K Yang

A real time dose monitoring and dose reconstruction tool for patient specific VMAT QA and delivery

PURPOSE To develop a real time dose monitoring and dose reconstruction tool to identify and quantify sources of errors during patient specific volumetric modulated arc therapy (VMAT) delivery and quality assurance. METHODS The authors develop a VMAT delivery monitor tool called linac data monitor that connects to the linac in clinical mode and records, displays, and compares real time machine parameters with the planned parameters. A new measure, called integral error, keeps a running total of leaf overshoot and undershoot errors in each leaf pair, multiplied by leaf width, and the amount of time during which the error exists in monitor unit delivery. Another tool reconstructs Pinnacle(3)™ format delivered plan based on the saved machine logfile and recalculates actual delivered dose in patient anatomy. Delivery characteristics of various standard fractionation and stereotactic body radiation therapy (SBRT) VMAT plans delivered on Elekta Axesse and Synergy linacs were quantified. RESULTS The MLC and gantry errors for all the treatment sites were 0.00 ± 0.59 mm and 0.05 ± 0.31°, indicating a good MLC gain calibration. Standard fractionation plans had a larger gantry error than SBRT plans due to frequent dose rate changes. On average, the MLC errors were negligible but larger errors of up to 6 mm and 2.5° were seen when dose rate varied frequently. Large gantry errors occurred during the acceleration and deceleration process, and correlated well with MLC errors (r = 0.858, p = 0.0004). PTV mean, minimum, and maximum dose discrepancies were 0.87 ± 0.21%, 0.99 ± 0.59%, and 1.18 ± 0.52%, respectively. The organs at risk (OAR) doses were within 2.5%, except some OARs that showed up to 5.6% discrepancy in maximum dose. Real time displayed normalized total positive integral error (normalized to the total monitor units) correlated linearly with MLC (r = 0.9279, p < 0.001) and gantry errors (r = 0.742, p = 0.005). There is a strong correlation between total integral error and PTV mean (r = 0.683, p = 0.015), minimum (r = 0.6147, p = 0.033), and maximum dose (r = 0.6038, p = 0.0376). CONCLUSIONS Errors may exist during complex VMAT planning and delivery. Linac data monitor is capable of detecting and quantifying mechanical and dosimetric errors at various stages of planning and delivery.

Comparing measurement-derived (3DVH) and machine log file-derived dose reconstruction methods for VMAT QA in patient geometries

The purpose of this study was to compare the measurement‐derived (3DVH) dose reconstruction method with machine log file‐derived dose reconstruction method in patient geometries for VMAT delivery. A total of ten patient plans were selected from a regular fractionation plan to complex SBRT plans. Treatment sites in the lung and abdomen were chosen to explore the effects of tissue heterogeneity on the respective dose reconstruction algorithms. Single‐ and multiple‐arc VMAT plans were generated to achieve the desired target objectives. Delivered plan in the patient geometry was reconstructed by using ArcCHECK Planned Dose Perturbation (ACPDP) within 3DVH software, and by converting the machine log file to Pinnacle3 9.0 treatment plan format and recalculating dose with CVSP algorithm. In addition, delivered gantry angles between machine log file and 3DVH 4D measurement were also compared to evaluate the accuracy of the virtual inclinometer within the 3DVH. Measured ion chamber and 3DVH‐derived isocenter dose agreed with planned dose within 0.4%±1.2% and ‐1.0%±1.6%, respectively. 3D gamma analysis showed greater than 98% between log files and 3DVH reconstructed dose. Machine log file reconstructed doses and TPS dose agreed to within 2% in PTV and OARs over the entire treatment. 3DVH reconstructed dose showed an average maximum dose difference of 3% ± 1.2% in PTV, and an average mean difference of ‐4.5%±10.5% in OAR doses. The average virtual inclinometer error (VIE) was ‐0.65° ± 1.6° for all patients, with a maximum error of ‐5.16° ± 4.54° for an SRS case. The time averaged VIE was within 1°–2°, and did not have a large impact on the overall accuracy of the estimated patient dose from ACPDP algorithm. In this study, we have compared two independent dose reconstruction methods for VMAT QA. Both methods are capable of taking into account the measurement and delivery parameter discrepancy, and display the delivered dose in CT patient geometry rather than the phantom geometry. The dose discrepancy can be evaluated in terms of DVH of the structures and provides a more intuitive understanding of the dosimetric impact of the delivery errors on the target and normal structure dose. PACS number: 87.55

Sensitivity analysis of physics and planning SmartArc parameters for single and partial arc VMAT planning

We investigate the sensitivity of various physics and planning SmartArc parameters to generate single and partial arc VMAT plans with equivalent or better plan quality as IMRT. Patients previously treated with step‐and‐shoot IMRT for several treatment sites were replanned using SmartArc. These treatment sites included head and neck, prostate, lung, and spine. Effect of various physics and planning SmartArc parameters, such as continuous vs. binned dose rate, dynamic leaf gap, leaf speed, maximum delivery time, number of arcs, and control point spacing, were investigated for Elekta Axesse and Synergy linacs. Absolute dose distribution was measured by using the ArcCHECK 3D cylindrical diode array. For all cases investigated, plan metrics such as conformity indices and dose homogeneity indices increased, while plan QA decreased with increasing leaf speed. Leaf speed had a significant impact on the segment size for low dose per fractionation cases. Constraining leaf motion to a lower speed not only avoids tiny large leaf travel and low‐dose rate value, but also achieves better PTV coverage (defined as the volume receiving prescription dose) with less total MUs. Maximum delivery time, the number of arcs, and the spacing of control points all had similar effects as the leaf motion constraint on dose rate and segment size. The maximum delivery time had a significant effect on the optimization, acting as a hard constraint. Increasing the control point spacing from 2 to 6 degrees increased the PTV coverage, but reduced the absolute dose gamma passing rate. Plans generated using continuous and binned dose rate modes did not show any difference in the quality and the delivery for the Elekta machines. Dosimetric analysis with a 3D cylindrical QA phantom resulted in 93.6%–99.3% of detectors with a gamma index 3%/2 mm <1 for all cases. PACS number: 80

External beam pulsed low dose radiotherapy using volumetric modulated arc therapy: Planning and delivery

PURPOSE To evaluate the feasibility of planning and delivering pulsed low dose radiotherapy (PLRT) using volumetric modulated arc therapy (VMAT) on Elektalinacs. METHODS Ten patients previously treated for glioblastomamultiforme (GBM) were replanned using PLRT VMAT to deliver ten 0.2 Gy pulses separated by 3 min intervals with an effective dose rate of 0.067 Gy∕min. VMAT parameters such as leaf speed and arc length were investigated to deliver 2 Gy∕fraction to a total of 60 Gy to the target volume in ten subfractions or pulses. Plan quality was assessed using conformity and homogeneity indices. Absolute dose distribution for individual pulses was measured using ArcCHECK diode array. Individual pulses were analyzed for reproducibility and stability using machine log files. Machine characteristics at low monitor units and low dose rate were also investigated. RESULTS An optimal arc length of 140°-160° and a leaf speed of 0.18-0.25 cm∕° were sufficient to provide equivalent plan coverage and stable delivery. The average time and dose rate required to deliver a single 0.2 Gy pulse was 39.5 ± 2.3 s and 49 ± 32.3 cGy∕min. Average reductions in MUs for the VMAT PLRT plan compared to IMRT for PTV was 16% (Range: -5.5%-36.1%) and 10.9% (Range: -18.4%-32.3%) for the initial and boost plan. A significant improvement was seen in maximum doses to all sensitive structures when planned with VMAT PLRT. The average absolute dose gamma passing rate for the 10 pulses combined and 2 Gy plan were 91.6 ± 2.5% and 97.3 ± 1.2%, respectively. Cumulative monitor units, dose rate, gantry angles, and leaf positions evaluated using machine log files were within 2% for all pulses. CONCLUSIONS Elekta linacs are capable of delivering reproducible and stable PLRT plans. A prospective clinical study employing PLRT is currently in development.

Evaluation of image guided motion management methods in lung cancer radiotherapy

PURPOSE To evaluate the accuracy and reliability of three target localization methods for image guided motion management in lung cancer radiotherapy. METHODS Three online image localization methods, including (1) 2D method based on 2D cone beam (CB) projection images, (2) 3D method using 3D cone beam CT (CBCT) imaging, and (3) 4D method using 4D CBCT imaging, have been evaluated using a moving phantom controlled by (a) 1D theoretical breathing motion curves and (b) 3D target motion patterns obtained from daily treatment of 3 lung cancer patients. While all methods are able to provide target mean position (MP), the 2D and 4D methods can also provide target motion standard deviation (SD) and excursion (EX). For each method, the detected MP/SD/EX values are compared to the analytically calculated actual values to calculate the errors. The MP errors are compared among three methods and the SD/EX errors are compared between the 2D and 4D methods. In the theoretical motion study (a), the dependency of MP/SD/EX error on EX is investigated with EX varying from 2.0 cm to 3.0 cm with an increment step of 0.2 cm. In the patient motion study (b), the dependency of MP error on target sizes (2.0 cm and 3.0 cm), motion patterns (four motions per patient) and EX variations is investigated using multivariant linear regression analysis. RESULTS In the theoretical motion study (a), the MP detection errors are -0.2 ± 0.2, -1.5 ± 1.1, and -0.2 ± 0.2 mm for 2D, 3D, and 4D methods, respectively. Both the 2D and 4D methods could accurately detect motion pattern EX (error < 1.2 mm) and SD (error < 1.0 mm). In the patient motion study (b), MP detection error vector (mm) with the 2D method (0.7 ± 0.4) is found to be significantly less than with the 3D method (1.7 ± 0.8,p < 0.001) and the 4D method (1.4 ± 1.0, p < 0.001) using paired t-test. However, no significant difference is found between the 4D method and the 3D method. Based on multivariant linear regression analysis, the variances of MP error in SI direction explained by target sizes, motion patterns, and EX variations are 9% with the 2D method, 74.4% with the 3D method, and 27% with the 4D method. The EX/SD detection errors are both < 1.0 mm for the 2D method and < 2.0 mm for the 4D method. CONCLUSIONS The 2D method provides the most accurate MP detection regardless of the motion pattern variations, while its performance is limited by the accuracy of target identification in the projection images. The 3D method causes the largest error in MP determination, and its accuracy significantly depends on target sizes, motion patterns, and EX variations. The 4D method provides moderate MP detection results, while its accuracy relies on a regular motion pattern. In addition, the 2D and 4D methods both provide accurate measurement of the motion SD/EX, providing extra information for motion management.

SU‐E‐T‐164: Comparing Measurement Derived (3DVH) and Machine Log File Derived Dose Reconstruction Methods for VMAT QA in Heterogeneous Patient Geometries

PURPOSE To extend the 3DVH analysis to heterogeneous patient geometries for VMAT delivery and comparing its accuracy using machine log file derived dose reconstruction method Methods: A total of 10 patient plans were selected from a regular fractionation plan to complex SBRT plans. Treatment sites in the lung and abdomen were chosen to explore the effects of tissue heterogeneity on the respective dose reconstruction algorithms. Delivered plan in the patient geometry was reconstructed by using ArcCheck Planned Dose Perturbation (ACPDP) within 3DVH software, and by converting the machine logfile to Pinnacle3 9.0 treatment plan format. In addition, delivered gantry angles between machine logfile and 3DVH 4D measurement was also compared to evaluate the accuracy of the virtual inclinometer within the 3DVH. RESULTS Measured ion chamber and 3DVH derived isocenter dose agreed with planned dose within 0.3±0.7 Gy and -0.5±0.8 Gy respectively. Machine log file reconstructed doses and TPS dose agreed to within 2 Gy in PTV and OARs over the entire treatment course. 3DVH reconstructed dose showed a difference of up to 3.2 Gy in maximum PTV doses compared to planned dose in hypo lung patients due to plan heterogeneity. For majority of normal structures, dose differences were within 1 Gy except for few cases, where a maximum point dose difference of up to 2.2 Gy in proximal bronchial tree dose for a hypo lung patient was seen. The average Virtual Inclinometer Error (VIE) was - 0.65±1.6° for all patients, with a maximum error of -5.16±4.54° for an SRS case. CONCLUSION Both methods are capable of taking into account the plan delivery errors. 3DVH is more sensitive to these errors compared to machine log file.

WE‐E‐BRB‐04: VMAT Dynamic QA for Moving Lung Tumors

Purpose: Development of a novel QA phantom and technique designed to evaluate the accuracy of VMAT delivered dose to the GTV when tumor motion is present. Materials and Methods: We have modified the Arccheck cylindrical QA phantom for VMAT delivery by designing a dynamic insert that can be accommodated in the central cavity of the detector and dosimetrically monitored. This was achieved by the use of a custom made water equivalent sphere with 5 imbedded mosfets. This sphere was encapsulated in a lung insert that can be accommodated by both the cylindrical QA phantom AND a thorax dynamic phantom. The motion of the dynamic phantom was preprogrammed for different trajectories including prerecorded traces of lung implanted fiducials from a previous study. A 4DCT scan was performed on the static/moving target and a VMAT treatment plan was correspondingly generated. These plans were mapped and calculated on the corresponding VMAT QA phantom (static/dynamic). The mosfets and the arccheck dose measurements from the treatment delivery were compared with the expected values obtained from the TPS. Results: Arccheck absolute dose analysis between measurement and calculation using γ (3%/3mm) shows more than 98% of diodes passing for both static and dynamic phantom with and without the lung phantom insert. Mosfet static measurement showed 2% agreement with the calculated value (5450 ± 120 vs 5250 ± 20 cGy). The dynamic measurement showed a larger spread than calculated (6900 ± 140 vs 7000 ± 250 cGy) indicating an accentuated interplay effect between MLC motion and tumor motion. Conclusion: We have developed a novel device to perform VMAT QA on moving tumors and to quantitatively estimate the dosimetric difference between the treatment plan and delivery. Our method is used to investigate a large variety of dose discrepancies including interplay effect and respiratory motion variation.

TH‐C‐BRB‐03: External Beam Pulsed Low Dose Radiotherapy Using Volumetric Modulated Arc Therapy: Planning, Delivery and Verification

Purpose: To evaluate the mechanical stability and dosimetric accuracy of planning and delivering pulsed low‐dose radiotherapy (PLRT) using volumetric modulated arc therapy (VMAT) on Elekta linacsMethods: Ten patients previously treated for Glioblastoma Multiforme were replanned using PLRT VMAT to deliver ten 0.2 Gy pulses separated by 3 min intervals with an effective dose rate of 0.067 Gy/min. VMAT parameters such as leaf speed and arc length were optimized to deliver 2 Gy/fraction to a total of 60 Gy to the target volume in ten sub‐fractions or pulses. Plan quality was assessed using conformity and homogeneity indices. Absolute dose distribution for individual pulses was measured using the Arccheck cylindrical diode array. Individual pulses were analyzed for reproducibility and stability using machine log file saved in clinical mode. Machine characteristics at low monitor units and low dose rate were also investigated. Results: An optimal arc length of 140 – 160 degree and a leaf speed of 0.18 − 0.25 cm/degree were sufficient to provide stable delivery and equivalent plan coverage to IMRT. The average time and dose rate required to deliver a single 0.2 Gy pulse was 39.5 ± 2.3 seconds and 49 ± 32.3 cGy/min. Average reduction in MUs for the PLRT plan compared to IMRT for PTV was 16.0% (Range: −5.5% to 36.1%). Significant improvement was seen in maximum doses to all sensitive structures when planned with VMAT PLRT. The average absolute dose gamma passing rate for the 10 pulses combined and 2 Gy plan were 91.6 ± 2.5% and 97.3 ± 1.2%. Cumulative monitor units, dose rate, gantry angles and leaf positions evaluated using machine log files were within 2% for all pulses. Flatness and symmetry were within Elekta specifications. Conclusions: Elekta linacs are capable of delivering reproducible and stable PLRT plans. Prospective clinical study employing PLRT is currently in process.

MO‐D‐BRB‐08: BEST IN PHYSICS (THERAPY) ‐ A Real Time Dose Monitoring and Dose Reconstruction Tool for Patient Specific VMAT QA and Delivery

PURPOSE To develop a real time dose monitoring and dose reconstruction tool to identify and quantify sources of errors during patient specific VMAT delivery and QAMethods: The VMAT delivery monitor tool called Linac Data Monitor (LDM) has been developed that connects to the linac in clinical mode and displays, records and compares real-time machine parameters to the planned parameters. A new quantity called integral error keeps a running total of leaf overshoot and undershoots errors in each leaf pair multiplied by leaf width and the amount of time during which error exists in MU delivery. Another tool reconstructs pinnacle format delivered plan based on the saved machine logfile and recalculates actual delivered dose in patient anatomy. Delivery characteristics of various standard and hypofractionation VMAT plans delivered on Elekta Axesse and Synergy linacs were quantified. RESULTS The MLC and gantry errors for all the treatment sites were 0.00±0.59mm and 0.05±0.31°, indicating a good MLC gain calibration. Standard fractionation plans had a larger gantry error than hypofractionation plans due to frequent dose rate changes. On average the MLC errors were negligible but larger errors of 4-6 mm and 2.5° were seen when dose rate varied frequently. Large gantry errors occurred during the acceleration and deceleration process, and correlated well with MLC errors (p<0.0001). PTV mean, minimum, maximum dose discrepancy were 0.87±0.21%, 0.99±0.59% and 1.18±0.52%. The other OAR doses were within 2.5% except a few that showed up to 5.6% discrepancy in maximum dose. Realtime displayed normalized total positive integral error (normalized to the total MUs) correlated linearly with MLC and gantry errors (p<0.001) and dosimetric discrepancy (PTVmean: p<0.01; PTVmax: p<0.067 and PTVmax: p<0.046). CONCLUSIONS Errors may exist during complex VMAT planning and delivery. LDM is capable of detecting and quantifying mechanical and dosimetric errors at various stages of planning and delivery.

SU‐E‐T‐429: Clinical Use of ArcCHECK for IMRT and VMAT Delivery QA

Purpose: To validate the dosimetric accuracy and sensitivity of ArcCheck for IMRT and VMAT QA. Methods: Arccheck is a cylindrical 3D phantom for IMRT and arc delivery. It is a 3D array of 1386 diodes arranged in a helical geometry at a depth of 2.9 cm. The dosimeter was evaluated for dose rate, angular dependence and systematic setup errors. Effect of planning parameters such as couch attenuation and control point (CP) spacing was investigated for various IMRT and VMAT plans. Diodes sensitivity to leaf position errors and gantry errors was studied by simulating errors in various arc delivery plans. Gamma evaluation criteria of 3%, 2mm was used for absolute dose comparison for clinical IMRT and VMAT plans. Results: Diodes show a dose rate dependency of 2% from 600 to 200 MU/min and 5% below 50 MU/min. An angular dependence of 5% was seen for a 20x30 cm2 field size. Prostate and HN IMRT plans delivered at planned angles showed lower gamma passing rate (94.3%) compared to mapcheck measurements (99.3%) delivered at 0° gantry angles. Accounting for couch attenuation in the calculation improved the average passing rate to 97 %. For a 3×40 cm2 static arc the gamma passing rate dropped from 92% to 83% when CP spacing changed from 2° to 6° but larger field sizes were not affected. Introducing a 1 mm setup error in AP, LR and SI direction resulted in 5%, 4.5% and 10% drop in passing rate for a HN VMAT plan. A 2mm leaf position error in one leaf and 3° error in one CP showed 6% and 5% difference in profile comparison for various arc deliveries. Conclusions: ArcCHECK is a robust and sensitive QA tool for IMRT and VMAT delivery. Future work will evaluate a CP by CP analysis to understand complex VMAT deliveries.