VMAT Cases Research Articles

The effect of dose gradients on gamma comparison insensitivity in patient specific QA comparisons.

While many have speculated on the reasons for gamma comparison insensitivity for patient-specific quality assurance analysis, the true reasons for insensitivity have not yet been elucidated. Failing to understand the reasons for this technique's insensitivity limits our ability to either improve the gamma metric to increase sensitivity of the comparison or the capacity to develop new comparison techniques that circumvent the limitations of the gamma comparison. To understand the underlying cause(s) for gamma comparison insensitivity and determine if simple plan characteristics can quantitatively predict for gamma comparison sensitivity. Known MLC and MU errors of varying magnitudes were induced on simple test fields to preliminarily investigate where gamma failures first begin to appear as error magnitude is increased. Gamma value maps between error-induced plan calculations and error-free plan calculations were created for 20 IMRT and 20 VMAT cases, each on three different detector geometries-ArcCHECK, MapCHECK, and Delta4. Gamma value maps were qualitatively compared to dose-gradient maps, and quantitative comparisons were performed between various plan descriptors and the computed gamma sensitivity for five different classes of induced errors were utilized to determine if any plan descriptor could predict the gamma sensitivity on a case-by-case basis. All comparisons were performed in a calculation-only scenario to remove uncertainties introduced by comparisons made with real patient specific QA measurements. Gamma value maps with increasing induced error magnitude illustrated that gamma comparisons fail first in high-dose, low-gradient regions of the field. Conversely, in areas of high gradient, gamma values typically remain low, even in the presence of large errors, regardless of detector geometry and gamma normalization setting. Thus, the complex, and often overlapping, high dose gradients in plans appear to be a limiting factor in gamma comparison sensitivity as the number of points along these gradients may often outnumber the points available for failing the comparison in lower gradient regions of the field. None of the simple plan descriptors studied were able to quantitively predict gamma comparison sensitivity, suggesting that quantitatively predicting the sensitivity of gamma comparisons on a case-by-case basis may require a combination of multiple factors or metrics not studied here. Simple plan descriptors and the number of points in high-dose, low-gradient regions of the field did not quantitively predict for gamma comparison sensitivity. However, it is clear from gradient and gamma value maps that gamma comparisons fail first in high-dose, low-gradient regions of the field in the presence of known induced errors, which we have shown to be independent of detector geometry and gamma comparison normalization setting. Gamma comparison sensitivity is thus limited by the ever-increasing complexity of plans and is particularly important to consider as treatment volumes become smaller and the complexity of overlapping plan gradients increases. This suggests that new methods for patient-specific QA comparisons are required to circumvent this limitation.

Improved planning efficiency in multiple brain lesion SRS VMAT cases using Eclipse scripting.

Though dosimetry has a multitude of treatment modalities, software, and workflows to aid in the treatment planning process, treatment planners are still responsible for several tedious and monotonous tasks that could decrease their planning efficiency. The purpose of this study was to determine if scripting could improve treatment planning efficiency for multiple brain lesion stereotactic radiosurgery (SRS) volumetric arc therapy cases by reducing planning time commitment. A script was developed for multiple brain lesion SRS cases using Eclipse scripting application programming interface with the intention of improving treatment planning efficiency by creating optimization structures and importing prescription and suggested OS dose metrics to the optimizer. Nine treatment planners were each provided with 3 different multiple brain lesion, single-isocenter SRS cases. Each planner created 2 plans for each case. One of these 2 plans used the SRS script, and the other did not. There were 54 treatment plans developed, totaling 27 plan comparisons. Each of the 54 treatment plans were considered clinically acceptable based on the participating institution's plan quality guidelines. Statistical analyses of planning time commitment with and without the SRS script were performed using RStudio. The mean and median planning times with and without the SRS script were compared using a paired T-test and Wilcoxon Signed Rank test, respectively, and effect size was evaluated using Cohen's classification. Using the SRS script resulted in statistically significant reduction in total contouring time (11.3 vs 2.8 minutes, p < 0.001), optimizer preparation time (7.7 vs 2.1 minutes, p < 0.001), and overall planning time (105.1 vs 77.9 minutes, p < 0.001). This study concluded that scripts developed using Eclipse scripting application programming interface offer an opportunity to improve treatment planning efficiency by reducing the planning time commitment for treatment planners.

Mean Arc Distance (MAD): a quantity to compare trajectory 4π sampling in single target cranial stereotactic radiotherapy

Purpose. C-arm linac-based radiotherapy has seen a recent interest in 4π methods of delivery using simultaneous rotations of couch and gantry to reduce doses to organs-at-risk (OARs) and increase dose compactness. While many methods use heuristics to generate trajectories that avoid OARs, combined with arbitrary trajectory restrictions to prevent oversampling, a quantity has not yet been developed to succinctly compare sampling of the 4π space for candidate trajectories as a surrogate for dosimetric compactness. Methods. Evenly spaced sampling points were distributed across a 4π sphere centred on isocentre. A metric, mean arc distance (MAD), was defined that quantifies the average arc distance between all sampling points and their nearest field in a radiotherapy trajectory. The relationship between isodose volume and MAD was examined in 2,047 plans: 900 unique trajectories of fixed port DCA plans, 900 unique trajectories of contiguous field DCA plans, 192 VMAT plans (eight volumes in four locations, each with six trajectories) in matRad with 5 VMAT plans repeated for validation in a clinical planning system, and 10 clinical VMAT cases replanned with five trajectories in a clinical treatment planning system. Results. All isodose volumes greater than 10% of the prescription dose decreased with decreasing MAD in all comparisons. In the range of 10% to 100% of the prescription dose, the rate of isodose volume decrease was exponential as a function of MAD in all comparisons. Reduction of absolute isodose volume is seen with increased 4π sampling, with larger target volumes exhibiting larger absolute reductions. Very low isodoses (0% to 10% of prescription) increased with decreasing MAD. Conclusions. MAD is a 4π sampling quantity useful in quantifying the decrease of isodose volume, relevant for sparing normal tissues. By quantifying this feature, candidate dynamic trajectories can be efficiently compared for 4π sampling. This quantity is explored here for single target cranial radiotherapy but may have applications to other radiotherapy treatment sites.

Evaluating suggested stricter gamma criteria for linac-based patient-specific delivery QA in the conventional and SBRT environments

PurposeTo evaluate AAPM TG-218 recommended tolerances for IMRT QA for conventional and SBRT delivery. MethodsQA analysis was repeated for 150 IMRT/VMAT patients with varying gamma criteria. True composite delivery was utilized, corrected for detector and output variation. Universal tolerance (TLuniv) and action limits (ALuniv) were compared with statistical process control (SPC) TLSPC and ALSPC values.Analysis was repeated as a function of plan complexity for 250 non-stereotactic body radiotherapy (SBRT) VMAT patients at 3%/2mm and a threshold of 10% and for 75 SBRT VMAT patients at 2%/2 mm and a threshold of 50% with results plotted as a function of PTV volume. Regions of failure were dose-scaled on the planning CT data sets based on delivery results. ResultsThe IMRT/VMAT TLSPC and ALSPC for gamma criteria of 3%/3 mm were 96.5% and 95.6% and for 3%/2 mm were 91.2% and 89.2%, respectively. Correlation with plan complexity for conventional fractionation VMAT was “low” for all sites with pelvis having the highest r value at −0.35. The equivalent SBRT PTV diameter ranged from 2.0 cm to 5.6 cm. Negative low correlation was found for 38 of 75 VMAT cases below ALuniv. ConclusionsThe ALuniv and ALSPC are similar for 3%/2 mm. However, our 5% failure rate for ALuniv, may result in treatment start delays approximately 2 times/month, given 40 new cases/month. VMAT QA failure at stricter criteria did not correlate strongly with plan complexity. Site-specific action limits vary less than 3% from the average. SBRT QA results do not strongly correlate with target size over the range studied.

Region-specific three-dimensional dose distribution prediction: a feasibility study on prostate VMAT cases

Purpose: To investigate region-specific models for organ’s three-dimensional dose distribution prediction with neural network.Methods: The dose distribution from different bladder regions for 52 pr...

Novel knowledge-based treatment planning model for hypofractionated radiotherapy of prostate cancer patients

PurposeTo demonstrate the strength of an innovative knowledge-based model-building method for radiotherapy planning using hypofractionated, multi-target prostate patients. Material and methodsAn initial RapidPlan model was trained using 48 patients who received 60 Gy to prostate (PTV60) and 44 Gy to pelvic nodes (PTV44) in 20 fractions. To improve the model's goodness-of-fit, an intermediate model was generated using the dose-volume histograms of best-spared organs-at-risk (OARs) of the initial model. Using the intermediate model and manual tweaking, all 48 cases were re-planned. The final model, trained using these re-plans, was validated on 50 additional patients. The validated final model was used to determine any planning advantage of using three arcs instead of two on 16 VMAT cases and tested on 25 additional cases to determine efficacy for single-PTV (PTV60-only) treatment planning. ResultsFor model validation, PTV V95% of 99.9% was obtained by both clinical and knowledge-based planning. D1% was lower for model plans: by 1.23 Gy (PTV60, CI = [1.00, 1.45]), and by 2.44 Gy (PTV44, CI = [1.72, 3.16]). OAR sparing was superior for knowledge-based planning: ΔDmean = 3.70 Gy (bladder, CI = [2.83, 4.57]), and 3.22 Gy (rectum, CI = [2.48, 3.95]); ΔD2% = 1.17 Gy (bowel bag, CI = [0.64, 1.69]), and 4.78 Gy (femoral heads, CI = [3.90, 5.66]). Using three arcs instead of two, improvements in OAR sparing and PTV coverage were statistically significant, but of magnitudes < 1 Gy. The model failed at reliable DVH predictions for single PTV plans. ConclusionsOur knowledge-based model delivers efficient, consistent plans with excellent PTV coverage and improved OAR sparing compared to clinical plans.

Evaluation of Delta 4 system in patient specific QA for VMAT technique: Retrospective lung VMAT cases

The objective of this study was to evaluate the Delta 4 system for lung VMAT technique. The standard fields and VMAT plans were generated into the homogeneous and heterogeneous phantoms and were evaluated using 2%/2 mm criterion to validate the Delta 4 system. For retrospective study, five lung VMAT plans were evaluated using 2%/2 mm criterion. The point dose was measured to ensure the accuracy of beam delivery. Moreover, the 1.0 mm MLC position error was introduced into the VMAT plan of homogeneous phantom and was evaluated using 2%/2 mm and 1%/1 mm criteria to evaluate the small error detection. The range of percentage gamma passing rate for standard fields and VMAT in both phantoms was 94.6% to 99.8%. For five retrospective lung VMAT plans, the range of percentage gamma passing rate was 96.6% to 99.5%. The point dose differences were within ± 3%. For 1.0 mm MLC position error, the percentage gamma passing rate for 2%/2 mm and 1%/1 mm were 99.1% and 83.6%, respectively. In conclusion, the Delta 4 system can be used in patient specific QA for lung VMAT cases using 2%/2 mm criterion. The error magnitude 1.0 mm can be detected by 1%/1 mm criterion.

Issues on VMAT QA Program with EBT3 Radiochromic Films: An improvement proposal

EBT3 radiochromic film is renowned in medical physics field due its dose verification for Quality Assurance (QA) program in intensity-modulated radiotherapy (IMRT). However, it is known that QA programs, which include these films, are expensive and relatively time-consuming. Furthermore, it was noted that dosimetry with EBT3 film has been suffering with lack of accuracy. In that way, it was evaluated its possible dosimetric inaccuracy, the time expenditure in the whole QA process, as well as the cost-benefit of such a program and it is suggested efforts to improve this process based on our institution expertise. A retrospective study was performed on VMATplans, rating how many cases got success by absolute dosimetric Gamma analysis. Furthermore, in order to verify the film intra-lot aging, the calibration curves were compared to the lot fabrication date. On the other hand, with single- and multi-channel dosimetric analyses, further trials were performed over the post-irradiationsaturation curves up to 35 h after irradiation. The retrospective Gamma analyses exhibited that about 23% of the total VMAT cases did not passed by absolute Gamma evaluation. It was seen that the fabrication date influences the dose response, which prerequisites a calibration curve for each film box. For the multichannelanalyses, as excepted, the red channel sensitivity was the highest for doses up to 5 Gy. Moreover, it was reinforced that the dose response is a direct function of dose level for both types of analyses. In that way, it was chosen a post-irradiation time interval of 12h-16h, with dose response variation criteria less than 2%. Finally, it was verified, based on our routine, that there were about 23h/month expended in a film QA program and directly costs bordering US$20,000.00/year.

Dose calibration uncertainty and plan-specific dose calibration for IMRT QA

Purpose: This study was to investigate IMRT QA dose calibration uncertainty that has not been realized before, and develop plan-specific calibration methods to reduce such uncertainty. Methods: IMRT plans were created on the Varian Eclipse treatment planning system. IMRT QA used a PTW Octavius phantom with an Octavius 2D array ionization chamber detector and was also tested for a solid water cube with a Farmer chamber. Dose calibration for a solid phantom was analyzed using the AAPM TG-51/TG-21 formulism. The reference fields used for IMRT dose calibration included squared fields and IMRT plan-specific fields that included geometric equivalent fields, and dose-equivalent fields defined by the 50% isodose distributions. Results: Thirteen IMRT QA cases including three VMAT cases were used for this study. For the single field calibration on the PTW Octavius device, the calibration factors varied with field size by up to 5.6%, and varied with gantry angle by up to 4.9%. Those variations changed IMRT QA passing rates by up to 16.2%. Using the plan-specific calibration, the Gamma passing rate was increased by up to 27.7%. The VMAT passing rates varied by up to 30%, and the equivalent field dose calibration gave the highest passing rate. For absolute point dose measurement using a Farmer chamber, calibration factors varied by up to 3.7% with different field sizes and different gantry angles. The dose equivalent field calibration achieved the best IMRT QA results. Conclusion: Different fields can have different dose calibration factors. Conventional dose calibration methods for IMRT QA using simple reference fields include significant uncertainty that would affect IMRT QA results. Dose calibration uncertainty is caused by the complex composition of phantom materials and complex design and characteristics of the detectors. The plan-specific dose calibration can reduce dose calibration uncertainty and improve IMRT QA accuracy.

Sub-Arc Collimator Angle Optimization for VMAT Planning of Multiple Metastases Brain SRS Treatments

To investigate the impact of collimator angle optimization in multiple metastases VMAT SRS treatments with respect to treatment delivery efficiency and OAR doses. We hypothesize that optimized collimator angle selection will improve treatment delivery efficiency (i.e. reduce required monitor units) while preserving similar OAR dose levels relative to plans in which the collimator angle has not been optimized. Using a previously published collimator angle optimization algorithm, 11 VMAT cases with 3 metastases (n=7) and 4 metastases (n=4) were planned using a standard 4 arc template (1 full axial arc and 3 partial arcs with couch rotations of 45, 315 and 90 degrees). A total of 10 sub arcs were distributed among the 4 arcs with optimized collimator angles (4 sub-arcs in the full axial arc, 2 sub-arcs in each of 3 partial arcs) . These cases were also planned using the 4 arc template with a single, standardized collimator angle per arc. The primary planning objective was to achieve 99.5% volume covered with the prescription dose (24 Gy to all targets in this study). The secondary objective was to meet the maximum OAR dose constraints as per QUANTEC; otherwise as low as achievable. Total monitor units were reduced on average by 15% for both the 3 met (p = 0.017) and 4 met cases (p = 0.057) using the collimator optimization method. This corresponded to reductions of 1132 and 1239 MU on average for the 3 and 4 met cases respectively. There were no statistically significant differences in the maximum doses received by any contoured OAR (brainstem, chiasm, eyes, optic nerves, optic tracks) (3 met: p > 0.46, 4 met p > 0.2). Although V12Gy was lower with optimized collimator angle, the results were not significant (3 met: p = 0.46, 4 met: p = 0.2). Increased treatment delivery efficiency is of significant interest in high dose per fraction treatments in which intrafraction motion could be particularly deleterious. The MU reductions achieved in this study translate into beam on time reductions on the order of a minute (depending on the dose rate used to deliver the treatment).

SU‐F‐T‐306: Validation of Mobius 3D and FX for Elekta Linear Accelerators

Purpose:Log file based IMRT and VMAT QA is a system that analyzes treatment log files and uses delivery parameters to compute the dose to the patient/phantom. This system was previously commissioned for Varian machines, the purpose of this work is to describe the process for commissioning Mobius for use with Elekta machines.Methods:Twelve IMRT and VMAT plans (6×) were planned and delivered and dose was measured using MapCheck, the results were compared to that computed by Mobius. For 10x and 18x, plans were generated, copied to a phantom and delivered, the dose was measured using a single ion chamber. The difference in measured dose to computed dose (Mobius) was used to adjust the dynamic leaf gap (DLG) in Mobius to achieve optimal agreement between measurements, Mobius and treatment plans.Results:For the measured dose comparison, the average 3%/3mm gamma 97.1% of pixels passed criteria using MapCheck where Mobius computed 96.9% of voxels passing. For 10×, a DLG of −5.5 was determined to achieve optimal results for TPS and measured ion chamber data with an average 0.1% difference and −1.7% respectively. For 18×, a DLG of −3 was determined to achieve optimal results from the TPS and measured data with an average of −0.7% and −1.4% difference on average from a set of IMRT and VMAT plans. The 6x data needed no DLG correction to arrive at agreement with the TPS and the MapCheck measured data.Conclusion:We have validated with measurements for IMRT and VMAT cases the use of Mobius FX with Elekta treatment machines for IMRT and VMAT QA. For 6×, no adjustments to the DLG were required to obtain good results utilizing Mobius whereas for 10× and 18×, the DLG had to be adjusted to obtain optimum agreement with measured data and our TPS.

SU‐F‐T‐236: Comparison of Two IMRT/VMAT QA Systems Using Gamma Index Analysis

Purpose:The goal of this study is to assess differences in the Gamma index pass rates when using two commercial QA systems and provide optimum Gamma index parameters for pre‐treatment patient specific QA.Methods:Twenty‐two VMAT cases that consisted of prostate, lung, head and neck, spine, brain and pancreas, were included in this study. The verification plans have been calculated using AcurosXB(V11) algorithm for different dose grids (1.5mm, 2.5mm, 3mm). The measurements were performed on TrueBeam(Varian) accelerator using both EPID(S1000) portal imager and ArcCheck(SunNuclearCorp) devices. Gamma index criteria variation of 3%/3mm, 2%/3mm, 2%/2mm and threshold (TH) doses of 5% to 50% were used in analysis.Results:The differences in Gamma pass rates between two devices are not statistically significant for 3%/3mm, yielding pass rate higher than 95%. Increase of lower dose TH showed reduced pass rates for both devices. ArcCheck's more pronounced effect can be attributed to higher contribution of lower dose region spread. As expected, tightening criteria to 2%/2mm (TH: 10%) decreased Gamma pass rates below 95%. Higher EPID (92%) pass rates compared to ArcCheck (86%) probably due to better spatial resolution. Portal Dosimetry results showed lower Gamma pass rates for composite plans compared to individual field pass rates. This may be due to the expansion in the analyzed region which includes pixels not included in the separate field analysis. Decreasing dose grid size from 2.5mm to 1.5mm did not show statistically significant (p&lt;0.05) differences in Gamma pass rates for both QA devices.Conclusion:Overall, both system measurements agree well with calculated dose when using gamma index criteria of 3%/3mm for a variety of VMAT cases. Variability between two systems increases using different dose GRID, TH and tighter gamma criteria and must be carefully assessed prior to clinical use.

SU‐F‐T‐393: Using Commercially Available Software to Prove That Providing Planning Objectives to the Planner Improves Plan Quality

Purpose:Use commercially available software to evaluate whether providing planning objectives during the treatment planning process improves overall plan quality in head and neck casesMethods:The commercially available software, PlanIQ, was used to evaluate the plan quality of head and neck cases before and after the introduction of an inhouse developed evaluation tool, Integrated Clinical Information System (ICIS). Twenty patients were selected; ten from before the introduction of ICIS and ten that used ICIS during the treatment planning process. These plans only included oropharynx cases. All plans were two or three arc VMAT cases prescribed at two dose levels, 60 Gy and 54 Gy. A PlanIQ algorithm was created with the assistance of physician input. The algorithm consisted of seventy‐four PTV and OAR plan quality metrics and objectives. Each plan was scored based on meeting the objectives provided. Plans were then evaluated by looking at the Plan Quality Metric (PQM) and Adjusted Plan Quality Metric (APQM) score. The PQM score is the overall score which evaluated the treatment plan against the PlanIQ algorithm. The APQM score was customized to the patient‐specific feasibility analysis, adjusting the PQM scores to account for unique challenges like patient‐specific anatomical challenges.Results:After introducing ICIS into the workflow, PlanIQ scores improved. On average, target coverage remained the same (p&gt;.05). Individual OAR sparing differed when looking at plans evaluated before ICIS versus plans created with the assistance of the ICIS template. PQM scores increased from 65.16% to 74.07% (p=.004), while the APQM scores increased from 78.26% to 84.05% (p=.008). Overall plan quality improved after providing planners with additional information during the treatment planning process.Conclusion:Overall plan quality improves when providing the treatment planner with additional information and guidance during the treatment planning process.

Evaluation of fluence-based dose delivery incorporating the spatial variation of dosimetric leaf gap (DLG).

The Eclipse treatment planning system uses a single dosimetric leaf gap (DLG) value to retract all multileaf collimator leaf positions during dose calculation to model the rounded leaf ends. This study evaluates the dosimetric impact of the 2D variation of DLG on clinical treatment plans based on their degree of fluence modulation. In‐house software was developed to retrospectively apply the 2D variation of DLG to 61 clinically treated VMAT plans, as well as to several test plans. The level of modulation of the VMAT cases were determined by calculating their modulation complexity score (MCS). Dose measurements were done using the MapCHECK device at a depth of 5.0 cm for plans with and without the 2D DLG correction. Measurements were compared against predicted dose planes from the TPS using absolute 3%/3 mm and 2%/2 mm gamma criteria for test plans and for VMAT cases, respectively. The gamma pass rate for the 2 mm, 4 mm, and 6 mm sweep test plans increased by 23.2%, 28.7%, and 26.0%, respectively, when the measurements were corrected with 2D variation of DLG. The clinical anal VMAT cases, which had very high MLC modulation, showed the most improvement. The majority of the improvement occurred for doses created by the 1.0 cm width leaves for both the test plans and the VMAT cases. The gamma pass rates for the highly modulated head and neck (H&N) cases, moderately modulated prostate and esophageal cases, and minimally modulated brain cases improved only slightly when corrected with 2D variation of DLG. This is because these cases did not employ the 1.0 cm width leaves for dose calculation and delivery. These data suggest that, at the very least, the TPS plans with highly modulated fluences created by the 1.0 cm fields require 2D DLG correction. Incorporating the 2D variation of DLG for the highly modulated clinical treatment plans improves their planar dose gamma pass rates, especially for fields employing the outer 1.0 cm width MLC leaves. This is because there are differences in DLG between the true DLG exhibited by the 1.0 cm width outer leaves and the constant DLG value modeled by the TPS for dose calculation.PACS numbers: 87.55.D, 87.55.Qr, 87.56.Fc, 87.56.N, 87.56.nk

Catching Errors With Trajectory Log Files

To present our single-institution experience on catching errors with trajectory log file analysis. To present measurement based tolerances for treatment type specific machine parameters as well as site-specific imaging shifts that could be used as a beam shutdown signal in the event parameters exceed tolerances. The reported causes of failures, probability of occurrences (O), severity of effects (S), and the probability of the failures to be undetected (D) could be added to guidelines of FMEA analysis. From March 2013 through March 2014, 19 569 patient treatment fields/arcs were analyzed for the treatment of brain, chest wall, esophagus, head and neck, lung, pelvis, prostate, spine, extremities, and breast. This work included checking all 131 treatment delivery parameters for all treatment sites and all treatment delivery fractions. Radiation therapy system trajectory log files for all treatment field types as well as all imaging types were accessed, read in every 20ms, and every control point (total of 37 million parameters) compared to the physician approved plan in the planning system. Couch angle outlier occurrence: N=327, range=-1.7–1.2 deg; gantry angle outlier occurrence: N=59, range=0.09-5.61 deg, collimator angle outlier occurrence: N=13, range=-0.2–0.2 deg. Volumetric modulated arc therapy cases had slightly larger variations in mechanical parameters; MLC: 3D single control point fields have a maximum deviation of 0.04mm, 39 step and shoot IMRT cases have MLC -0.3 – 0.5mm deviations, all (1286) VMAT cases have -0.9–0.7mm deviations. Two possible serious errors were found: 1) A 4cm isocenter shift for the PA beam of an AP-PA pair, underdosing a portion of PTV by 25%; 2) Delivery with MLC leaves abutted behind the jaws as opposed to the midline as planned, leading to an underdosing of a small volume of the PTV by 25%, by just the boost plan. Due to their error origin, neither of these errors could have been detected by pretreatment verification. This work shows the importance of performing trajectory log file analysis for all treatment plans, and for all treatment fractions, preferably real time to catch typically undetected errors to avoid potentially adverse incidents.

Analysis of direct clinical consequences of MLC positional errors in volumetric-modulated arc therapy using 3D dosimetry system.

In advanced, intensity‐modulated external radiotherapy facility, the multileaf collimator has a decisive role in the beam modulation by creating multiple segments or dynamically varying field shapes to deliver a uniform dose distribution to the target with maximum sparing of normal tissues. The position of each MLC leaf has become more critical for intensity‐modulated delivery (step‐and‐shoot IMRT, dynamic IMRT, and VMAT) compared to 3D CRT, where it defines only field boundaries. We analyzed the impact of the MLC positional errors on the dose distribution for volumetric‐modulated arc therapy, using a 3D dosimetry system. A total of 15 VMAT cases, five each for brain, head and neck, and prostate cases, were retrospectively selected for the study. All the plans were generated in Monaco 3.0.0v TPS (Elekta Corporation, Atlanta, GA) and delivered using Elekta Synergy linear accelerator. Systematic errors of +1,+0.5,+0.3,0,−1,−0.5,−0.3 mm were introduced in the MLC bank of the linear accelerator and the impact on the dose distribution of VMAT delivery was measured using the COMPASS 3D dosimetry system. All the plans were created using single modulated arcs and the dose calculation was performed using a Monte Carlo algorithm in a grid size of 3 mm. The clinical endpoints D95%,D50%,D2%, and Dmax,D20%, D50% were taken for the evaluation of the target and critical organs doses, respectively. A significant dosimetric effect was found for many cases even with 0.5 mm of MLC positional errors. The average change of dose D95% to PTV for ±1 mm,±0.5 mm, and ±0.3 mm was 5.15%, 2.58%, and 0.96% for brain cases; 7.19%, 3.67%, and 1.56% for head and neck cases; and 8.39%, 4.5%, and 1.86% for prostate cases, respectively. The average deviation of dose Dmax was 5.4%, 2.8%, and 0.83% for brainstem in brain cases; 8.2%, 4.4%, and 1.9% for spinal cord in H&N; and 10.8%, 6.2%, and 2.1% for rectum in prostate cases, respectively. The average changes in dose followed a linear relationship with the amount of MLC positional error, as can be expected. MLC positional errors beyond ±0.3 mm showed a significant influence on the intensity‐modulated dose distributions. It is, therefore, recommended to have a cautious MLC calibration procedure to sufficiently meet the accuracy in dose delivery.PACS number: 87.56

SU‐E‐T‐158: Catching Errors with Trajectory Log Files

Purpose:To present our single institution experience on catching errors with trajectory log file analysis. The reported causes of failures, probability of occurrences (O), severity of effects (S), and the probability of the failures to be undetected (D) could be added to guidelines of FMEA analysis.Methods:From March 2013 to March 2014, 19569 patient treatment fields/arcs were analyzed. This work includes checking all 131 treatment delivery parameters for all patients, all treatment sites and all treatment delivery fractions. TrueBeam trajectory log files for all treatment field types as well as all imaging types were accessed, read in every 20ms, and every control point (total of 37 million parameters) compared to the physician approved plan in the planning system.Results:Couch angle outlier occurrence: N= 327, range = −1.7 −1.2 deg; gantry angle outlier occurrence: N =59, range = 0.09 – 5.61 deg, collimator angle outlier occurrence: N = 13, range = −0.2 – 0.2 deg. VMAT cases have slightly larger variations in mechanical parameters. MLC: 3D single control point fields have a maximum deviation of 0.04 mm, 39 step and shoot IMRT cases have MLC −0.3 – 0.5 mm deviations, all (1286) VMAT cases have −0.9 – 0.7 mm deviations. Two possible serious errors were found: 1) A 4 cm isocenter shift for the PA beam of an AP‐PA pair, under‐dosing a portion of PTV by 25%. 2) Delivery with MLC leaves abutted behind the jaws as opposed to the midline as planned, leading to a under‐dosing of a small volume of the PTV by 25%, by just the boost plan. Due to their error origin, neither of these errors could have been detected by pre‐treatment verification.Conclusion:Performing Trajectory Log file analysis could catch typically undetected errors to avoid potentially adverse incidents.

SU-E-T-290: Dosimetric Accuracy of Acuros XB and Analytical Anisotropic Algorithm in Stereotactic Ablative Radiotherapy of Small Lung Lesions

Purpose: The accuracy of dose calculation for lung stereotactic ablative radiotherapy (SABR) of small lesions critically depends on the proper modeling of the lateral scatter in heterogeneous media. In recent years, grid-based Boltzman solvers such as Acuros XB (AXB) have been introduced for enhanced modeling of radiation transport in heterogeneous media. The purpose of this study is to evaluate the dosimetric impact of dose calculation between AXB and convolution-superposition algorithms such as analytical anisotropic algorithm (AAA) for small lesion sizes and different beam energies. Methods: Five lung SABR VMAT cases with GTV ranged from 0.8cm to 2.5cm in diameter were studied. For each case, doses were calculated by AAA, AXB (V11031) and Monte Carlo simulation (MC) with the same plan parameters for 10MV and 6MV. The dose calculation accuracy were evaluated by comparing DVHs and dose distributions with MC as the benchmark. The accuracy of calculated dose was also validated by EBT3 film measurement with a field size of 3cmx3cm in a thorax phantom. Results: For 10MV and GTV less than 1cm, dose calculated by AXB agrees well with MC compared to AAA. Dose difference calculated with AXB and AAA could be up to 30%. For GTV greater than 2cm, calculation results of AXB and AAA agree within 5% in GTV. For 6MV, the difference between calculated doses by AXB and AAA is less 10% for GTV less than 1cm. Based on film measurements, lung dose was overestimated 10% and 20% by AAA for 6MV and 10MV. Conclusion: Lateral scatter and transport is modeled more accurately by AXB than AAA in heterogeneous media, especially for small field size and high energy beams. The accuracy depends on the assigned material in calculation. If grid-based Boltzman solvers or MC are not available for calculation, lower energy beams should be used for treatment.

Sci—Thur PM: Planning &amp; Delivery — 06: Real‐Time Interactive Treatment Planning

Purpose:To describe and evaluate a novel system for generalized Real‐Time Interactive Planning (RTIP) applied to head &amp; neck (H&amp;N) VMAT. Methods: The clinician interactively manipulates dose distributions using DVHs, isodoses, or rate of dose fall‐off, which may be subjected to user‐defined constraints. Dose is calculated using a fast Achievable Dose Estimate (ADE) algorithm, which simulates the limits of what can be achieved during treatment. After each manipulation contributing fluence elements are modified and the dose distribution updates in effectively real‐time. For H&amp;N VMAT planning, structure sets for 11 patients were imported into RTIP. Each dose distribution was interactively modified to minimize OAR dose while constraining target DVHs. The resulting RTIP DVHs were transferred to the Eclipse™ VMAT optimizer, and conventional VMAT optimization was performed.Results:Dose calculation and update times for the ADE algorithm ranged from 2.4 to 22.6 milliseconds, thus facilitating effectively real‐time manipulation of dose distributions. For each of the 11 H&amp;N VMAT cases, the RTIP process took ∼2–10 minutes. All RTIP plans exhibited acceptable PTV coverage, mean dose, and max dose. 10 of 11 RTIP plans achieved substantially improved sparing of one or more OARs without compromising dose to targets or other OARs. Importantly, 10 of the 11 RTIP plans required only one or two post‐RTIP optimizations.Conclusions:RTIP is a novel system for manipulating and updating achievable dose distributions in real‐time. H&amp;N VMAT plans generated using RTIP demonstrate improved OAR sparing and planning efficiency.Disclosures:One author has a commercial interest in the presented materials.

SU‐E‐T‐440: Elliptical Source Model for IMRT and VMAT with Complex Modulation

Purpose:To improve dose calculation accuracy for highly modulated fields in IMRT and VMAT using an elliptical source model.Methods:For 2 TrueBeam linacs equipped with Millennium and HD MLCs, the source model of 6 and 15 MV beams was optimized with adjustments of the focal spot dimensions in the Varian Eclipse TPS. Starting from the measured focal spot size of 1.75 × 1.75 mm, the y‐dimension of the source was varied while the x‐dimension was kept constant. In addition, all other modeling parameters such as the extrafocal source and dosimetric leaf gap remained unchanged. The different models were compared with measured penumbra for jaws and MLC using ion chamber and diode. For 2 IMRT and 2 VMAT cases, the calculated doses were compared with radiochromic film and EPID measurements.Results:A source size of 1.75 × 0.75 mm was found to be optimal for beam modeling. The calculated penumbra (80–20) of a MLCdefined open field was within 0.8 mm compare to measurement for both the inline and crossline profile for various field sizes and depths. The dose gradients in the direction perpendicular to the MLC travel direction was better modeled compared to the source model with a circular focal spot of 1.75 mm for the fields with rigorous MLC modulation. Local differences up to 8% were found in those regions.Conclusion:The conventional source model using a symmetric focal spot tends to overestimate the penumbra perpendicular to the MLC travel direction, resulting in an inaccurate dose calculation especially for fields with complex MLC modulation. This can be improved with an elliptical source model and it can be considered as a standard as IMRT and VMAT treatments are becoming a norm for the clinic.