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At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Application of Geant4 Monte Carlo simulation in dose calculations for small radiosurgical fields

The Geant4 toolkit was used to develop a Monte Carlo (MC)-based engine for accurate dose calculations in small radiation field sizes. The Geant4 toolkit (version 10.1.p02) was used to simulate 6-MV photon beam of a Varian2100C linear accelerator that is being used for stereotactic radiosurgery (SRS) treatment with small radiation fields. Geometric models of 3 in-house designed radiosurgical divergent cones, with the diameters of their projections at the isocenter being 10, 20, and 30 mm, were simulated. The accuracy of the MC simulation technique was examined by reproducing several different simulated dosimetric parameters of the primary beams with the experimental data. The dose distributions are first checked for single beams for each cone, then standard multiple field (SMF) techniques are applied. A sample set of DICOM files from computed tomography (CT) scan imaging of a patient's head was converted to the Geant4 geometry format to implement MC-based engine for a clinical test. To validate the accuracy of the MC-based calculations for SMF arrangements, the isodose lines from MC simulation in water phantom were compared with the measured isodose lines using EBT3 Gafchromic film in Solid Water phantoms. Agreements between measured and simulated depth dose values and beam profiles for SRS cones were generally within 2%/2 mm. For output factors, the largest discrepancy was observed for 10 mm SRS cone, which was 1.7%. For SMF techniques, in SRS cones, the MC simulation and EBT3 Gafchromic film dosimetry were in acceptable agreement (5%/5 mm). Excellent agreement between the results of the MC-based and measured dose values for both single and SMF techniques in SRS cones indicates the ability of the Geant4 toolkit to be applied as the platform for treatment planning of advanced radiotherapy techniques.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2016

SU-G-TeP4-08: Automating the Verification of Patient Treatment Parameters

Purpose: To automate the daily verification of each patient’s treatment by utilizing the trajectory log files (TLs) written by the Varian TrueBeam linear accelerator while reducing the number of false positives including jaw and gantry positioning errors, that are displayed in the Treatment History tab of Varian’s Chart QA module. Methods: Small deviations in treatment parameters are difficult to detect in weekly chart checks, but may be significant in reducing delivery errors, and would be critical if detected daily. Software was developed in house to read TLs. Multiple functions were implemented within the software that allow it to operate via a GUI to analyze TLs, or as a script to run on a regular basis. In order to determine tolerance levels for the scripted analysis, 15,241 TLs from seven TrueBeams were analyzed. The maximum error of each axis for each TL was written to a CSV file and statistically analyzed to determine the tolerance for each axis accessible in the TLs to flag for manual review. The software/scripts developed were tested by varying the tolerance values to ensure veracity. After tolerances were determined, multiple weeks of manual chart checks were performed simultaneously with the automated analysis to ensure validity. Results: The tolerance values for the major axis were determined to be, 0.025 degrees for the collimator, 1.0 degree for the gantry, 0.002cm for the y-jaws, 0.01cm for the x-jaws, and 0.5MU for the MU. The automated verification of treatment parameters has been in clinical use for 4 months. During that time, no errors in machine delivery of the patient treatments were found. Conclusion: The process detailed here is a viable and effective alternative to manually checking treatment parameters during weekly chart checks.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

A study of IMRT planning parameters on planning efficiency, delivery efficiency, and plan quality

Purpose: To improve planning and delivery efficiency of head and neck IMRT without compromising planning quality through the evaluation of inverse planning parameters.Methods: Eleven head and neck patients with pre-existing IMRT treatment plans were selected for this retrospective study. The Pinnacle treatment planning system (TPS) was used to compute new treatment plans for each patient by varying the individual or the combined parameters of dose/fluence grid resolution, minimum MU per segment, and minimum segment area. Forty-five plans per patient were generated with the following variations: 4 dose/fluence grid resolution plans, 12 minimum segment area plans, 9 minimum MU plans, and 20 combined minimum segment area/minimum MU plans. Each plan was evaluated and compared to others based on dose volume histograms (DVHs) (i.e., plan quality), planning time, and delivery time. To evaluate delivery efficiency, a model was developed that estimated the delivery time of a treatment plan, and validated through measurements on an Elekta Synergy linear accelerator. Results: The uncertainty (i.e., variation) of the dose-volume index due to dose calculation grid variation was as high as 8.2% (5.5 Gy in absolute dose) for planning target volumes (PTVs) and 13.3% (2.1 Gy in absolute dose) for planning at risk volumes (PRVs). Comparison results of dose distributions indicated that smaller volumes were more susceptible to uncertainties. The grid resolution of a 4 mm dose grid with a 2 mm fluence grid was recommended, since it can reduce the final dose calculation time by 63% compared to the accepted standard (2 mm dose grid with a 2 mm fluence grid resolution) while maintaining a similar level of dose-volume index variation. Threshold values that maintained adequate plan quality (DVH results of the PTVs and PRVs remained satisfied for their dose objectives) were 5 cm{sup 2} for minimum segment area and 5 MU for minimum MU. As the minimum MU parameter was increased, the number of segments and delivery time were decreased. Increasing the minimum segment area parameter decreased the plan MU, but had less of an effect on the number of segments and delivery time. Our delivery time model predicted delivery time to within 1.8%. Conclusions: Increasing the dose grid while maintaining a small fluence grid allows for improved planning efficiency without compromising plan quality. Delivery efficiency can be improved by increasing the minimum MU, but not the minimum segment area. However, increasing the respective minimum MU and/or the minimum segment area to any value greater than 5 MU and 5 cm{sup 2} is not recommended because it degrades plan quality.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2013

Accuracy and Consistency of Respiratory Gating in Abdominal Cancer Patients

Purpose: To evaluate respiratory gating accuracy and intrafractional consistency for abdominal cancer patients treated with respiratory gated treatment on a regular linear accelerator system. Methods and Materials: Twelve abdominal patients implanted with fiducials were treated with amplitude-based respiratory-gated radiation therapy. On the basis of daily orthogonal fluoroscopy, the operator readjusted the couch position and gating window such that the fiducial was within a setup margin (fiducial-planning target volume [f-PTV]) when RPM indicated “beam-ON.” Fifty-five pre- and post-treatment fluoroscopic movie pairs with synchronized respiratory gating signal were recorded. Fiducial motion traces were extracted from the fluoroscopic movies using a template matching algorithm and correlated with f-PTV by registering the digitally reconstructed radiographs with the fluoroscopic movies. Treatment was determined to be “accurate” if 50% of the fiducial area stayed within f-PTV while beam-ON. For movie pairs that lost gating accuracy, a MATLAB program was used to assess whether the gating window was optimized, the external-internal correlation (EIC) changed, or the patient moved between movies. A series of safety margins from 0.5 mm to 3 mm was added to f-PTV for reassessing gating accuracy. Results: A decrease in gating accuracy was observed in 44% of movie pairs from daily fluoroscopic movies of 12 abdominal patients. Three main causes for inaccurate gating were identified as change of global EIC over time (∼43%), suboptimal gating setup (∼37%), and imperfect EIC within movie (∼13%). Conclusions: Inconsistent respiratory gating accuracy may occur within 1 treatment session even with a daily adjusted gating window. To improve or maintain gating accuracy during treatment, we suggest using at least a 2.5-mm safety margin to account for gating and setup uncertainties.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2014

A practical and theoretical definition of very small field size for radiotherapy output factor measurements

Purpose: This work introduces the concept of very small field size. Output factor (OPF) measurements at these field sizes require extremely careful experimental methodology including the measurement of dosimetric field size at the same time as each OPF measurement. Two quantifiable scientific definitions of the threshold of very small field size are presented. Methods: A practical definition was established by quantifying the effect that a 1 mm error in field size or detector position had on OPFs and setting acceptable uncertainties on OPF at 1%. Alternatively, for a theoretical definition of very small field size, the OPFs were separated into additional factors to investigate the specific effects of lateral electronic disequilibrium, photon scatter in the phantom, and source occlusion. The dominant effect was established and formed the basis of a theoretical definition of very small fields. Each factor was obtained using Monte Carlo simulations of a Varian iX linear accelerator for various square field sizes of side length from 4 to 100 mm, using a nominal photon energy of 6 MV. Results: According to the practical definition established in this project, field sizes ≤15 mm were considered to be very small for 6 MV beams for maximal field size uncertainties of 1 mm. If the acceptable uncertainty in the OPF was increased from 1.0% to 2.0%, or field size uncertainties are 0.5 mm, field sizes ≤12 mm were considered to be very small. Lateral electronic disequilibrium in the phantom was the dominant cause of change in OPF at very small field sizes. Thus the theoretical definition of very small field size coincided to the field size at which lateral electronic disequilibrium clearly caused a greater change in OPF than any other effects. This was found to occur at field sizes ≤12 mm. Source occlusion also caused a large change in OPF for field sizes ≤8 mm. Based on the results of this study, field sizes ≤12 mm were considered to be theoretically very small for 6 MV beams. Conclusions: Extremely careful experimental methodology including the measurement of dosimetric field size at the same time as output factor measurement for each field size setting and also very precise detector alignment is required at field sizes at least ≤12 mm and more conservatively≤15 mm for 6 MV beams. These recommendations should be applied in addition to all the usual considerations for small field dosimetry, including careful detector selection.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2016

MO-FG-BRA-06: Electromagnetic Beacon Insertion in Lung Cancer Patients and Resultant Surrogacy Errors for Dynamic MLC Tumour Tracking

Purpose: To assess endo-bronchial electromagnetic beacon insertion and to quantify the geometric accuracy of using beacons as a surrogate for tumour motion in real-time multileaf collimator (MLC) tracking of lung tumours. Methods: The LIGHT SABR trial is a world-first clinical trial in which the MLC leaves move with lung tumours in real time on a standard linear accelerator. Tracking is performed based on implanted electromagnetic beacons (CalypsoTM, Varian Medical Systems, USA) as a surrogate for tumour motion. Five patients have been treated and have each had three beacons implanted endo-bronchially under fluoroscopic guidance. The centre of mass (C.O.M) has been used to adapt the MLC in real-time. The geometric error in using the beacon C.O.M as a surrogate for tumour motion was measured by measuring the tumour and beacon C.O.M in all phases of the respiratory cycle of a 4DCT. The surrogacy error was defined as the difference in beacon and tumour C.O.M relative to the reference phase (maximum exhale). Results: All five patients have had three beacons successfully implanted with no migration between simulation and end of treatment. Beacon placement relative to tumour C.O.M varied from 14 to 74 mm and in one patient spanned two lobes. Surrogacy error was measured in each patient on the simulation 4DCT and ranged from 0 to 3 mm. Surrogacy error as measured on 4DCT was subject to artefacts in mid-ventilation phases. Surrogacy error was a function of breathing phase and was typically larger at maximum inhale. Conclusion: Beacon placement and thus surrogacy error is a major component of geometric uncertainty in MLC tracking of lung tumours. Surrogacy error must be measured on each patient and incorporated into margin calculation. Reduction of surrogacy error is limited by airway anatomy, however should be taken into consideration when performing beacon insertion and planning. This research is funded by Varian Medical Systems via a collaborative research agreement.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Proton dose calculation on scatter-corrected CBCT image: Feasibility study for adaptive proton therapy

Purpose: To demonstrate the feasibility of proton dose calculation on scatter-corrected cone-beam computed tomographic (CBCT) images for the purpose of adaptive proton therapy. Methods: CBCT projection images were acquired from anthropomorphic phantoms and a prostate patient using an on-board imaging system of an Elekta infinity linear accelerator. Two previously introduced techniques were used to correct the scattered x-rays in the raw projection images: uniform scatter correction (CBCT{sub us}) and a priori CT-based scatter correction (CBCT{sub ap}). CBCT images were reconstructed using a standard FDK algorithm and GPU-based reconstruction toolkit. Soft tissue ROI-based HU shifting was used to improve HU accuracy of the uncorrected CBCT images and CBCT{sub us}, while no HU change was applied to the CBCT{sub ap}. The degree of equivalence of the corrected CBCT images with respect to the reference CT image (CT{sub ref}) was evaluated by using angular profiles of water equivalent path length (WEPL) and passively scattered proton treatment plans. The CBCT{sub ap} was further evaluated in more realistic scenarios such as rectal filling and weight loss to assess the effect of mismatched prior information on the corrected images. Results: The uncorrected CBCT and CBCT{sub us} images demonstrated substantial WEPL discrepancies (7.3 ± 5.3 mm and 11.1 ± 6.6 mm, respectively) with respect to the CT{sub ref}, while the CBCT{sub ap} images showed substantially reduced WEPL errors (2.4 ± 2.0 mm). Similarly, the CBCT{sub ap}-based treatment plans demonstrated a high pass rate (96.0% ± 2.5% in 2 mm/2% criteria) in a 3D gamma analysis. Conclusions: A priori CT-based scatter correction technique was shown to be promising for adaptive proton therapy, as it achieved equivalent proton dose distributions and water equivalent path lengths compared to those of a reference CT in a selection of anthropomorphic phantoms.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-319: Dosimetric Evaluation of IMRT with Mix-Energy Beam for Deep Seated Targets

Purpose: IMRT is preferred in the range of 6–10MV X-rays. Partially adding high energy (>10MV) treatment fields, may provide advantage of both higher and lower energies. To study IMRT dose distribution obtained from treatment plans with single (6MV) and mixed-energy (6MV and 15MV) for deep seated targets (separation more than 30cm). Methods: Five patients of carcinoma of cervix were studied using eclipse planning system. Two different dynamic IMRT plans were generated for Varian CL2300C/D linear accelerator; one is by using 6MV X-ray with seven equally spaced coplanar beams. In second plan, 2 lateral oblique fields (gantry angle 102°, 255°) beam energy was modified to 15MV by keeping all other parameters and dose volume constraints constant. Dose prescription for the planning target volume (PTV) was (5040cGy/28f). For plan comparison, dose volume histogram (DVH) was used and PTV coverage index (CI=Target volume covered by prescription dose/Target volume), heterogeneity index (D5/D95), mean dose to organ at risk (OAR) and normal tissue integral dose (NTID, liter-Gray) was also noted. Total monitor unit (MU) required to deliver a plan was also noted. Results: Mixed-energy plan showed a better conformity and CI values were 0.942±0.032 and 0.960±0.040 for single-energy and mixed-energy plan, respectively. In addition, HI value of mixed energy beam is comparable to that of single energy and the values were within 1.084±0.034 and 1.082±0.032 for single energy and mixed-energy plan, respectively. Variation in mean dose to bladder, rectum and bowel were within 1.05%, 0.87% and 0.90%. NTID was lesser for mixed-energy beam due to use of two high-energy fields. NTID were 1573.40±214.60 and 1510.20±249.80 litre-Gray for single energy and mixed-energy plan. MU needed to deliver a plan was similar in both plans and MUs were 238±45 and 237±47. Conclusion: Partial use of 15MV treatment fields in IMRT plan for deep seated targets showed dosimetric advantage over 6MV plan.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-F-P-34: Commission of Enhanced Dynamic Wedge of Varian Truebeam Linac System with Feature Study

Purpose: To evaluate an Enhanced Dynamic Wedge (EDW) as part of machine commission process with feature study. Methods: The EDW system in this study was from a Truebeam, which is the Linear accelerator manufactured by Varian Medical Systems. The EDW feature vectors includes selected elements. These elements were dosimetric output spots check, field size, wedge angles, dose rate, collimator orientation, and different energy settings. Point dose measurement was done by a PTW farmer chamber, and profiles were measured by Gafchromic EBT2 films positing at different depths of the Solidwater based on the study elements. The output spot measurements were done with PTW farmer chamber with Solidwater setting for all orientation and wedge angles in the EDW system. The profiles comparisons were done by IMRT measurement function in RIT software at version 6.3. And the films were scanned by Vidar scanner. Dosimetry calculation were done by using the same Solidwater scanned by GE LightSpeed CT in Eclipse Treatment Planning System (TPS). Then measurements were compared to simulation results in TPS. Results: The energy average percentage difference between chamber measurement and TPS was 0.16% with standard deviation (SD) at 0.93%. For selected features, the average percentage difference between film measurement and computation was 0.93% with SD at 1.55% in horizontal profiles, and 1.18% with SD at 0.98% at vertical profiles. The average gamma difference for film measurement and TPS computing results was at 0.924 with SD at 0.314. Conclusion: A feature vector was developed to describe the commission of EDW, and developing a complete set of features for sufficiency of commission of a LINAC function could provide optimal commission instance with acceptable confident level of clinical application of the machine. Given the institution specific vector pattern and big data process, it could provide wide range clinical outcome comparison information in application of EDW.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-202: Impact of Monte Carlo Dose Calculation Algorithm On Prostate SBRT Treatments

Purpose: The purpose of this work was to quantify the dosimetric impact of using Monte Carlo algorithm on pre calculated SBRT prostate treatment with pencil beam dose calculation algorithm. Methods: A 6MV photon beam produced by a Novalis TX (BrainLAB-Varian) linear accelerator equipped with HDMLC was used. Treatment plans were done using 9 fields with Iplanv4.5 (BrainLAB) and dynamic IMRT modality. Institutional SBRT protocol uses a total dose to the prostate of 40Gy in 5 fractions, every other day. Dose calculation is done by pencil beam (2mm dose resolution), heterogeneity correction and dose volume constraint (UCLA) for PTV D95%=40Gy and D98%>39.2Gy, Rectum V20Gy<50%, V32Gy<20%, V36Gy<10% and V40Gy<5%, Bladder V20Gy<40% and V40Gy<10%, femoral heads V16Gy<5%, penile bulb V25Gy<3cc, urethra and overlap region between PTV and PRV Rectum Dmax<42Gy. 10 SBRT treatments plans were selected and recalculated using Monte Carlo with 2mm spatial resolution and mean variance of 2%. DVH comparisons between plans were done. Results: The average difference between PTV doses constraints were within 2%. However 3 plans have differences higher than 3% which does not meet the D98% criteria (>39.2Gy) and should have been renormalized. Dose volume constraint differences for rectum, bladder, femoral heads and penile bulb were les than 2% and within tolerances. Urethra region and overlapping between PTV and PRV Rectum shows increment of dose in all plans. The average difference for urethra region was 2.1% with a maximum of 7.8% and for the overlapping region 2.5% with a maximum of 8.7%. Conclusion: Monte Carlo dose calculation on dynamic IMRT treatments could affects on plan normalization. Dose increment in critical region of urethra and PTV overlapping region with PTV could have clinical consequences which need to be studied. The use of Monte Carlo dose calculation algorithm is limited because inverse planning dose optimization use only pencil beam.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

SU-D-213-02: Characterization of the Effect of a New Commercial Transmission Detector On Radiotherapy Beams

Purpose: To evaluate the influence of a new commercial transmission detector on radiotherapy beams of various energies. Methods: A transmission detector designed for online treatment monitoring was characterized on a TrueBeam STx linear accelerator with 6MV, 6FFF, 10MV, and 10FFF beams. Measurements of beam characteristics including percentage depth doses (PDDs), inplane and crossplane off-axis profiles at different depths, transmission factors, and skin dose were acquired at field sizes of 3×3cm, 5×5m, 10×10cm, and 20×20cm at 100cm and 80cm source-to-surface distance (SSD). All measurements were taken with and without the transmission detector in the path of the beam. A CC04 chamber was used for all profile and transmission factor measurements. Skin dose was assessed at 100cm, 90cm, and 80cm SSD and using a variety of detectors (Roos and Markus parallel-plate chambers, and OSLD). Results: The PDDs showed small differences between the unperturbed and perturbed beams for both 100cm and 80cm SSD (≤4mm dmax difference and <1.2% average profile difference). The differences were larger for the flattened beams and at larger field sizes. The off-axis profiles showed similar trends. The penumbras looked similar with and without the transmission detector. Comparisons in the central 80% of the profile showed a maximum average (maximum) profile difference between all field sizes of 0.756% (1.535%) and 0.739% (3.682%) for 100cm and 80cm SSD, respectively. The average measured skin dose at 100cm (80cm) SSD for 10×10cm field size was <4% (<35%) dose increase for all energies. For 20×20cm field size, this value increased to <10% (≤45%). Conclusion: The transmission detector has minimal effect on the clinically relevant radiotherapy beams for IMRT and VMAT (field sizes 10×10cm and less). For larger field sizes, some perturbations are observable which would need to be assessed for clinical impact. The authors of this publication has research support from IBA Dosimetry.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Evaluation of the dosimetric impact of applying flattening filter-free beams in intensity-modulated radiotherapy for early-stage upper thoracic carcinoma of oesophagus

Flattening filter-free (FFF) radiation beams have recently become clinically available on modern linear accelerators in radiation therapy. This study aimed to evaluate the dosimetric impact of using FFF beams in intensity-modulated radiotherapy (IMRT) for early-stage upper thoracic oesophageal cancer. Eleven patients with primary stage upper thoracic oesophageal cancer were recruited. For each patient, two IMRT plans were computed using conventional beams (Con-P) and FFF beams (FFF-P), respectively. Both plans employed a five-beam arrangement and were prescribed with 64 Gy to (planning target volume) PTV1 and 54 Gy to PTV2 in 32 fractions using 6 MV photons. The dose parameters of the target volumes and organs at risks (OARs), and treatment parameters including the monitor units (MU) and treatment time (TT) for Con-P and FFF-P were recorded and compared. The mean D{sub 5} of PTV1 and PTV2 were higher in FFF-P than Con-P by 0.4 Gy and 0.3 Gy, respectively. For the OARs, all the dose parameters did not show significant difference between the two plans except the mean V{sub 5} and V{sub 10} of the lung in which the FFF-P was lower (46.7% vs. 47.3% and 39.1% vs. 39.6%, respectively). FFF-P required 54% more MU but 18.4% less irradiation time when compared to Con-P. The target volume and OARs dose distributions between the two plans were comparable. However, FFF-P was more effective in sparing the lung from low dose and reduced the mean TT compared with Con-P. Long-term clinical studies are suggested to evaluate the radiobiological effects of FFF beams.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2012

On-Line Use of Three-Dimensional Marker Trajectory Estimation From Cone-Beam Computed Tomography Projections for Precise Setup in Radiotherapy for Targets With Respiratory Motion

Purpose: To develop and evaluate accurate and objective on-line patient setup based on a novel semiautomatic technique in which three-dimensional marker trajectories were estimated from two-dimensional cone-beam computed tomography (CBCT) projections. Methods and Materials: Seven treatment courses of stereotactic body radiotherapy for liver tumors were delivered in 21 fractions in total to 6 patients by a linear accelerator. Each patient had two to three gold markers implanted close to the tumors. Before treatment, a CBCT scan with approximately 675 two-dimensional projections was acquired during a full gantry rotation. The marker positions were segmented in each projection. From this, the three-dimensional marker trajectories were estimated using a probability based method. The required couch shifts for patient setup were calculated from the mean marker positions along the trajectories. A motion phantom moving with known tumor trajectories was used to examine the accuracy of the method. Trajectory-based setup was retrospectively used off-line for the first five treatment courses (15 fractions) and on-line for the last two treatment courses (6 fractions). Automatic marker segmentation was compared with manual segmentation. The trajectory-based setup was compared with setup based on conventional CBCT guidance on the markers (first 15 fractions). Results: Phantom measurements showed that trajectory-based estimation of the mean marker position was accurate within 0.3 mm. The on-line trajectory-based patient setup was performed within approximately 5 minutes. The automatic marker segmentation agreed with manual segmentation within 0.36 {+-} 0.50 pixels (mean {+-} SD; pixel size, 0.26 mm in isocenter). The accuracy of conventional volumetric CBCT guidance was compromised by motion smearing ({<=}21 mm) that induced an absolute three-dimensional setup error of 1.6 {+-} 0.9 mm (maximum, 3.2) relative to trajectory-based setup. Conclusions: The first on-line clinical use of trajectory estimation from CBCT projections for precise setup in stereotactic body radiotherapy was demonstrated. Uncertainty in the conventional CBCT-based setup procedure was eliminated with the new method.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2013

Feasibility of an online adaptive replanning method for cranial frameless intensity-modulated radiosurgery

To introduce an approach for online adaptive replanning (i.e., dose-guided radiosurgery) in frameless stereotactic radiosurgery, when a 6-dimensional (6D) robotic couch is not available in the linear accelerator (linac). Cranial radiosurgical treatments are planned in our department using intensity-modulated technique. Patients are immobilized using thermoplastic mask. A cone-beam computed tomography (CBCT) scan is acquired after the initial laser-based patient setup (CBCT{sub setup}). The online adaptive replanning procedure we propose consists of a 6D registration-based mapping of the reference plan onto actual CBCT{sub setup}, followed by a reoptimization of the beam fluences (“6D plan”) to achieve similar dosage as originally was intended, while the patient is lying in the linac couch and the original beam arrangement is kept. The goodness of the online adaptive method proposed was retrospectively analyzed for 16 patients with 35 targets treated with CBCT-based frameless intensity modulated technique. Simulation of reference plan onto actual CBCT{sub setup}, according to the 4 degrees of freedom, supported by linac couch was also generated for each case (4D plan). Target coverage (D99%) and conformity index values of 6D and 4D plans were compared with the corresponding values of the reference plans. Although the 4D-based approach does not always assure the target coverage (D99% between 72% and 103%), the proposed online adaptive method gave a perfect coverage in all cases analyzed as well as a similar conformity index value as was planned. Dose-guided radiosurgery approach is effective to assure the dose coverage and conformity of an intracranial target volume, avoiding resetting the patient inside the mask in a “trial and error” way so as to remove the pitch and roll errors when a robotic table is not available.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2012

Generation of a novel phase-space-based cylindrical dose kernel for IMRT optimization

Purpose: Improving dose calculation accuracy is crucial in intensity-modulated radiation therapy (IMRT). We have developed a method for generating a phase-space-based dose kernel for IMRT planning of lung cancer patients. Methods: Particle transport in the linear accelerator treatment head of a 21EX, 6 MV photon beam (Varian Medical Systems, Palo Alto, CA) was simulated using the EGSnrc/BEAMnrc code system. The phase space information was recorded under the secondary jaws. Each particle in the phase space file was associated with a beamlet whose index was calculated and saved in the particle's LATCH variable. The DOSXYZnrc code was modified to accumulate the energy deposited by each particle based on its beamlet index. Furthermore, the central axis of each beamlet was calculated from the orientation of all the particles in this beamlet. A cylinder was then defined around the central axis so that only the energy deposited within the cylinder was counted. A look-up table was established for each cylinder during the tallying process. The efficiency and accuracy of the cylindrical beamlet energy deposition approach was evaluated using a treatment plan developed on a simulated lung phantom. Results: Profile and percentage depth doses computed in a water phantom for an open, square field size were within 1.5% of measurements. Dose optimized with the cylindrical dose kernel was found to be within 0.6% of that computed with the nontruncated 3D kernel. The cylindrical truncation reduced optimization time by approximately 80%. Conclusions: A method for generating a phase-space-based dose kernel, using a truncated cylinder for scoring dose, in beamlet-based optimization of lung treatment planning was developed and found to be in good agreement with the standard, nontruncated scoring approach. Compared to previous techniques, our method significantly reduces computational time and memory requirements, which may be useful for Monte-Carlo-based 4D IMRT or IMAT treatment planning.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2016

SU-F-T-72: Experimental Determination of the Positionuncertainties for ROOS Ionization Chambers in Clinical Electron Beams

Purpose: National and international dosimetry protocols assume a position accuracy for ionization chambers of less than 0.2mm. To follow this precept the manufacturer PTW-Freiburg introduced a positioning assistance system (TRUFIX) for their particular ion chambers. Aim of this study is an experimental investigation of the positioning uncertainties for ROOS-type ionization chambers. Methods: For all measurements a linear accelerator Elekta Synergie was used. The experiments were performed in a water-phantom. To collimate the electron beam a 10×10cm{sup 2} applicator was installed. All measured depth dose curves were normalized to their maximum. In all cases the TRUFIX system was applied for chamber positioning. For the first measurement series, to determine the positioning reproducibility of a ROOS chamber, one person placed the chamber three times in a 6 MeV electron beam. The mean value of this three measurements was the reference for further six random persons who repeated this procedure. The results were compared for different depths (R{sub 50}, z{sub ref} and R{sub p}). To investigate the impact of different individual chambers of the same type 10 different ROOS chambers were placed by the same person in a 6, 12 and 18MeV electron beam and the measured reference depths z{sub ref} were compared. Results: The absolute positioning reproducibility is less than 0.1mm for the same person. The positioning uncertainties are increasing up to +/−0.3mm if different persons perform the chamber’s positioning within the water phantom. The comparison of the 10 different ROOS chambers resulted in reference depths z{sub ref} with deviations in the range of +/−0.45mm for all energies. Conclusion: The position accuracy of 0.2mm can be fulfilled with the TRUFIX system. The comparison of the 10 different ROOS ionization chambers showed noticeable deviations in the determined reference depth. The impact of a positioning uncertainty of about 0.3–0.4mm on the total perturbation correction will be considered.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

SU-E-J-235: Varian Portal Dosimetry Accuracy at Detecting Simulated Delivery Errors

Purpose: To use receiver operating characteristic (ROC) analysis to quantify the Varian Portal Dosimetry (VPD) application's ability to detect delivery errors in IMRT fields. Methods: EPID and VPD were calibrated/commissioned using vendor-recommended procedures. Five clinical plans comprising 56 modulated fields were analyzed using VPD. Treatment sites were: pelvis, prostate, brain, orbit, and base of tongue. Delivery was on a Varian Trilogy linear accelerator at 6MV using a Millenium120 multi-leaf collimator. Image pairs (VPD-predicted and measured) were exported in dicom format. Each detection test imported an image pair into Matlab, optionally inserted a simulated error (rectangular region with intensity raised or lowered) into the measured image, performed 3%/3mm gamma analysis, and saved the gamma distribution. For a given error, 56 negative tests (without error) were performed, one per 56 image pairs. Also, 560 positive tests (with error) with randomly selected image pairs and randomly selected in-field error location. Images were classified as errored (or error-free) if percent pixels with γ<κ was < (or ≥) τ. (Conventionally, κ=1 and τ=90%.) A ROC curve was generated from the 616 tests by varying τ. For a range of κ and τ, true/false positive/negative rates were calculated. This procedure was repeated for inserted errors of different sizes. VPD was considered to reliably detect an error if images were correctly classified as errored or error-free at least 95% of the time, for some κ+τ combination. Results: 20mm{sup 2} errors with intensity altered by ≥20% could be reliably detected, as could 10mm{sup 2} errors with intensity was altered by ≥50%. Errors with smaller size or intensity change could not be reliably detected. Conclusion: Varian Portal Dosimetry using 3%/3mm gamma analysis is capable of reliably detecting only those fluence errors that exceed the stated sizes. Images containing smaller errors can pass mathematical analysis, though may be detected by visual inspection. This work was not funded by Varian Oncology Systems. Some authors have other work partly funded by Varian Oncology Systems.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Defining the Optimal Planning Target Volume in Image-Guided Stereotactic Radiosurgery of Brain Metastases: Results of a Randomized Trial

Purpose: To identify an optimal margin about the gross target volume (GTV) for stereotactic radiosurgery (SRS) of brain metastases, minimizing toxicity and local recurrence. Methods and Materials: Adult patients with 1 to 3 brain metastases less than 4 cm in greatest dimension, no previous brain radiation therapy, and Karnofsky performance status (KPS) above 70 were eligible for this institutional review board–approved trial. Individual lesions were randomized to 1- or 3- mm uniform expansion of the GTV defined on contrast-enhanced magnetic resonance imaging (MRI). The resulting planning target volume (PTV) was treated to 24, 18, or 15 Gy marginal dose for maximum PTV diameters less than 2, 2 to 2.9, and 3 to 3.9 cm, respectively, using a linear accelerator–based image-guided system. The primary endpoint was local recurrence (LR). Secondary endpoints included neurocognition Mini-Mental State Examination, Trail Making Test Parts A and B, quality of life (Functional Assessment of Cancer Therapy-Brain), radionecrosis (RN), need for salvage radiation therapy, distant failure (DF) in the brain, and overall survival (OS). Results: Between February 2010 and November 2012, 49 patients with 80 brain metastases were treated. The median age was 61 years, the median KPS was 90, and the predominant histologies were non–small cell lung cancer (25 patients) and melanoma (8). Fifty-five, 19, and 6 lesions were treated to 24, 18, and 15 Gy, respectively. The PTV/GTV ratio, volume receiving 12 Gy or more, and minimum dose to PTV were significantly higher in the 3-mm group (all P<.01), and GTV was similar (P=.76). At a median follow-up time of 32.2 months, 11 patients were alive, with median OS 10.6 months. LR was observed in only 3 lesions (2 in the 1 mm group, P=.51), with 6.7% LR 12 months after SRS. Biopsy-proven RN alone was observed in 6 lesions (5 in the 3-mm group, P=.10). The 12-month DF rate was 45.7%. Three months after SRS, no significant change in neurocognition or quality of life was observed. Conclusions: SRS was well tolerated, with low rates of LR and RN in both cohorts. However, given the higher potential risk of RN with a 3-mm margin, a 1-mm GTV expansion is more appropriate.

62 RADIOLOGY AND NUCLEAR MEDICINE↗