MLC Leaf Research Articles

Dose Impact of Systemic MLC Position Error for Esophagus Cancer Plan: Volumetric Modulated Arc Therapy Versus Step and Shoot Modulated Therapy

Dose Impact of Systemic MLC Position Error for Esophagus Cancer Plan: Volumetric Modulated Arc Therapy Versus Step and Shoot Modulated Therapy

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

Optimizing the MLC model parameters for IMRT in the RayStation treatment planning system.

Unlike other commercial treatment planning systems (TPS) which model the rounded leaf end differently (such as the MLC dosimetric leaf gap (DLG) or rounded leaf‐tip radius), the RayStation TPS (RaySearch Laboratories, Stockholm, Sweden) models transmission through the rounded leaf end of the MLC with a step function, in which the radiation transmission through the leaf end is the square root of the average MLC transmission factor. We report on the optimization of MLC model parameters for the RayStation planning system. This (TPS) models the rounded leaf end of the MLC with the following parameters: leaf‐tip offset, leaf‐tip width, average transmission factor, and tongue and groove. We optimized the MLC model parameters for IMRT in the RayStation v. 4.0 planning system and for a Varian C‐series linac with a 120‐leaf Millennium MLC, and validated the model using measured data. The leaf‐tip offset is the geometric offset due to the rounded leaf‐end design and resulting divergence of the light/radiation field. The offset value is a function of the leaf‐tip position, and tabulated data are available from the vendor. The leaf‐tip width was iteratively evaluated by comparing computed and measured transverse dose profiles of MLC defined fields at dmax in water. In‐water profile comparisons were also used to verify the MLC leaf position (leaf‐tip offset). The average transmission factor and leaf tongue‐and‐groove width were derived iteratively by maximizing the agreement between measurements and RayStation TPS calculations for five clinical IMRT QA plans. Plan verifications were performed by comparing MapCHECK2 measurements and Monte Carlo calculations. The MLC model was validated using five test IMRT cases from the AAPM Task Group 119 report. Absolute gamma analyses (3 mm/3% and 2 mm/2%) were applied. In addition, computed output factors for MLC‐defined small fields (2×2,3×3,4×4,6×6 cm2) of both 6 MV and 18 MV photons were compared to those independently measured by the Imaging and Radiation Oncology Core (IROC), Houston, TX. 6 MV and 18 MV models were both determined to have the same MLC parameters: leaf‐tip offset=0.3 cm,2.5% transmission, and leaf tongue‐and‐groove width=0.05 cm. IMRT QA analysis for five test cases in TG‐119 resulted in a 100% passing rate with 3 mm/3% gamma analysis for 6 MV, and >97.5% for 18 MV. The passing rate was >94.6% for 6 MV and >90.9% for 18 MV when the 2 mm/2% gamma analysis criteria was applied. These results compared favorably with those published in AAPM Task Group 119. The reported MLC model parameters serve as a reference for other users.PACS number(s): 87.55.D, 87.56.nk

SU‐E‐T‐111: An Iterative Approach to Modelling Dosimetric Properties of a Varian MLC

Purpose:In effort to efficiently and accurately model MLC parameters for use in Varian Eclipse treatment planning system (TPS), an iterative technique for characterizing the dosimetric properties of MLCs has been developed.Methods:The initial characterization of the dosimetric leaf gap (DLG) and MLC transmission was performed using varied sizes of moving gaps and transmission measurements through central axis of the radiation beam. These values were determined during the initial data collection. After the initial values were determined and entered into the TPS, two plans were calculated to test the veracity of DLG and transmission independently. The plan used to determine the DLG has openings of varied sizes which are affected differently by the DLG. A large field uniform fluence that required a MLC carriage shift was used to determine the transmission value independent of DLG. After characterizing the dosimetric properties of the MLC independently, clinical IMRT plans were tested and used to tweak the final values.Results:This approach to modelling MLC dosimetric properties was employed for: standard 120 leaf MLC (SDMLC) and 120 leaf high definition MLC (HDMLC). Both types were successfully commissioned, tested, and approved for clinical use on 4 SDMLC machines and 2 HDMLC machines. The pass rate of clinical IMRT plans was 96.9% for 6MV and 98.3% for 10MV for SDMLC and 96.3% for 6MV, 96.5% for 6FFF, and 97.5% for 10FFF for HDMLCs using MapCheck2 with gamma analysis of 2%/2mm and 10% threshold. The final DLG and transmission parameters for 6X were 0.5mm and 0.01, and 1.35mm and 0.02 for HD and SD MLCs respectively.Conclusion:This iterative approach of determining dosimetric properties of MLCs is viable for both SDMLC and HDMLC and multiple energies. This approach accurately and efficiently determines the values needed for commissioning MLCs when they are employed for modulated therapies.

SU‐E‐T‐126: Application of PIPSpro and EPID for Quality Assurance of Medical Accelerators

Purpose:The purpose is to study applying PIPSpro and EPID for routine quality assurance of medical accelerators.Methods:(1) The coincidence of Light field and radiation field: Placed the FC‐2 phantom on the treatment couch at the 100cm SSD. Using jaw set 10×10 cm2 or 15×15 cm2 field. Verified that the light field aligns with the engraved squares on the phantom. Placed the Crosshair Marker phantom on the top of the FC‐2 phantom with the cross‐hair and spherical marker aligned to the center of the light field. Acquired images with EPID and the images should be exported preferably in DICOM format. Analyzed Follow the recommended procedures in the radiation light field module of PIPSpro software.(2)MLC QA:Set up the MLC phantom on the treatment couch at isocenter. Positioned the EPID imager at the highest possible position without colliding with either the couch or the phantom. Created and delivered four different patterns for testing the MLC leaf position, leaf width, multi‐port and leaf transmission. (3)Radiation isocenter test: Included the isocenter of collimator rotation, gantry rotation and couch rotation. Set the jaws of the collimator to create a thin field as 0.4cm × 20cm.Irradiated film or EPID using an equidistant 15° spacing angles with rotating collimator, gantry and couch respectively.Results:We analyzed imagers from EPID and film with PIPSPro software. As a Result, we acquired the specifications of medical accelerators quickly. In the meantime, the results were saved to database for trending and future analysis.Conclusion:Quality assurance of medical accelerator is very important. We used PIPSpro and EPID for routine detection, not only improved our efficiency, but also saved the cost. It is an good method.

WE-AB-BRB-09: Real Time In Vivo Scintillating Fiber Array Detector for Medical LINACS

Purpose: An in vivo transmission scintillation fiber detector was developed to monitor patient treatment in real time for the enhancement of patient safety and treatment accuracy. The detector system is capable of monitoring each pulse from a medical LINAC during treatment to determine the dose delivered as treatment progresses. Methods: The detector system consists of 60 parallel scintillating fibers coupled to fast data processing optoelectronics that can monitor the beam fluence in real time. Each 2.5mm2 square fiber is aligned with an MLC leaf pair and is long enough to capture a 40cm field. The fibers are embedded within a water equivalent polymer substrate that is secured in the LINAC accessory tray. The fibers are coupled to high speed photosensors and front end amplifiers that filter noise and pass each pulse to a high speed analog-to-digital converter. The system components are capable of detecting pulse repetition times shorter than what is delivered by a medical LINAC to ensure true real time data acquisition. Results: The system was able to capture and record the signal from each linac pulse and display the information in real time with no pulse pile up. It was found that the fiber array attenuates 2.65% of the beam which can easily be compensated for in treatment planning. The fibers responded linearly with dose, are independent of clinical beam energies, and are independent of dose rate. Calibration of the system was performed as a function of beam energy, beam size, dose rate, and monitor units to optimize beam fluence error detection. Conclusion: The detector system presented provides true real time in vivo beam monitoring to enhance patient safety and treatment delivery accuracy. Furthermore, the detector can be used for current patient specific QA.

SU‐E‐T‐347: Effect of MLC Leaf Position Inaccuracy On Dose Distribution for Spinal SBRT with Different Energies and Dose Rates

Purpose:To evaluate dosimetric impact of spinal SBRT when MLC leaf positions deviate from planning positions for different energies and doserates.Methods and Materials:18 localized spinal metastases patients were selected for SBRT using IMRT planning with 9 posterior beams delivered at gantry angles ranging between 100°–260°. A modern linear accelerator(Varian Turebeam STX with HDMLC 2.5 mm thick leaf at isocenter) IMRT plans were generated using both 6X and 6X_FFF(Flattening filter free) beams with a nominal prescription dose of 6 Gy/fraction to PTV. Doserates ranging from 200–600 MU/min for 6X and 400–1400 MU/min for 6X_FFF, with 200 increments were examined. A fixed amount(0.3, 0.5, 1, and 2 mm) of MLC‐leaf position deviation was simulated to each plan under following conditions: 1)only along X1 collimator; 2)with increments at both X1 and X2 collimator directions;3)with reductions at both X1 and X2 collimator directions. Dose was recalculated for each modified plans. Both original and modified plans were delivered using Turebeam STX machine and measured using both portal dosimetry and a 3D dosimeter(Delta4 of ScandiDos). Each field's Result were compared using following three parameters: the 95% iso‐dose level Conformal Index(95%CI), the spinal cord maximum dose(SCDmax), and the planned target volume(PTV) mean dose.Results:Dosimetric impacts on the 95%CI, SCDmax and the PTV mean dose are: 1)negligible if MLC‐leaf position deviation only along a single collimator direction ≥1.0 mm,2)substantial if MLC‐leaf position increment along both collimator directions ≥0.3 mm(95% CI decreases while SCDmax and PTV mean‐dose increase), 3)substantial if MLC‐leaf position reduction along both collimator directions ≥0.3 mm(95% CI first increases and then decreases while SCDmax and PTV mean‐dose decrease). Different energies and doserates demonstrated comparable dosimetric impacts.Conclusion:Substantial dose deviations could happen for spinal SBRT using IMRT plan with HD‐MLC if leaf position deviation ≥0.3 mm. The effects of different energy and doserate are negligible.

TH-AB-204-05: Real-Time Prevention of Radiation ‘mistakes’ Using Cherenkov Light Emission

Purpose: To determine the feasibility of using real-time imaging of Cherenkov light emission from an object’s surface for the qualitative identification of radiation ‘mistakes’. Methods: A highly sensitive camera was used to image the Cherenkov light emitted for a solid water phantom. A Varian 2100CD was used to deliver 500 MU for a 6 MV photon beam operating a dose rate of 600 MU/min, and the chosen imaging system and geometry provided data with a field of view of 30×30 cm, at 10 FPS, at a resolution of approximately 0.6 mm. Three mistake scenarios were imaged, two static errors (1) incorrect jaw size for stereotactic delivery using a 20 mm cone, (2) incorrect wedge angle applicator for a 10×10 cm field, and a dynamic errors (3) MLC leaf error and (4) incorrect collimator or gantry angle for IMRT delivery. Results: Using the real-time results of Cherenkov light emission imaging, the incorrect jaw size setting for SRS or SRT and incorrect wedge angle applicator were clearly identified in the first captured image. For the detection of a MLC leaf error, the proposed method was able to provide 4 frames per field, and detect leaf errors down to 1 mm and collimator and gantry errors down to 1 deg. Discussion and Conclusions: While the results in this feasibility study were performed using a solid water phantom, the technique could be readily applied in-vivo, as Cherenkov light emission imaging is completely non-invasive and requires no external materials to be introduced into the Linac or radiation beam path. For the detection of static errors, by providing real-time information after a single MU of delivery, further incorrect radiation field delivery could be avoided. Furthermore, dynamic errors in leaf positioning collimator or gantry angle could be detected in real-time down to 1 mm or 1 deg.

SU‐E‐T‐483: In Vivo Dosimetry of Conventional and Rotational Intensity Modulated Radiotherapy Using Integral Quality Monitor (IQM)

Purpose:To investigate the accuracy, sensitivity and constancy of integral quality monitor (IQM), a new system for in vivo dosimetry of conventional intensity modulated radiation therapy (IMRT) or rotational volumetric modulated arc therapy (VMAT)Methods:A beta‐version IQM system was commissioned on an Elekta Infinity LINAC equipped with 160‐MLCs Agility head. The stationary and rotational dosimetric constancy of IQM was evaluated, using five‐field IMRT and single‐or double‐arc VMAT plans for prostate and head‐and‐neck (H&N) patients. The plans were delivered three times over three days to assess the constancy of IQM response. Picket fence (PF) fields were used to evaluate the sensitivity of detecting MLC leaf errors. A single leaf offset was intentionally introduced during delivery of various PF fields with segment apertures of 3×1, 5×1, 10×1, and 24×1cm2. Both 2mm and 5mm decrease in the field width were used.Results:Repeated IQM measurements of prostate and H&N IMRT deliveries showed 0.4 and 0.5% average standard deviation (SD) for segment‐by‐segment comparison and 0.1 and 0.2% for cumulative comparison. The corresponding SDs for VMAT deliveries were 6.5, 9.4% and 0.7, 1.3%, respectively. Statistical analysis indicates that the dosimetric differences detected by IQM were significant (p < 0.05) in all PF test deliveries. The largest average IQM signal response of a 2 mm leaf error was found to be 2.1% and 5.1% by a 5mm leaf error for 3×1 cm2 field size. The same error in 24×1 cm2 generates a 0.7% and 1.4% difference in the signal.Conclusion:IQM provides an effective means for real‐time dosimetric verification of IMRT/ VMAT treatment delivery. For VMAT delivery, the cumulative dosimetry of IQM needs to be used in clinical practice.

SU‐E‐T‐54: A New Method for Optimizing Radiation Isocenter for Linac‐Based SRS

Purpose:To develop a new method to minimize deviation of linac x‐ray beams from the centroid of the volumetric radiation isocenter for all combinations of gantry and table angle.Methods:A set of ball‐bearing (Winston‐Lutz) images was used to determine the gantry radiation isocenter as the midrange of deviation values. Deviations in the cross‐plane direction were minimized by calibration of MLC leaf position offset, and by adjusting beam position steering for each energy. Special attention was also paid to matching the absolute position of isocenter across all energies by adjusting position steering in the gun‐target axis. Displacement of table axis from the gantry isocenter, and recommended table axis adjustment for contemporary Elekta linacs, was also determined. Eight images were used to characterize the volumetric isocenter for the full range of gantry and table rotations available. Tabulation of deviation for each beam was used to test compliance with isocenter tolerance.Results:Four contemporary Elekta linacs were evaluated and the radius in the gun‐target axis of the radiation isocenter was 0.5 to 0.7 mm. After beam steering adjustment, the radius in the cross‐plane direction was typically 0.2 to 0.4 mm. Position matching between energies can be reduced to 0.28 mm. Maximum total deviation was 0.68 to 1.07 mm, depending primarily on the effect of systematic table axis wobble with rotation.Conclusion:This new method effectively facilitates minimization of deviation between beam center and target position. The test, which requires a few minutes to perform, can be easily incorporated into a routine machine QA program. A tighter radiation isocenter for contemporary Elekta linacs would require reducing the effect of gantry arm flex and/or table axis wobble that are the two main components of deviation from the designated isocenter point.

SU-E-T-562: Motion Tracking Optimization for Conformal Arc Radiotherapy Plans: A QUASAR Phantom Based Study

Purpose: This study is to use plan parameters optimization (Dose rate, collimator angle, couch angle, initial starting phase) to improve the performance of conformal arc radiotherapy plans with motion tracking by increasing the plan performance score (PPS). Methods: Two types of 3D conformal arc plans were created based on QUASAR respiratory motion phantom with spherical and cylindrical targets. Sinusoidal model was applied to the MLC leaves to generate motion tracking plans. A MATLAB program was developed to calculate PPS of each plan (ranges from 0–1) and optimize plan parameters. We first selected the dose rate for motion tracking plans and then used simulated annealing algorithm to search for the combination of the other parameters that resulted in the plan of the maximal PPS. The optimized motion tracking plan was delivered by Varian Truebeam Linac. In-room cameras and stopwatch were used for starting phase selection and synchronization between phantom motion and plan delivery. Gaf-EBT2 dosimetry films were used to measure the dose delivered to the target in QUASAR phantom. Dose profiles and Truebeam trajectory log files were used for plan delivery performance evaluation. Results: For spherical target, the maximal PPS (PPSsph) of the optimized plan was 0.79: (Dose rate: 500MU/min, Collimator:more » 90°, Couch: +10°, starting phase: 0.83π). For cylindrical target, the maximal PPScyl was 0.75 (Dose rate: 300MU/min, Collimator: 87°, starting phase: 0.97π) with couch at 0°. Differences of dose profiles between motion tracking plans (with the maximal and the minimal PPS) and 3D conformal plans were as follows: PPSsph=0.79: %ΔFWHM: 8.9%, %Dmax: 3.1%; PPSsph=0.52: %ΔFWHM: 10.4%, %Dmax: 6.1%. PPScyl=0.75: %ΔFWHM: 4.7%, %Dmax: 3.6%; PPScyl=0.42: %ΔFWHM: 12.5%, %Dmax: 9.6%. Conclusion: By achieving high plan performance score through parameters optimization, we can improve target dose conformity of motion tracking plan by decreasing total MLC leaf travel distance and leaf speed.« less

SU‐E‐P‐21: Impact of MLC Position Errors On Simultaneous Integrated Boost Intensity‐Modulated Radiotherapy for Nasopharyngeal Carcinoma

Purpose:To investigate the impact of MLC position errors on simultaneous integrated boost intensity‐modulated radiotherapy (SIB‐IMRT) for patients with nasopharyngeal carcinoma.Methods:To compare the dosimetric differences between the simulated plans and the clinical plans, ten patients with locally advanced NPC treated with SIB‐IMRT were enrolled in this study. All plans were calculated with an inverse planning system (Pinnacle3, Philips Medical System₎. Random errors −2mm to 2mm₎,shift errors₍ 2mm,1mm and 0.5mm) and systematic extension/ contraction errors (±2mm, ±1mm and ±0.5mm) of the MLC leaf position were introduced respectively into the original plans to create the simulated plans. Dosimetry factors were compared between the original and the simulated plans.Results:The dosimetric impact of the random and system shift errors of MLC position was insignificant within 2mm, the maximum changes in D95% of PGTV,PTV1,PTV2 were‐0.92±0.51%,1.00±0.24% and 0.62±0.17%, the maximum changes in the D0.1cc of spinal cord and brainstem were 1.90±2.80% and −1.78±1.42%, the maximum changes in the Dmean of parotids were1.36±1.23% and −2.25±2.04%.However,the impact of MLC extension or contraction errors was found significant. For 2mm leaf extension errors, the average changes in D95% of PGTV,PTV1,PTV2 were 4.31±0.67%,4.29±0.65% and 4.79±0.82%, the averaged value of the D0.1cc to spinal cord and brainstem were increased by 7.39±5.25% and 6.32±2.28%,the averaged value of the mean dose to left and right parotid were increased by 12.75±2.02%,13.39±2.17% respectively.Conclusion:The dosimetric effect was insignificant for random MLC leaf position errors up to 2mm. There was a high sensitivity to dose distribution for MLC extension or contraction errors.We should pay attention to the anatomic changes in target organs and anatomical structures during the course,individual radiotherapy was recommended to ensure adaptive doses.

TH-AB-303-03: Real-Time Error Estimation for Real-Time Motion Prediction

Purpose: A critical component of real-time MLC tracking is to incorporate motion prediction in order to account for system latency, the time between the occurrence of motion and MLC leaf repositioning, so as to avoid geometric and dosimetric errors. In this work, we address a major limitation of current motion prediction algorithms – estimating the inherent error in prediction in real-time. We propose using an ensemble of real-time predictors to reduce overall prediction error, and provide a quantitative metric to trigger beam holds based on a user-defined error threshold. Methods: An ensemble of eighteen kernel density estimation (KDE)-based estimators was implemented in parallel and deployed on motion traces recorded from ten lung cancer patients. For each trace, the ensemble generated a sequence of predictions for a look-ahead time of 250ms, typical for many real-time systems. Each predictor, and the ensemble as a whole, was evaluated based on their root-mean-square error (RMSE) with respect to the original data, and assigned a beam hold score between 0 (perfect) and 1 to quantify the number of false beam holds and false beam releases. Results: The average execution time of the KDE-ensemble was 16ms, well below the 250ms look-ahead. For all traces, the ensemble produced a lower RMSE and hold score than any of the individual constituent predictors. For the worst-case motion trace, the minimum RMSE(hold score) for individual predictors was 0.912cm(0.340) compared to 0.854cm(0.305) for the KDE-ensemble predictor. The hold score for individual estimators was ∼0.6 when error estimates were not used to assert beam holds. Conclusion: The KDE-ensemble approach results in improved motion prediction. More importantly, it also provides real-time estimations of prediction error, thus addressing a major gap in current motion prediction algorithms. Such error prediction can be utilized to assert beam-holds during MLC tracking, preventing serious geometric and dosimetric errors. Funding for this work was provided by Elekta Limited.

TH‐AB‐BRB‐03: Automated Complexity Analysis of Volumetric Modulated Arc Therapy Plans

Purpose:Volumetric modulated arc therapy (VMAT) plans can become excessively complex with little additional dosimetric benefit due to the degenerative nature of inverse planning. Our goal is to improve the efficiency and quality of the VMAT planning process through the use of an automated complexity metric script.Methods:A metric that quantifies the complexity of VMAT plans by evaluating individual segment modulation was developed at our institution. This metric finds the ratio of the MLC leaf side length over aperture area for each control point, and calculates an MU‐weighted average of all controls points in the plan. A plug‐in script was created to enable use of the metric within our commercial treatment planning system. The script calculates the complexity metric and then compares it to metrics from previous clinical VMAT plans, all of which have passed patient specific pre‐treatment quality assurance. To set a threshold complexity value, we used the complexity metric script to analyze 517 clinical VMAT plans created in 2013–14 that passed QA (composite analysis, 95% of points passing 4%/1mm) and 57 that failed QA. This threshold was optimized based on the resulting true and false positive rates.Results:With the clinically set threshold, the complexity metric has a true positive rate of 44% for identifying plans that failed QA with a corresponding false positive rate of 6%. The false positives correspond to highly modulated plans, which may benefit from being reoptimized even though they initially passed patient specific QA.Conclusion:Our in‐house complexity metric script has the ability to streamline the VMAT planning and treatment process by alerting the planner when overly complex plans are generated. Those plans can be reoptimized before pre‐treatment QA is performed to attempt to reduce the plan complexity and therefore improve the plan delivery accuracy.This project was supported in part by P01CA059827.

TU‐G‐BRD‐08: In‐Vivo EPID Dosimetry: Quantifying the Detectability of Four Classes of Errors

Purpose:EPID dosimetry is an emerging method for treatment verification and QA. Given that the in‐vivo EPID technique is in clinical use at some centers, we investigate the sensitivity and specificity for detecting different classes of errors. We assess the impact of these errors using dose volume histogram endpoints. Though data exist for EPID dosimetry performed pre‐treatment, this is the first study quantifying its effectiveness when used during patient treatment (in‐vivo).Methods:We analyzed 17 patients; EPID images of the exit dose were acquired and used to reconstruct the planar dose at isocenter. This dose was compared to the TPS dose using a 3%/3mm gamma criteria. To simulate errors, modifications were made to treatment plans using four possible classes of error: 1) patient misalignment, 2) changes in patient body habitus, 3) machine output changes and 4) MLC misalignments. Each error was applied with varying magnitudes. To assess the detectability of the error, the area under a ROC curve (AUC) was analyzed. The AUC was compared to changes in D99 of the PTV introduced by the simulated error.Results:For systematic changes in the MLC leaves, changes in the machine output and patient habitus, the AUC varied from 0.78–0.97 scaling with the magnitude of the error. The optimal gamma threshold as determined by the ROC curve varied between 84–92%. There was little diagnostic power in detecting random MLC leaf errors and patient shifts (AUC 0.52–0.74). Some errors with weak detectability had large changes in D99.Conclusion:These data demonstrate the ability of EPID‐based in‐vivo dosimetry in detecting variations in patient habitus and errors related to machine parameters such as systematic MLC misalignments and machine output changes. There was no correlation found between the detectability of the error using the gamma pass rate, ROC analysis and the impact on the dose volume histogram.Funded by grant R18HS022244 from AHRQ

MO‐F‐CAMPUS‐T‐01: Radiosurgery of Multiple Brain Metastases with Single‐Isocenter VMAT: Optimizing Treatment Geometry to Reduce Normal Brain Dose

Purpose:To develop an optimization algorithm to reduce normal brain dose by optimizing couch and collimator angles for single isocenter multiple targets treatment of stereotactic radiosurgery.Methods:Three metastatic brain lesions were retrospectively planned using single‐isocenter volumetric modulated arc therapy (VMAT). Three matrices were developed to calculate the projection of each lesion on Beam's Eye View (BEV) by the rotating couch, collimator and gantry respectively. The island blocking problem was addressed by computing the total area of open space between any two lesions with shared MLC leaf pairs. The couch and collimator angles resulting in the smallest open areas were the optimized angles for each treatment arc. Two treatment plans with and without couch and collimator angle optimization were developed using the same objective functions and to achieve 99% of each target volume receiving full prescription dose of 18Gy. Plan quality was evaluated by calculating each target's Conformity Index (CI), Gradient Index (GI), and Homogeneity index (HI), and absolute volume of normal brain V8Gy, V10Gy, V12Gy, and V14Gy.Results:Using the new couch/collimator optimization strategy, dose to normal brain tissue was reduced substantially. V8, V10, V12, and V14 decreased by 2.3%, 3.6%, 3.5%, and 6%, respectively. There were no significant differences in the conformity index, gradient index, and homogeneity index between two treatment plans with and without the new optimization algorithm.Conclusion:We have developed a solution to the island blocking problem in delivering radiation to multiple brain metastases with shared isocenter. Significant reduction in dose to normal brain was achieved by using optimal couch and collimator angles that minimize total area of open space between any of the two lesions with shared MLC leaf pairs. This technique has been integrated into Eclipse treatment system using scripting API.

SU‐E‐T‐445: Gross Error Detection Based On EPID‐Image‐Extracted MLC‐Leaf‐Position Monitoring

Purpose:To develop a robust real‐time, log‐file independent MLC position validation metric capable of preventing gross treatment delivery errors.Methods:MLC leaf positions are identified on individual during‐treatment EPID images acquired in real time at 10 Hz during VMAT/IMRT patient treatments. MLC positions are identified as follows: (1) the image is rotated counterclockwise by 90°‐collimatorAngle so that MLC motion direction is vertical; (2) pixel‐columns corresponding to individual leaves are assigned based upon the imager SDD and known leaf positions on the isocenter plane; (3) inflection points are identified and averaged across pixel‐column intensity‐profiles for all central (50 % of total) pixel‐columns of each leaf; (4) an empirical position‐based offset is added to each leaf's average inflection point to arrive at its final estimated position. These leaf positions are appropriately interpolated between EPID images and compared to their planned positions at every control point and statistically analyzed for in‐field active (moving) leaves on image. Machine log and planned positions were compared for the same deliveries.Results:The mean EPID‐detected leaf offset from 51 fractions was 0.03±0.25 cm. 90.3 % of EPID‐detected leaves were within 3 mm. Machine‐log‐based analysis found an average of 0.0002±0.02 cm and 100% within.7 mm. Large (>3mm) EPID‐detected deviations resulted from algorithm failure in cases where multiple inflection points persisted in the profiles.Conclusions:Real‐time EPID‐based quality control can provide an independent validation of MLC leaf positions with sufficient resolution for gross error detection. Although further algorithmic improvements are required to match log‐file‐based accuracy, the current EPID‐based leaf position algorithm is suitable for clinical gross error detection.

SU‐E‐T‐145: Beam Characteristics of Flattening Filter Free Beams Including Low Dose Rate Setting

Purpose:In commissioning of volumetric modulated arc therapy (VMAT), it is necessary to evaluate the beam characteristics of various dose rate settings with potential to use. The aim of this study is to evaluate the beam characteristics of flattened and flattening filter free (FFF) including low dose rate setting.Methods:We used a Varian TrueBeam with Millennium 120 MLC. Both 6 and 10 MV beams with or without flattening filter were used for this study. To evaluate low‐dose rate FFF beams, specially‐designed leaf sequence files control out‐of‐field MLC leaf pair at constant dose rate ranging from 80 to 400 MU/min. For dose rate from 80 MU/min to the maximum usable value of all energies, beam output were measured using ionization chamber (CC04, IBA). The ionization chamber was inserted into water equivalent phantom (RT3000‐New, R‐tech), and the phantom was set with SAD of 100cm. The beam profiles were performed using the 2D diode array (Profiler2, Sun Nuclear). The SSD was set to 90cm and a combined 30cmx30cmx9cm phantom which consisted of solid water slabs was put on the device. All measurement were made using 100MU irradiation for 10cmx10cm jaw‐defined field size with a gantry angle of 0°.Results:In all energies, the dose rate dependences with beam output and variation coefficient were within 0.2% and 0.07%, respectively. The flatness and symmetry exhibited small variations (flatness ≤0.1 point and symmetry≤0.3 point at absolute difference).Conclusion:We had studied the characteristics of flattened and FFF beam over the 80 MU/min. Our results indicated that the beam output and profiles of FFF of TrueBeam linac were highly stable at low dose rate setting.

SU‐E‐T‐179: Clinical Impact of IMRT Failure Modes at Or Near TG‐142 Tolerance Criteria Levels

Purpose:Quantitatively assess the clinical impact of 11 critical IMRT dose delivery failure modes.Methods:Eleven step‐and‐shoot IMRT failure modes (FMs) were introduced into twelve Pinnacle v9.8 treatment plans. One standard and one highly modulated plan on the IROC IMRT phantom and ten previous H&N patient treatment plans were used. FMs included physics components covered by basic QA near tolerance criteria levels (TG‐142) such as beam energy, MLC positioning, and MLC modeling. Resultant DVHs were compared to those of failure‐free plans and the severity of plan degradation was assessed considering PTV coverage and OAR and normal tissue tolerances and used for FMEA severity scoring. Six of these FMs were physically simulated and phantom irradiations performed. TLD and radiochromic film results are used for comparison to treatment planning studies.Results:Based on treatment planning studies, the largest clinical impact from the phantom cases was induced by 2 mm systematic MLC shift in one bank with the combination of a D95% target under dose near 16% and OAR overdose near 8%. Cord overdoses of 5%–11% occurred with gantry angle, collimator angle, couch angle, MLC leaf end modeling, and MLC transmission and leakage modeling FMs. PTV coverage and/or OAR sparing was compromised in all FMs introduced in phantom plans with the exception of CT number to electron density tables, MU linearity, and MLC tongue‐and‐groove modeling. Physical measurements did not entirely agree with treatment planning results. For example, symmetry errors resulted in the largest physically measured discrepancies of up to 3% in the PTVs while a maximum of 0.5% deviation was seen in the treatment planning studies. Patient treatment plan study results are under analysis.Conclusion:Even in the simplistic anatomy of the IROC phantom, some basic physics FMs, just outside of TG‐142 tolerance criteria, appear to have the potential for large clinical implications.

SU‐E‐T‐571: Newly Emerging Integrated Transmission Detector Systems Provide Online Quality Assurance of External Beam Radiation Therapy

Purpose:Two newly emerging transmission detectors positioned upstream from the patient have been evaluated for online quality assurance of external beam radiotherapy. The prototype for the Integral Quality Monitor (IQM), developed by iRT Systems GmbH (Koblenz, Germany) is a large‐area ion chamber mounted on the linac accessory tray to monitor photon fluence, energy, beam shape, and gantry position during treatment. The ion chamber utilizes a thickness gradient which records variable response dependent on beam position. The prototype of Delta4 Discover™, developed by ScandiDos (Uppsala, Sweden) is a linac accessory tray mounted 4040 diode array that measures photon fluence during patient treatment. Both systems are employable for patient specific QA prior to treatment delivery.Methods:Our institution evaluated the reproducibility of measurements using various beam types, including VMAT treatment plans with both the IQM ion chamber and the Delta4 Discover diode array. Additionally, the IQM's effect on photon fluence, dose response, simulated beam error detection, and the accuracy of the integrated barometer, thermometer, and inclinometer were characterized. The evaluated photon beam errors are based on the annual tolerances specified in AAPM TG‐142.Results:Repeated VMAT treatments were measured with 0.16% reproducibility by the IQM and 0.55% reproducibility by the Delta4 Discover. The IQM attenuated 6, 10, and 15 MV photon beams by 5.43±0.02%, 4.60±0.02%, and 4.21±0.03% respectively. Photon beam profiles were affected <1.5% in the non‐penumbra regions. The IQM's ion chamber's dose response was linear and the thermometer, barometer, and inclinometer agreed with other calibrated devices. The device detected variations in monitor units delivered (1%), field position (3mm), single MLC leaf positions (13mm), and photon energy.Conclusion:We have characterized two new transmissions detector systems designed to provide in‐vivo like measurements upstream from the patient. Both systems demonstrate substantial utility for online treatment verification and QA of photon external beam radiotherapy.