of GSI monochromatic images at 65 keV agree with those of the regular protocol within 0.1 lp/cm. GSI monochromatic images at 65 keV demonstrated the lowest noise level among GSI images of different monochromatic energies and showed very similar noise magnitude and noise power distribution as compared to the regular protocol images. The CNR of 60 and 65 keV GSI monoimages are approximately 100% of those of the regular protocol images. Estimated iodine concentration levels agreed with the actual values within 2% when the iodine solutions were placed at 3, 9, 12 o’clock positions of the phantom; when iodine solutions were placed at the phantom center and at 6 o’clock position, higher discrepancies of 2%–10% were observed. The observed dependency of HU on keV and iodine concentration levels agreed with the expectation from x-ray attenuations.Conclusions:
Equivalent performances were observed in the comparison between GSI 65 keV monochromatic images and images from a regular abdomen scan protocol. This suggests the possibility of GSI to be employed in routine abdominal scans, which would potentially offer more information through its capabilities of material decomposition.
38(2011); http://dx.doi.org/10.1118/1.3553408View Description Hide DescriptionPurpose:
In this work, the authors investigate how beam hardening affects the image formation in x-ray phase-contrast imaging and consecutively develop a correction algorithm based on the results of the analysis.Methods:
The authors’ approach utilizes a recently developed x-ray imaging technique using a gratinginterferometer capable of visualizing the differential phase shift of a wave front traversing an object. An analytical description of beam hardening is given, highlighting differences between attenuation and phase-contrast imaging. The authors present exemplary beam hardening artifacts for a number of well-defined samples in measurements at a compact laboratory setup using a polychromatic source.Results:
Despite the differences in image formation, the authors show that beam hardening leads to a similar reduction of image quality in phase-contrast imaging as in conventional attenuation-contrast imaging. Additionally, the authors demonstrate that for homogeneous objects, beam hardening artifacts can be corrected by a linearization technique, applicable to all kinds of phase-contrast methods using polychromatic sources.Conclusions:
The evaluated correction algorithm is shown to yield good results for a number of simple test objects and can thus be advocated in medical imaging and nondestructive testing.