Volume 39, Issue 8, August 2012
- medical physics letters
- radiation therapy physics
- radiation imaging physics
- radiation measurement physics
- magnetic resonance physics
- nuclear medicine physics
- ultrasound physics
- infrared and microwave imaging
- thermotherapy physics
- tissue measurements
- anatomy and physiology
- radiation protection physics
- books and publications
Index of content:
Performing drug-delivery with an ultrasonic imagingscannerin situ could drastically simplify treatment and improve its specificity. Our objective is to deliver large amounts of an encapsulated agent in vivo using a clinical ultrasoundscanner with a millimetric resolution. This study describes the encapsulation of fluorescein within ultrasound-inducible composite droplets and its targeted release in predefined zones in the liver of rats.Methods:
An aqueous solution of fluorescein was encapsulated within perfluorocarbon liquid in 4μm monodisperse droplets using a microfluidic system. The agent was then injected within the femoral vein of 12 rats. After exploratory ultrasound imaging, the sonographer defined five zones in the liver and a release sequence was initiated on the same apparatus. The surface of the liver was observed under fluorescence macroscopy and intraoperative fluorescence camera in vivo, before liver samples were sliced for pathology.Results:
Following the conversion of the droplets, a 25 dB increase in contrast was observed in the zones selected by the sonographer. These hyperechoic regions were colocalized with the bright fluorescent spots observed on the surface of the liver. A minimum peak-negative pressure of 2.6 MPa, which is within regulations for imaging pulses, was required for the delivery of the content of the droplets. The tissue and cellular structures were not affected by the exposure to the release sequence.Conclusions:
Since composite droplets can carry various therapeutic and imaging agents, they could deliver such agents specifically in any organ accessible to ultrasound.
- MEDICAL PHYSICS LETTERS
39(2012); http://dx.doi.org/10.1118/1.4736827View Description Hide DescriptionPurpose
: The generation of digitally reconstructedradiographs(DRRs) is the most time consuming step on the CPU in intensity based two-dimensional x-ray to three-dimensional (CT or 3D rotational x-ray)medical image registration, which has application in several image guided interventions. This work presents optimized DRR rendering on graphical processor units (GPUs) and compares performance achievable on four commercially available devices.Methods
: A ray-cast based DRR rendering was implemented for a 512 × 512 × 72 CT volume. The block size parameter was optimized for four different GPUs for a region of interest (ROI) of 400 × 225 pixels with different sampling ratios (1.1%–9.1% and 100%). Performance was statistically evaluated and compared for the four GPUs. The method and the block size dependence were validated on the latest GPU for several parameter settings with a public gold standard dataset (512 × 512 × 825 CT) for registration purposes.Results
: Depending on the GPU, the full ROI is rendered in 2.7–5.2 ms. If sampling ratio of 1.1%–9.1% is applied, execution time is in the range of 0.3–7.3 ms. On all GPUs, the mean of the execution time increased linearly with respect to the number of pixels if sampling was used.Conclusions
: The presented results outperform other results from the literature. This indicates that automatic 2D to 3D registration, which typically requires a couple of hundred DRR renderings to converge, can be performed quasi on-line, in less than a second or depending on the application and hardware in less than a couple of seconds. Accordingly, a whole new field of applications is opened for image guided interventions, where the registration is continuously performed to match the real-time x-ray.
- RADIATION THERAPY PHYSICS
EchoSeed Model 6733 Iodine-125 brachytherapy source: Improved dosimetric characterization using the MCNP5 Monte Carlo code39(2012); http://dx.doi.org/10.1118/1.4736418View Description Hide Description
This study primarily aimed to obtain the dosimetric characteristics of the Model 6733 125I seed (EchoSeed) with improved precision and accuracy using a more up-to-date Monte-Carlo code and data (MCNP5) compared to previously published results, including an uncertaintyanalysis. Its secondary aim was to compare the results obtained using the MCNP5, MCNP4c2, and PTRAN codes for simulation of this low-energy photon-emitting source. The EchoSeed geometry and chemical compositions together with a published 125I spectrum were used to perform dosimetric characterization of this source as per the updated AAPM TG-43 protocol. These simulations were performed in liquid water material in order to obtain the clinically applicable dosimetric parameters for this source model. Dose rate constants in liquid water, derived from MCNP4c2 and MCNP5 simulations, were found to be 0.993 cGyh−1 U−1 (±1.73%) and 0.965 cGyh−1 U−1 (±1.68%), respectively. Overall, the MCNP5 derived radial dose and 2D anisotropy functions results were generally closer to the measured data (within ±4%) than MCNP4c and the published data for PTRAN code (Version 7.43), while the opposite was seen for dose rate constant. The generally improved MCNP5 Monte Carlo simulation may be attributed to a more recent and accurate cross-section library. However, some of the data points in the results obtained from the above-mentioned Monte Carlo codes showed no statistically significant differences. Derived dosimetric characteristics in liquid water are provided for clinical applications of this source model.
Monte Carlo simulation of a compact microbeam radiotherapy system based on carbon nanotube field emission technology39(2012); http://dx.doi.org/10.1118/1.4728220View Description Hide DescriptionPurpose:
Microbeam radiation therapy(MRT) is an experimental radiotherapy technique that has shown potent antitumor effects with minimal damage to normal tissue in animal studies. This unique form of radiation is currently only produced in a few large synchrotron accelerator research facilities in the world. To promote widespread translational research on this promising treatment technology we have proposed and are in the initial development stages of a compact MRT system that is based on carbon nanotube field emission x-ray technology. We report on a Monte Carlo based feasibility study of the compact MRT system design.Methods:
Monte Carlo calculations were performed using EGSnrc-based codes. The proposed small animal research MRT device design includes carbon nanotubecathodes shaped to match the corresponding MRTcollimator apertures, a common reflection anode with filter, and a MRTcollimator. Each collimator aperture is sized to deliver a beam width ranging from 30 to 200μm at 18.6 cm source-to-axis distance. Design parameters studied with Monte Carlo include electron energy, cathode design, anode angle, filtration, and collimator design. Calculations were performed for single and multibeam configurations.Results:
Increasing the energy from 100 kVp to 160 kVp increased the photon fluence through the collimator by a factor of 1.7. Both energies produced a largely uniform fluence along the long dimension of the microbeam, with 5% decreases in intensity near the edges. The isocentric dose rate for 160 kVp was calculated to be 700 Gy/min/A in the center of a 3 cm diameter target. Scatter contributions resulting from collimator size were found to produce only small (<7%) changes in the dose rate for field widths greater than 50μm. Dose vs depth was weakly dependent on filtration material. The peak-to-valley ratio varied from 10 to 100 as the separation between adjacent microbeams varies from 150 to 1000 μm.Conclusions:
Monte Carlo simulations demonstrate that the proposed compact MRT system design is capable of delivering a sufficient dose rate and peak-to-valley ratio for small animal MRT studies.
39(2012); http://dx.doi.org/10.1118/1.4736527View Description Hide DescriptionPurpose:
To evaluate how changes in the measured small field output factors affect the doses in intensity-modulated treatment planning.Methods:
IMRT plans were created using Philips Pinnacle treatment planning system. The plans were optimized to treat a cylindrical target 2 cm in diameter and 2 cm in length. Output factors for 2 × 2 and 3 × 3 cm2field sizes were changed by ±5%, ±10%, and ±20% increments from the baseline measurements and entered into the planning system. The treatment units were recommissioned in the treatment planning system after each modification of the output factors and treatment plans were reoptimized. All plans were delivered to a solid water phantom and dose measurements were made using an ionization chamber. The percentage differences between measured and computed doses were calculated. An Elekta Synergy and a Varian 2300CD linear accelerator were separately evaluated.Results:
For the Elekta unit, decreasing the output factors resulted in higher measured than computed doses by 0.8% for −5%, 3.6% for −10%, and 8.7% for −20% steps. Increasing the output factors resulted in lower doses by 2.9% for +5%, 5.4% for +10%, and 8.3% for +20% steps. For the Varian unit no changes were observed for either increased or decreased output factors.Conclusions:
The measurement accuracy of small field output factors are of importance especially when the treatment plan consists of small segments as in IMRT. The method proposed here could be used to verify the accuracy of the measured small field output factors for certain linear accelerators as well as to test the beam model. The Pinnacle treatment planning system model uses output factors as a function of jaw setting. Consequently, plans using the Elekta unit, which conforms the jaws to the segments, are sensitive to small field measurement accuracy. On the other hand, for the Varian unit, jaws are fixed and segments are modeled as blocked fields hence, the impact of small field output factors on IMRT monitor unit calculation is not evaluable by this method.
Verification and dosimetric impact of Acuros XB algorithm on intensity modulated stereotactic radiotherapy for locally persistent nasopharyngeal carcinoma39(2012); http://dx.doi.org/10.1118/1.4736819View Description Hide DescriptionPurpose:
The main aim of the current study was to assess the dosimetric impact on intensity modulated stereotactic radiotherapy (IMSRT) for locally persistent nasopharyngeal carcinoma (NPC) due to the recalculation from the Anisotropic Analytical Algorithm (AAA) to the recently released Acuros XB (AXB) algorithm. The dosimetric accuracy of using AXB in predicting air/tissue interface doses from an open single small field in a simple geometric phantom and intensity modulated small fields in an anthropomorphic phantom was also investigated.Methods:
The central axis percentage depth doses (PDD) of a rectangular phantom containing an air cavity were calculated by both AAA and AXB from 6 MV beam with small field sizes (2 × 2 to 5 × 5 cm2). These data were compared to PDD measured by thin thermoluminescent dosimeters(TLDs) and Monte Carlo simulations. The doses predicted by AAA and AXB near air/tissue interfaces from five different IMSRT plans were compared to the TLD measured doses in an anthropomorphic phantom. The PTV coverage, conformity and doses to organs at risk (OARs) calculated by AAA and AXB were compared for 12 patients, using identical beam setup, leaves movement and monitor units.Results:
Testing using the simple rectangular phantom demonstrated that AAA and AXB overestimated the PDD at the air/tissue interfaces by up to 41% and 6%, respectively, from a 2 × 2 cm2 field. The secondary build-up curves predicted by AXB caught up well with the measured data at around 2 mm beyond the air cavity. Testing using the anthropomorphic phantom showed that AAA overestimated the doses by up to 10%, while the measured doses matched those of the AXB to within 3%. Using AAA, the planning target coverage represented by 100% of the reference dose was estimated to be 4% higher than using AXB. The averaged minimum dose to the PTV predicted by AAA was about 4% higher and OARs doses 3% to 6% higher compared to AXB.Conclusions:
AXB should be used whenever possible as the standard reference for IMSRT boost of NPC cases. The more accurate AXB indicating lower target coverage and lower minimum target dose compared to AAA should be noted.
39(2012); http://dx.doi.org/10.1118/1.4736825View Description Hide DescriptionPurpose:
EcCk, which stands forElectronic Chart ChecK, is a computer software and database system. It was developed to improve quality and efficiency of patient chart checking in radiation oncology departments. The core concept is to automatically collect and analyze patient treatment data, and to report discrepancies and potential concerns.Methods:
EcCk consists of several different computer technologies, including relational database, DICOM, dynamicHTML, and image processing. Implemented in MATLAB and C#, EcCk processes patient data in DICOM, PDF, Microsoft Word, database, and Pinnacle native formats. Generated reports are stored on the storage server and indexed in the database. A standalone report-browser program is implemented to allow users to view reports on any computer in the department. Checks are performed according to predefined logical rules, and results are presented through color-coded reports in which discrepancies are summarized and highlighted. Users examine the reports and take appropriate actions. The core design is intended to automate human task and to improve the reliability of the performed tasks. The software is not intended to replace human audits but rather to aid as a decision support tool.Results:
The software was successfully implemented in the clinical environment and has demonstrated the feasibility of automation of this common task with modern clinical tools. The software integrates multiple disconnected systems and successfully supports analysis of data in diverse formats.Conclusions:
While the human is the ultimate expert, EcCk has a significant potential to improve quality and efficiency of patient treatment record audits, and to allow verification of tasks that are not easily performed by humans. EcCk can potentially relieve human experts from simple and repetitive tasks, and allow them to work on other important tasks, and in the end to improve the quality and safety of radiation therapytreatments.
Proton therapy dose distribution comparison between Monte Carlo and a treatment planning system for pediatric patients with ependymomaa)39(2012); http://dx.doi.org/10.1118/1.4736413View Description Hide DescriptionPurpose
: Compare dose distributions for pediatric patients with ependymoma calculated using a Monte Carlo(MC) system and a clinical treatment planning system (TPS).Methods
: Plans from ten pediatric patients with ependymoma treated using double scatter proton therapy were exported from the TPS and calculated in our MC system. A field by field comparison of the distal edge (80% and 20%), distal fall off (80% to 20%), field width (50% to 50%), and penumbra (80% to 20%) were examined. In addition, the target dose for the full plan was compared.Results
: For the 32 fields from the 10 patients, the average differences of distal edge at 80% and 20% on central axis between MC and TPS are −1.9 ± 1.7 mm (p < 0.001) and −0.6 ± 2.3 mm (p = 0.13), respectively. Excluding the fields that ranged out in bone or an air cavity, the 80% difference was −0.9 ± 1.7 mm (p = 0.09). The negative value indicates that MC was on average shallower than TPS. The average difference of the 63 field widths of the 10 patients is −0.7 ± 1.0 mm (p < 0.001), negative indicating on average the MC had a smaller field width. On average, the difference in the penumbra was 2.3 ± 2.1 mm (p < 0.001). The average of the mean clinical target volume dose differences is −1.8% (p = 0.001), negative indicating a lower dose for MC.Conclusions
: Overall, the MC system and TPS gave similar results for field width, the 20% distal edge, and the target coverage. For the 80% distal edge and lateral penumbra, there was slight disagreement; however, the difference was less than 2 mm and occurred primarily in highly heterogeneous areas. These differences highlight that the TPS dose calculation cannot be automatically regarded as correct.
Improvements in dose calculation accuracy for small off-axis targets in high dose per fraction tomotherapy39(2012); http://dx.doi.org/10.1118/1.4736811View Description Hide DescriptionPurpose:
A recent field safety notice from TomoTherapy detailed the underdosing of small, off-axis targets when receiving high doses per fraction. This is due to angular undersampling in the dose calculation gantry angles. This study evaluates a correction method to reduce the underdosing, to be implemented in the current version (v4.1) of the TomoTherapy treatment planning software.Methods:
The correction method, termed “Super Sampling” involved the tripling of the number of gantry angles from which the dose is calculated during optimization and dose calculation. Radiochromic film was used to measure the dose to small targets at various off-axis distances receiving a minimum of 21 Gy in one fraction. Measurements were also performed for single small targets at the center of the Lucy phantom, using radiochromic film and the dose magnifying glass (DMG).Results:
Without super sampling, the peak dose deficit increased from 0% to 18% for a 10 mm target and 0% to 30% for a 5 mm target as off-axis target distances increased from 0 to 16.5 cm. When super sampling was turned on, the dose deficit trend was removed and all peak doses were within 5% of the planned dose. For measurements in the Lucy phantom at 9.7 cm off-axis, the positional and dose magnitude accuracy using super sampling was verified using radiochromic film and the DMG.Conclusions:
A correction method implemented in the TomoTherapy treatment planning system which triples the angular sampling of the gantry angles used during optimization and dose calculation removes the underdosing for targets as small as 5 mm diameter, up to 16.5 cm off-axis receiving up to 21 Gy.
An evaluation of interference of inflatable penile prostheses with electromagnetic localization and tracking system39(2012); http://dx.doi.org/10.1118/1.4736976View Description Hide DescriptionPurpose:
The Calypso system is stated by the manufacturer to be contraindicated for cases where the patient has been implanted with a penile prosthesis. This is due to concern for potential metal interference-related reduction of spatial localization and tracking accuracy. Here we quantify the localization and tracking accuracy of the Calypso system in the presence of inflatable penile prosthesis devices from three most widely used models which account for, essentially, 100% of implants in North America.Methods:
Phantom studies were first performed to quantify the interference of Calypso localization and tracking accuracy from both varying metal (steel) masses, and from the penile prosthetic devices themselves. The interference of varying steel masses was studied as a function of two factors: (a) the mass and (b) the location of steel material. The Calypso daily quality assurance (QA) phantom with three implanted Beacon® transponders was used to measure any aliasing of position that might occur due to metal interference. After confirming the safety of use in phantom, we implanted Calypso Beacon® transponders in one patient with a previously implanted AMS Model 700 inflatable penile prosthetic device. For each of the 42 delivered treatment fractions, redundant stereotactic ultrasound(US)image guidance was performed to ensure good agreement between US and Calypso guidance.Results:
We observed that a steel mass of less than 18 g did not cause any detectable positional aliasing for the Calypso tracking function. The mass of metal material measured to exist in the three penile prosthetic devices studied here (MP35N alloy) was approximately 1 g for each. No positional aliasing was observed for the three prosthetic devices in phantom, and good agreement between redundant US and Calypso was also observed in patient.Conclusions:
Both phantom and patient evaluations with the penile prosthetic devices showed no measurable interference with the Calypso system, thus indicating that accurate Calypso-based alignments can be performed in the presence of current industry standard inflatable penile prosthetic devices.
39(2012); http://dx.doi.org/10.1118/1.4736423View Description Hide DescriptionPurpose:
Despite promising research in modulated electron radiotherapy (MERT), an applicator to produce modulated electron beams and associated treatment planning software is still not commercially available. This work investigated an optimization process in treatment planning for the McGill few leaf electron collimator (FLEC) MERT delivery device. In addition, the possibility of combining MERT with photon fields was examined to investigate mixed beamradiotherapy.Methods:
A FLEC direct aperture optimization (DAO) method, in which FLEC apertures and weights were iteratively optimized was created. The authors evaluated the performance of DAO against our previous technique for generating FLEC plans and with commercially available photonbeamoptimization algorithms using a basic target and organ at risk geometry. The authors applied the DAO technique on a sarcoma treatment to evaluate clinical parameters. Finally, the authors examined the merit of mixing the DAO generated FLEC electron fields with photon fields to improve the dosimetry of the sarcoma treatment.Results:
In relation to the alternative plans, the DAO generated sarcoma MERT plan was competitive in its ability to reduce the dose to OAR but weaker in its ability to highly conform the dose to the target volume. The addition of photon fields improved the quality of the MERT plan in terms of OAR sparing and target conformality.Conclusions:
The DAO approach yielded deliverable FLEC-based MERT plans with a limited number of fields. The approach combined with photonoptimization added flexibility, where the mutual benefits of each radiation type was used in unison to improve plan quality.
High resolution 2D dose measurement device based on a few long scintillating fibers and tomographic reconstructiona)39(2012); http://dx.doi.org/10.1118/1.4736526View Description Hide DescriptionPurpose:
Patient-specific QA of highly conformal radiotherapy treatments are usually conducted using 2D or 3D dosimetry of the incident dose distribution in a water-equivalent phantom. However, dosimeters typically used for this task usually lack in either spatial resolution or dose accuracy. The purpose of this work is to develop and validate a novel type of high resolution 2D dosimeter based on the tomographic reconstruction of the dose projections obtained using long scintillating fibers for the quality assurance of modern radiotherapy techniques such as IMRT.Methods:
Fifty parallel scintillating fibers were aligned in a 30 cm diameter cylindrical masonite phantom with a 95 cm source-to-surface distance and a 100 cm source-to-fibers distance. The fibers were disposed so that the effective detection area of the scintillating fibers was a 20 cm diameter disk. Both ends of each scintillating fiber were coupled to clear optical fibers to enable light collection by a single CCD camera. Seven IMRT segments and two square fields were acquired using 18 projections over a 170° rotation of the device. Computation of the dose integrals was made for each scintillating fiber using the irradiation of known rectangular reference fields.Dose reconstructions were conducted using a total-variation minimization iterative reconstruction algorithm. Eight monitor units were programmed for each projection and the reconstructed dose grid pixel resolution was set to 1 × 1 mm2.Results:
3%/3 mm gamma tests conducted between the reconstructed IMRTdose distributions and the dose calculated with the treatment planning system Pinnacle3 were on average successful for 99.6% of the dose pixels with a predicted dose of at least 10% of the maximum dose. The dose profiles for both square fields and IMRT segments agreed within 2% to the dose calculated with Pinnacle3 except in high dose gradient regions, and were comparable to the dose measured using an ionization chamber array (IBA MatriXX) and radiographic films (Kodak XV2).Conclusions:
Using tomographic reconstruction on the projections acquired with rotating scintillating fibers, we were able to perform water-equivalent 2D dosimetry of square fields and IMRT segments with acceptable accuracy and high spatial resolution. The underlying concept of tomographic dosimetry and the small number of fibers needed to reconstruct a given 2D dose distribution offer new dosimetric possibilities, both applicable to 2D and 3D dosimetry.
39(2012); http://dx.doi.org/10.1118/1.4736800View Description Hide DescriptionPurpose:
Despite numerous advantages of radiochromic film dosimeter (high spatial resolution, near tissue equivalence, low energy dependence) to measure a relative dose distribution with film, one needs to first measure an absolute dose (following previously established reference dosimetry protocol) and then convert measured absolute dose values into relative doses. In this work, we present result of our efforts to obtain a functional form that would linearize the inherently nonlinear dose–response curve of the radiochromic film dosimetry system.Methods:
Functional form [ ζ = (−1)·netOD(2/3)/ln(netOD)] was derived from calibration curves of various previously established radiochromic film dosimetry systems. In order to test the invariance of the proposed functional form with respect to the film model used we tested it with three different GAFCHROMIC™ film models (EBT, EBT2, and EBT3) irradiated to various doses and scanned on a same scanner. For one of the film models (EBT2), we tested the invariance of the functional form to the scanner model used by scanning irradiated film pieces with three different flatbed scanner models (Epson V700, 1680, and 10000XL). To test our hypothesis that the proposed functional argument linearizes the response of the radiochromic film dosimetry system, verification tests have been performed in clinical applications: percent depth dose measurements, IMRT quality assurance (QA), and brachytherapy QA.Results:
Obtained R2 values indicate that the choice of the functional form of the new argument appropriately linearizes the dose response of the radiochromic film dosimetry system we used. The linear behavior was insensitive to both film model and flatbed scanner model used. Measured PDD values using the green channel response of the GAFCHROMIC™ EBT3 film model are well within ±2% window of the local relative dose value when compared to the tabulated Cobalt-60 data. It was also found that criteria of 3%/3 mm for an IMRT QA plan and 3%/2 mm for a brachytherapy QA plan are passing 95% gamma function points.Conclusions:
In this paper, we demonstrate the use of functional argument to linearize the inherently nonlinear response of a radiochromic film based reference dosimetry system. In this way, relative dosimetry can be conveniently performed using radiochromic film dosimetry system without the need of establishing calibration curve.
Integrated multicriterial optimization of beam angles and intensity profiles for coplanar and noncoplanar head and neck IMRT and implications for VMAT39(2012); http://dx.doi.org/10.1118/1.4736803View Description Hide DescriptionPurpose:
To quantify improved salivary gland sparing for head and neck cancer patients using intensity-modulated radiotherapy(IMRT) plans based on integrated computerized optimization of beam orientations and intensity profiles. To assess if optimized nonzero couch angles also improve VMAT plans.Methods:
Our in-house developed algorithm iCycle was used for automated generation of multicriterial optimized plans with optimized beam orientations and intensity profiles, and plans with optimized profiles for preselected beam arrangements. For 20 patients, five IMRT plans, based on one “wish-list,” were compared: (i) and (ii) seven- and nine-beam equiangular coplanar plans (iCycle7equi, iCycle9equi), (iii) and (iv) nine-beam plans with optimized coplanar and noncoplanar beam orientations (iCyclecopl, iCyclenoncopl), and (v) a nine-beam coplanar plan with optimized gantry angles and one optimized couch rotation (iCyclecouch). VMAT plans without and with this optimized couch rotation were evaluated.Results:
iCyclenoncopl resulted in the best salivary gland sparing, while iCyclecouch yielded similar results for 18 patients. For iCycle7equi, submandibular gland NTCP values were on average 5% higher. iCycle9equi performed better than iCycle7equi. iCyclecopl showed further improvement. Application of the optimized couch angle from iCyclecouch also improved NTCP values in VMAT plans.Conclusions:
iCycle allows objective comparison of competing planning strategies. Integrated optimization of beam profiles and angles can significantly improve normal tissue sparing, yielding optimal results for iCyclenoncopl.
On the output factor measurements of the CyberKnife iris collimator small fields: Experimental determination of the correction factors for microchamber and diode detectors39(2012); http://dx.doi.org/10.1118/1.4736810View Description Hide DescriptionPurpose:
To measure the output factors (OFs) of the small fields formed by the variable aperture collimator system (iris) of a CyberKnife (CK) robotic radiosurgery system, and determine the correction factors for a microchamber and four diode detectors.Methods:
OF measurements were performed using a PTW PinPoint 31014 microchamber, four diode detectors (PTW-60017, −60012, −60008, and the SunNuclear EDGE detector), TLD-100 microcubes, alanine dosimeters, EBT films, and polymergels for the 5 mm, 7.5 mm, 10 mm, 12.5 mm, and 15 mm iris collimators at 650 mm, 800 mm, and 1000 mm source to detector distance (SDD). The alanine OF measurements were corrected for volume averaging effects using the 3D dose distributions registered in polymergeldosimeters. correction factors for the PinPoint microchamber and the diode dosimeters were calculated through comparison against corresponding polymergel, EBT, alanine, and TLD results.Results:
Experimental OF results are presented for the array of dosimetric systems used. The PinPoint microchamber was found to underestimate small field OFs, and a correction factor ranging from 1.127 ± 0.022 (for the 5 mm iris collimator) to 1.004 ± 0.010 (for the 15 mm iris collimator) was determined at the reference SDD of 800 mm. The PinPoint correction factor was also found to increase with decreasing SDD; values equal to 1.220 ± 0.028 and 1.077 ± 0.016 were obtained for the 5 mm iris collimator at 650 mm and 1000 mm SDD, respectively. On the contrary, diode detectors were found to overestimate small field OFs and a correction factor equal to 0.973 ± 0.006, 0.954 ± 0.006, 0.937 ± 0.007, and 0.964 ± 0.006 was measured for the PTW-60017, −60012, −60008 and the EDGE diode detectors, respectively, for the 5 mm iris collimator at 800 mm SDD. The corresponding correction factors for the 15 mm iris collimator were found equal to 0.997 ± 0.010, 0.994 ± 0.009, 0.988 ± 0.010, and 0.986 ± 0.010, respectively. No correlation of the diode correction factors with SDD was observed.Conclusions:
This work demonstrates an experimental procedure for the determination of the correction factors required to obtain small field OF results of increased accuracy.
Reliable detection of fluence anomalies in EPID-based IMRT pretreatment quality assurance using pixel intensity deviations39(2012); http://dx.doi.org/10.1118/1.4736821View Description Hide DescriptionPurpose:
This work uses repeat images of intensity modulated radiation therapy(IMRT) fields to quantify fluence anomalies (i.e., delivery errors) that can be reliably detected in electronic portal images used for IMRT pretreatment quality assurance.Methods:
Repeat images of 11 clinical IMRT fields are acquired on a Varian Trilogy linear accelerator at energies of 6 MV and 18 MV. Acquired images are corrected for output variations and registered to minimize the impact of linear accelerator and electronic portal imaging device(EPID) positioning deviations. Detection studies are performed in which rectangular anomalies of various sizes are inserted into the images. The performance of detection strategies based on pixel intensity deviations (PIDs) and gamma indices is evaluated using receiver operating characteristic analysis.Results:
Residual differences between registered images are due to interfraction positional deviations of jaws and multileaf collimator leaves, plus imager noise. Positional deviations produce large intensity differences that degrade anomaly detection. Gradient effects are suppressed in PIDs using gradient scaling. Background noise is suppressed using median filtering. In the majority of images, PID-based detection strategies can reliably detect fluence anomalies of ≥5% in ∼1 mm2 areas and ≥2% in ∼20 mm2 areas.Conclusions:
The ability to detect small dose differences (≤2%) depends strongly on the level of background noise. This in turn depends on the accuracy of image registration, the quality of the reference image, and field properties. The longer term aim of this work is to develop accurate and reliable methods of detectingIMRT delivery errors and variations. The ability to resolve small anomalies will allow the accuracy of advanced treatment techniques, such as image guided, adaptive, and arc therapies, to be quantified.
39(2012); http://dx.doi.org/10.1118/1.4736951View Description Hide DescriptionPurpose:
Recently, the jaw size for the TomoTherapy Hi-Art II® (TomoTherapy Inc., Madison, WI) was reduced from 4 mm (J4) to 1 mm (J1) to improve the longitudinal (IEC-Y) resolution in megavoltage computed tomography (MVCT) images. This study evaluated the effect of jaw size on the image quality and dose, as well as the dose delivered to the lens of the eye, which is a highly radiosensitive tissue.Methods:
MVCT image quality (imagenoise, uniformity, contrast linearity, high-contrast resolution, and full width at half-maximum) and multiple scan average dose (MSAD) were measured at different jaw sizes. A head phantom and photoluminescence glass dosimeters(PLDs) were used to measure the exposed lens dose (cGy). Different MVCT scan modes (pitch = 1, 2, and 3) and scan lengths (108 mm, 156 mm, and 204 mm) were applied in the MSAD and PLDs measurements.Results:
The change in jaw size from J4 to J1 produced no change or only a slight improvement in imagenoise, uniformity, contrast linearity, and high-contrast resolution. However, the full-width at half-maximum reduced from approximately 7.2 at J4 to 4.5 mm at J1, which represents an enhancement in the longitudinal resolution. The MSAD at the center point changed from approximately 0.69–2.32 cGy (peripheral: 0.83–2.49 cGy) at J4 to 0.85–2.81 cGy (peripheral: 1.05–2.86 cGy) at J1. The measured lens dose increased from 0.92–3.36 cGy at J4 to 1.06–3.91 cGy at J1.Conclusions:
The change in jaw size improved longitudinal resolution. The MVCT imagingdose of approximately 3.86 cGy, 1.92 cGy, and 1.22 cGy was delivered at a pitch of 1, 2, and 3, respectively, per fraction in the head and neck treatment plans. Therefore, allowance for an approximately 15% increase in lens dose over that with J4 should be provided with J1.
39(2012); http://dx.doi.org/10.1118/1.4737024View Description Hide DescriptionPurpose:
Introducing intensity modulation into neutron radiotherapy (IMNRT) planning has the potential to mitigate some normal tissue complications seen in past neutron trials. While the hardware to deliver IMNRT plans has been in use for several years, until recently the IMNRT planning process has been cumbersome and of lower fidelity than conventional photon plans. Our in-house planning system used to calculate neutron therapy plans allows beam weight optimization of forward planned segments, but does not provide inverse optimization capabilities. Commercial treatment planning systems provide inverse optimization capabilities, but currently cannot model our neutron beam.Methods:
We have developed a methodology and software suite to make use of the robust optimization in our commercial planning system while still using our in-house planning system to calculate final neutron dose distributions. Optimized multileaf collimator (MLC) leaf positions for segments designed in the commercial system using a 4 MV photon proxy beam are translated into static neutron ports that can be represented within our in-house treatment planning system. The true neutron dose distribution is calculated in the in-house system and then exported back through theMATLAB software into the commercial treatment planning system for evaluation.Results:
The planning process produces optimized IMNRT plans that reduce dose to normal tissue structures as compared to 3D conformal plans using static MLC apertures. The process involves standard planning techniques using a commercially available treatment planning system, and is not significantly more complex than conventional IMRT planning. Using a photon proxy in a commercial optimization algorithm produces IMNRT plans that are more conformal than those previously designed at our center and take much less time to create.Conclusions:
The planning process presented here allows for the optimization of IMNRT plans by a commercial treatment planning optimization algorithm, potentially allowing IMNRT to achieve similar conformality in treatment as photon IMRT. The only remaining requirements for the delivery of very highly modulated neutron treatments are incremental improvements upon already implemented hardware systems that should be readily achievable.
39(2012); http://dx.doi.org/10.1118/1.4737095View Description Hide DescriptionPurpose:
The authors developed a realistic respiratory trace generating (RTG) tool for use with phantom and simulation studies.Methods:
The authors analyzed the extent of abdominal wall motion from a real-time position management system database comprised of 125 lung,liver, and abdominal patients to determine the shape and extent of motion. Using Akaike's information criterion (AIC), the authors compared different model types to find the optimal realistic model of respiratory motion.Results:
The authors compared a family of sigmoid curves and determined a four parameter sigmoid fit was optimal for over 98% patient inhale and exhale traces. This fit was also better than sin 2(x) for 98% of patient exhale and 70% of patient inhale traces and better than sin (x) for 100% of both patient inhale and exhale traces. This analysis also shows that sin 2(x) is better than sin (x) for over 95% of patient inhale and exhale traces. With results from shape and extent of motion analysis, we developed a realistic respiratory trace generating (RTG) software tool. The software can be run in two modes: population and user defined. In population mode, the RTG draws entirely from the population data including inter- and intra fraction amplitude and period variability and baseline drift. In user-defined mode, the user customizes the respiratory parameters by inputting the peak-to-peak amplitude, period, end exhale position, as well as controls variability in these parameters and baseline drift.Conclusions:
This work provides a method of generating custom respiratory data that can be used for initial implementation and testing of new technologies.
39(2012); http://dx.doi.org/10.1118/1.4736949View Description Hide DescriptionPurpose:
To evaluate methods of pretreatment IMRTanalysis, using real measurements performed with a commercial 2D detector array, for clinical relevance and accuracy by comparing clinical DVH parameters.Methods:
We divided the work into two parts. The first part consisted of six in-phantom tests aimed to study the sensitivity of the different analysis methods. Beam fluences, 3D dose distribution, and DVH of an unaltered original plan were compared to those of the delivered plan, in which an error had been intentionally introduced. The second part consisted of comparing gamma analysis with DVH metrics for 17 patient plans from various sites. Beam fluences were measured with the MapCHECK 2 detector, per-beam planar analysis was performed with the MapCHECK software, and 3D gamma analysis and the DVH evaluation were performed using 3DVH software.Results:
In a per-beam gamma analysis some of the tests yielded false positives or false negatives. However, the 3DVH software correctly described the DVH of the plan which included the error. The measured DVH from the plan with controlled error agreed with the planned DVH within 2% dose or 2% volume. We also found that a gamma criterion of 3%/3 mm was too lax to detect some of the forced errors. Global analysis masked some problems, while local analysis magnified irrelevant errors at low doses. Small hotspots were missed for all metrics due to the spatial resolution of the detector panel. DVH analysis for patient plans revealed small differences between treatment plan calculations and 3DVH results, with the exception of very small volume structures such as the eyes and the lenses. Target coverage (D98 and D95) of the measured plan was systematically lower than that predicted by the treatment planning system, while other DVH characteristics varied depending on the parameter and organ.Conclusions:
We found no correlation between the gamma index and the clinical impact of a discrepancy for any of the gamma index evaluation possibilities (global, local, 2D, or 3D). Some of the tests yielded false positives or false negatives in a per-beam gamma analysis. However, they were correctly accounted for in a DVH analysis. We also showed that 3DVH software is reliable for our tests, and is a viable method for correlating planar discrepancies with clinical relevance by comparing the measured DVH of target and OAR's with clinical tolerance.