N Tyagi

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

PURPOSEnTo 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.nnnMETHODSnThe 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.nnnRESULTSnThe 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).nnnCONCLUSIONSnErrors 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.

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%/2u2009mmu2009<1 for all cases. PACS number: 80

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

PURPOSEnTo evaluate the feasibility of planning and delivering pulsed low dose radiotherapy (PLRT) using volumetric modulated arc therapy (VMAT) on Elektalinacs.nnnMETHODSnTen 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.nnnRESULTSnAn 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.nnnCONCLUSIONSnElekta linacs are capable of delivering reproducible and stable PLRT plans. A prospective clinical study employing PLRT is currently in development.

Variability in spine radiosurgery treatment planning – results of an international multi-institutional study

BackgroundThe aim of this study was to quantify the variability in spinal radiosurgery (SRS) planning practices between five international institutions, all member of the Elekta Spine Radiosurgery Research Consortium.MethodsFour institutions provided one representative patient case each consisting of the medical history, CT and MR imaging. A step-wise planning approach was used where, after each planning step a consensus was generated that formed the basis for the next planning step. This allowed independent analysis of all planning steps of CT-MR image registration, GTV definition, CTV definition, PTV definition and SRS treatment planning. In addition, each institution generated one additional SRS plan for each case based on intra-institutional image registration and contouring, independent of consensus results.ResultsAveraged over the four cases, image registration variability ranged between translational 1.1xa0mm and 2.4xa0mm and rotational 1.1° and 2.0° in all three directions. GTV delineation variability was 1.5xa0mm in axial and 1.6xa0mm in longitudinal direction averaged for the four cases. CTV delineation variability was 0.8xa0mm in axial and 1.2xa0mm in longitudinal direction. CTV-to-PTV margins ranged between 0xa0mm and 2xa0mm according to institutional protocol. Delineation variability was 1xa0mm in axial directions for the spinal cord. Average PTV coverage for a single fraction18 Gy prescription was 87u2009±u20095xa0%; Dmin to the PTV was 7.5u2009±u20091.8xa0Gy averaged over all cases and institutions. Average Dmax to the PRV_SC (spinal cordu2009+u20091xa0mm) was 10.5u2009±u20091.6xa0Gy and the average Paddick conformity index was 0.69u2009±u20090.06.ConclusionsResults of this study reflect the variability in current practice of spine radiosurgery in large and highly experienced academic centers. Despite close methodical agreement in the daily workflow, clinically significant variability in all steps of the treatment planning process was demonstrated. This may translate into differences in patient clinical outcome and highlights the need for consensus and established delineation and planning criteria.

Determinants of intrafraction variation during image-guided hypofractionated radiotherapy of the prostate

PurposeThe purpose of this study is to examine intrafraction variation and residual error (IFVu2009+u2009RE) during conebeam CT (CBCT)-guided hypofractionated prostate radiotherapy and evaluate factors associated along with target margins.Materials and methodsForty-six patients underwent hypofractionated prostate radiotherapy receiving 64xa0Gy in 20 fractions with on-line CBCT correction. Analysis of factors associated with IFVu2009+u2009RE including patient characteristics, treatment characteristics, and organ volumes was performed using univariate and multivariate analyses.ResultsThe mean IFVu2009+u2009RE were 0.10u2009±u20091.1, −1.1u2009±u20092.5, and −1.5xa0±u20092.7xa0mm in the mediolateral (ML), anteroposterior (AP), and craniocaudad (CC) dimensions, respectively. The IFVu2009+u2009RE vector was 3.5u2009±u20092.5xa0mm. It was found that 1, 22, and 31xa0% of fractions had an IFVu2009+u2009RE greater than 3xa0mm in the ML, AP, and CC dimensions, respectively. Multivariate analysis found that age and rectal volumes are associated with increased IFVu2009+u2009RE. When evaluating IFVu2009+u2009RE greater than 3xa0mm, age, treatment time, and rectal volumes were associated. A treatment time of greater than 16.5xa0min was found to be a cut point for increased IFVu2009+u2009RE.ConclusionsIFV may represent a significant component of target margins utilized in hypofractionated prostate radiotherapy with target margins exceeding 3xa0mm in the CC and AP dimensions. Rectal volumes, treatment time, and age are associated with IFVu2009+u2009RE.

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.

SU‐E‐U‐12: Evaluation of a Time‐Resolved Ultrasound System for Prostate Tracking Using a Novel, Multimodality KV‐Ultrasound Dynamic Phantom

Purpose: Development of novel multi‐modality dynamic phantom and technique designed to evaluate the accuracy of a time‐resolved ultrasound device (4D‐US), used for localization and motion monitoring of the prostate during hypo‐fractionated radiotherapy. Methods: A modified dynamic thorax phantom was designed using a spherical, ultrasound compatible, prostate phantom. The phantom end of the 90° arm was submerged in a water filled plastic container. A coupling cavity between the ultrasound probe and the container wall was developed. The motion of the dynamic phantom was pre‐programmed to test the 4D‐US capability to accurately reproduce the input trajectories. The Clarity 4D‐US presents two types of scanning modes: a volumetric mode for reference volume acquisition; and an auto‐scan mode for real‐time monitoring of the target. Using the monitoring mode, the amplitude detected was compared with the input amplitude in order to determine speed‐dependent, amplitude under‐sampling effects. Using the volumetric mode, the system time lag and manifestation of volumetric distortion due to increase in the phantom speed was determined. The difference between the largest dimension of the detected volume and the reference volume diameter divided by the phantom speed, determined the time lag of the 4D‐US system. Results: In monitoring mode, the system was capable to accurately reproduce a Sup‐Inf sinusoidal phantom trajectory with 2cm amplitude and a period of ∼15s ( speed∼2.7mm/s), showing an increased amplitude under‐sampling effect as the motion period was gradually decreased to 4s. In volumetric acquisition mode, the system average time lag, calculated based on the speed induced volumetric distortion, was found to be 0.31s. Conclusion: We have developed a novel kV‐US dynamic phantom to establish the 4D‐US scanning system parameters in order to accurately detect prostate motion. The novel phantom can be used in a multi‐modality concomitant imaging scheme allowing a direct, on‐line comparison of different real‐time tracking modalities.

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

PURPOSEnTo 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.nnnRESULTSnThe 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).nnnCONCLUSIONSnErrors 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.