Results Of Gamma Analysis Research Articles

Dosimetric comparison of tools for intensity modulated radiation therapy with gamma analysis: a phantom study

Dosimetry of the Intensity Modulated Radiation Therapy (IMRT) is very important because of the complex dose distributions. Diode arrays are the most common and practical measurement tools for clinical usage for IMRT. Phantom selection is critical for QA process. IMRT treatment plans are recalculated for the phantom irradiation in QA. Phantoms are made in different geometrical shapes to measure the doses of different types of irradiation techniques. Comparison of measured and calculated dose distributions for IMRT can be made by using gamma analysis. In this study, 10 head-and-neck IMRT QA plans were created with Varian Eclipse 8.9 treatment planning system. Water equivalent RW3-slab phantoms, Octavius-2 phantom and PTW Seven29 2D-array were used for QA measurements. Gantry, collimator and couch positions set to 00 and QA plans were delivered to RW3 and Octavius phantoms. Then the positions set to original angles and QA plans irradiated again. Measured and calculated fluence maps were evaluated with gamma analysis for different DD and DTA criteria. The effect of different set-up conditions for RW3 and Octavius phantoms in QA plan delivery evaluated by gamma analysis. Results of gamma analysis show that using RW3-slab phantoms with setting parameters to 00 is more appropriate for IMRT QA.

SU-E-T-631: Commissioning and Comprehensive Evaluation of the ArcCHECK Cylindrical Diode Array for VMAT QA

Purpose: Commissioning and comprehensive evaluation of ArcCHECK phantom for dosimetry of VMAT QA, using 6MV photon beam with and without the flattening filter. Methods: ArcCHECK was evaluated for response dependency on linac dose rate, instantaneous dose rate, radiation field size, beam angle and couch insertion. Scatter dose characterization, consistency and symmetry of response, dosimetric accuracy of fixed aperture arcs and clinical VMAT plans were investigated. Measurements were done using TrueBeam™ STx accelerator (Console version 1.6) with a 6 MV beam with and without flattening filter. Reference dose-grids were calculated using Eclipse TPS Analytical Anisotropic Algorithm (AAA version 10.0.39). Planned doses were calculated using symmetric 2mm 3D dose grids with 4 degree angular resolution defaulted to each control point. Gamma evaluations were performed in absolute dose mode, with default normalization to maximum dose in the curved plane and a low dose threshold of 10% to restrict the analysis to clinically relevant areas. Global and local gamma indices at 3mm/3% and 2mm/2% level were computed using SNC software (version 6.0). Results: Results of gamma analysis demonstrated an overall agreement between ArcCHECK measured and TPS calculated reference doses. Field size dependency was within 0.5% of the reference. Dose-rate based dependency was well within 1% of the TPS reference and the angular dependency was ±3% of the reference, as tested for BEV angles. At the level of 3%/3mm, narrow and wide open arcs as well as clinical VMAT cases demonstrated high level of dosimetry accuracy in global gamma passing rates for both 6X and 6F beams. At the level of 2%/2mm two VMAT cases involving the narrow heavily modulated arcs demonstrated lower passing rates. Conclusion: ArcCHECK phantom with latest software and hardware upgrades is suitable for VMAT QA. For higher sensitivity of 2%/2mm gamma analysis, we intend to use it as one of the VMAT QA evaluation metrics.

SU‐E‐T‐468: A New 3D Gamma Analysis Algorithm for Better VMAT/IMRT QA Results

Purpose:The gamma dose distribution comparison method is used in VMAT QA to score the agreement between planned and measured dose distributions. Unfortunately, this dose comparison technique can Resultin compromised passing rates when the treatment plan includes a high dose gradient perpendicular to the measurement plane. To address this, we have developed and tested a 3D gamma analysis tool.Methods:An in‐house 3D‐gamma analysis algorithm was developed. Five VMAT patient specific QA plans were delivered and measured using a MatriXX 2D ion chamber array. A 3D dose volume was generated in the Pinnacle TPS, and exported to the analysis software. Both 2D and 3D gamma analysis were performed. The results determined using a 3%/3mm passing criteria were then evaluated and compared.Results:Cases that did not include a high dose gradient perpendicular and adjacent to the detector plane showed only minor differences between the 2D and 3D gamma analysis results. Cases including a high dose gradient (e.g. as a means to avoid an adjacent sensitive structure), the use of 3D gamma analysis resulted in improved passing rates with lower gamma values. For the five cases tested, the average gamma index value was reduced from 0.361 to 0.345 when 3D analysis was used. The average passing rate increased from 94.7% to 96%.Conclusion:A 3D gamma analysis tool has been developed and tested. For patient specific IMRT/VMAT QA performed using the composite dose measurement method, the use of 3D gamma analysis can effectively reduce the uncertainty in QA results due to factors such as setup errors. This technique can more effectively account for a high dose gradient perpendicular to the measurement plan. Consequently, improved and more meaningful QA results can be achieved using this technique.

2D vs 3D gamma analysis: Establishment of comparable clinical action limits

Purpose: As clinics begin to use 3D metrics for intensity-modulated radiation therapy (IMRT) quality assurance; these metrics will often produce results different from those produced by their 2D counterparts. 3D and 2D gamma analyses would be expected to produce different values, because of the different search space available. We compared the results of 2D and 3D gamma analysis (where both datasets were generated the same way) for clinical treatment plans. Methods : 50 IMRT plans were selected from our database and recalculated using Monte Carlo. Treatment planning system-calculated (“evaluated”) and Monte Carlo-recalculated (“reference”) dose distributions were compared using 2D and 3D gamma analysis. This analysis was performed using a variety of dose-difference (5%, 3%, 2%, and 1%) and distance-to-agreement (5, 3, 2, and 1 mm) acceptance criteria, low-dose thresholds (5%, 10%, and 15% of the prescription dose), and data grid sizes (1.0, 1.5, and 3.0 mm). Each comparison was evaluated to determine the average 2D and 3D gamma and percentage of pixels passing gamma. Results: Average gamma and percentage of passing pixels for each acceptance criterion demonstrated better agreement for 3D than for 2D analysis for every plan comparison. Average difference in the percentage of passing pixels between the 2D and 3D analyses with no low-dose threshold ranged from 0.9% to 2.1%. Similarly, using a low-dose threshold resulted in a differences ranging from 0.8% to 1.5%. No appreciable differences in gamma with changes in the data density (constant difference: 0.8% for 2D vs. 3D) were observed. Conclusion : We found that 3D gamma analysis resulted in up to 2.9% more pixels passing than 2D analysis. Factors such as inherent dosimeter differences may be an important additional consideration to the extra dimension of available data that was evaluated in this study. ------------------------------------ Cite this article as: Pulliam KB, Huang JY, Bosca R, Followill D, Kry SF. 2D vs. 3D gamma analysis: Establishment of comparable clinical action limits. Int J Cancer Ther Oncol 2014; 2(2):020231. DOI: 10.14319/ijcto.0202.31

Comparison of 2D and 3D gamma analyses

As clinics begin to use 3D metrics for intensity-modulated radiation therapy (IMRT) quality assurance, it must be noted that these metrics will often produce results different from those produced by their 2D counterparts. 3D and 2D gamma analyses would be expected to produce different values, in part because of the different search space available. In the present investigation, the authors compared the results of 2D and 3D gamma analysis (where both datasets were generated in the same manner) for clinical treatment plans. Fifty IMRT plans were selected from the authors' clinical database, and recalculated using Monte Carlo. Treatment planning system-calculated ("evaluated dose distributions") and Monte Carlo-recalculated ("reference dose distributions") dose distributions were compared using 2D and 3D gamma analysis. This analysis was performed using a variety of dose-difference (5%, 3%, 2%, and 1%) and distance-to-agreement (5, 3, 2, and 1 mm) acceptance criteria, low-dose thresholds (5%, 10%, and 15% of the prescription dose), and data grid sizes (1.0, 1.5, and 3.0 mm). Each comparison was evaluated to determine the average 2D and 3D gamma, lower 95th percentile gamma value, and percentage of pixels passing gamma. The average gamma, lower 95th percentile gamma value, and percentage of passing pixels for each acceptance criterion demonstrated better agreement for 3D than for 2D analysis for every plan comparison. The average difference in the percentage of passing pixels between the 2D and 3D analyses with no low-dose threshold ranged from 0.9% to 2.1%. Similarly, using a low-dose threshold resulted in a difference between the mean 2D and 3D results, ranging from 0.8% to 1.5%. The authors observed no appreciable differences in gamma with changes in the data density (constant difference: 0.8% for 2D vs 3D). The authors found that 3D gamma analysis resulted in up to 2.9% more pixels passing than 2D analysis. It must be noted that clinical 2D versus 3D datasets may have additional differences--for example, if 2D measurements are made with a different dosimeter than 3D measurements. Factors such as inherent dosimeter differences may be an important additional consideration to the extra dimension of available data that was evaluated in this study.

Sensitivity of volumetric modulated arc therapy patient specific QA results to multileaf collimator errors and correlation to dose volume histogram based metrics

This study investigates the impact of systematic multileaf collimator (MLC) positional errors on gamma analysis results used for quality assurance (QA) of Rapidarc treatments. In addition, this study evaluates the relationship of these gamma analysis results and clinical dose volume histogram metrics (DVH) for Rapidarc treatment plans. Five prostate plans were modified by the introduction of systematic MLC errors. The MLC shifts to each individual active leaf introduced were 0.25, 0.5, 0.75, and 1 mm. All QA verification plans were delivered and estimated 3D patient dose or high density phantom dose were obtained based on the ArcCHECK measurement files. QA gamma analysis of 3%/3 mm and 2%/2 mm were implemented and relationships to dose differences in DVH metrics encountered due to MLC errors were determined. Tolerances of 3% and 5% for DVH metric were implemented to determine the sensitivity of gamma analysis to MLC errors. A calculation of sensitivity was determined from the number of incidences of false negative and false positive cases in gamma analysis results. The sensitivity of global gamma analysis for criteria of 3%/3 mm was 0.78 and for 2%/2 mm was 0.82. A number of instances occurred for an acceptable VMAT QA gamma index which did not indicate a DVH metric dose error greater than 5%. The correlation between global gamma analysis using criteria 3%/3 mm and DVH metric dose error were all <0.8 indicating less than a strong correlation. There is a greater sensitivity for detection of dosimetric errors occurring in a Rapidarc plan using gamma criteria of 2%/2 mm than 3%/3 mm. However, there is lack of consistently strong correlation between global gamma indexes and clinical DVH metrics for PTV and bladder and rectum for Rapidarc plans. It is recommended that the sole use of gamma index for Rapidarc QA plan evaluation could be insufficient and a methodology for evaluation of delivered dose to patient is required.

SU‐E‐T‐169: Characterization and Evaluation of Octavius 4D System for Patient Specific Quality Assurance Using 3D Gamma Index Analysis in VMAT

Purpose: To characterize and evaluate Octavius 4D system for patient specific quality assurance using 3D gamma index analysis for patients treated with VMAT technique. Methods: Characterization of the 4D system was carried out by following tests. (i) Verification of calibration factor of each chambers, (ii) reproducibility of all the 729 chambers, (iii) linearity for static and arc mode, (iv) output factor dependency for static and arc mode, (v) dosimetric comparison between measured and TPS calculated (AcurosXB 11.0.21) dose distributions for combination of open fields and/or wedge fields and TG 119 benchmark plans for VMAT. Evaluation of Octavius 4D system is carried out by 3D gamma index analysis (3mm/3% DD) of 102 clinically approved VMAT patient plans of different sites. Results: Average percentage deviation between measured and manufacturer specified calibration factors for all the chambers is −0.28%. Reproducibility measurements of 729 chambers showed maximum standard deviation of 0.25% and 0.2% for 6MV and 15MV respectively. Linearity (test) for static and arc mode, output factor dependency (test) for static and arc mode tests agree very well with semiflux ionization chamber measurements. Average passing rates for combination of open fields and/or wedge fields are, 96.13% and 96.08% for coronal and sagittal plane 2D gamma evaluation respectively and 95.84% for volumetric 3D gamma evaluation. Isocentre dose measurements for TG 119 benchmark IMRT AND VMAT plans are within 2% and average volumetric 3D gamma analysis results are 97.77% and 96.25% for 6 and 15MV respectively. Volumetric global gamma evaluation shows 97.13%, 95.39% and 97.11% passing results for Head and Neck, Thorax and Pelvis sites respectively. Conclusion: Characterization and evaluation results indicate that Octavius 4D system is an efficient tool for 3D patient specific quality assurance.

SU-GG-T-247: A New Method to Compensate Angular Dependency of MapCheck Device in Intensity Modulated Arc Therapy

Purpose: To evaluate the performance of a new method to compensate angular dependency of QA devices in intensity modulated arc therapy. Methods and Materials: The QA phantom consisted of a MapCheck device sitting in an acrylic base, which is inserted between two 5‐cm solid water pieces. Two‐dimensional dose planes were measured by delivering one 10×10cm 6MV X‐ray beam for per 15 degree beam angle. Two‐dimensional doses at the diode detector plane were calculated in Eclipse using the AAA algorithm with inhomogeneity correction. Two types of halfpipe‐shape boluses were added to the planning CTimage. Bolus A has uniform thickness to attenuate the TPS dose to match the measured dose at the 180 degree beam angle. Bolus B has variable thickness to match the doses at every 15 degree beam angle. TPS doses were re‐calculated using the modified CTimages. Angular dependences were evaluated for the original phantom and with bolus A and B individually. Six patient QAs were performed to compare gamma analysis results. Results: Without compensation, the TPS dose was about 5% higher than the measured dose for most posterior angles (105 degree to 180 degree). It is probably due to angular dependence of diode detectors and CT metal artifacts. Bolus A slightly reduced the angular dependence for the posterior angles (RMS decreased from 4.0% to 3.4%). Bolus B reduced the dependence significantly (RMS=1.4%). The failure rate of gamma analysis was reduced by 39% and 41% in average. Conclusion: The new method has been shown to be able to compensate angular dependency for the MapCheck device.

SU‐DD‐A1‐03: On Rotational Measurements with the ArcCHECK 3D Diode Array

Purpose: The ArcCHECK 3D diode array dosimeter (Sun Nuclear Corporation) represents a novel approach to detector placement. We investigate the effects this design can have on the rotational measurements' accuracy and suggest correction factors that could sometimes improve it. Method and Materials: The ArcCHECK design places 1386 0.8×0.8mm2 diodes arranged on a 1×1cm helical grid on a cylindrical surface 21cm in diameter inside a doughnut‐shaped phantom.. Beam arrangements tested in this work include single static beams, simple open arcs, and VMAT plans based on the AAPM TG1 19 test suite. Field size dependence, angular dependence, phantom inhomogeneity effects, and calculation angular‐discretization effects were investigated separately.Results: Unlike an existing device using the same diodes (MapCHECK), ArcCHECK exhibits non‐negligible field size dependence in detector response, likely due to the increased phantom volume. Angular‐dependent detector response is also shown, evidenced by measured beam profiles deviating from the calculations and ion chamber measurements by more than 2% for field widths exceeding approximately 15 cm. The issue is being addressed by the manufacturer. There is an additional small disagreement between measured and calculated exit doses (1.5%) due to the inhomogeneous phantom structure. Simple arc measurements demonstrate strong influence of the small changes in field width (e.g. 9%/mm at 2cm), while the isocenter dose is essentially unchanged. This is inherent and expected given the device design. The average gamma analysis passing rates for a well‐commissioned VMAT system were 98.4 and 92.9% at the 3%/3mm and 2%/2mm error thresholds, respectively. They improved to 98.7 and 94.2%, respectively, after manual application of the field size correction factor. Conclusion: Despite hypersensitivity to field width in the simple arc geometry, the VMAT gamma analysis results are in line with expectations based on independent dosimetry measurements. Introduction of correction factors for angular and field‐size dependence would further improve measurement accuracy.

A new method of testing periodicity in cyclic sediments: application to the Newark Supergroup

We present a new, quantitative method for analyzing cyclic sediments. The gamma method (gamma = time/thickness) tests the assumption that the cycles are periodic and provides quantitative estimates for relative accumulation rates of the different sedimentary facies that compose the cycles. As such, it provides a means of estimating relative time at very high resolution. Application of the gamma method to Hettangian sediments of the Newark Supergroup indicates that the primary cycles were deposited in response to precessional climatic forcing, and that these cycles were modulated by an eccentricity cycle, with period ratios very similar to those of today. Results of gamma analysis also suggest that relative accumulation rates of the laminated, deep water lacustrine facies was about six times slower than that of the shallow water facies.