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Title: SU-F-T-298: The Impact of Modeling the Treatment Couch On Patient Specific VMAT QA

Abstract

Purpose: The aim of this work is to quantify the impact of modeling the treatment couch on the passing rate of ion chamber measurements during VMAT quality assurance. Methods: For the initial characterization, attenuation and surface dose measurements were performed following the guidelines of TG-176 for the Civco Universal couch top using an Elekta VersaHD accelerator at an energy of 6 MV. A simulation CT was performed to aid in the creation of contours for representing the shape and size of the couch top in the treatment planning system (TPS). A uniform value of density for the couch wall was determined by comparing the ratios of ion chamber measurements made in a 30×30×11 cm3 water phantom with the TPS dose values of a plan with the same geometry. At our institution, patient specific quality assurance is performed using a Sun Nuclear ArcCheck with a multi-plug for chamber measurements, a 0.125cc PTW TN31010 chamber, and a Sun Nuclear 1010 electrometer. Ten VMAT plans were transferred into the phantom geometry created in the TPS with two settings: with and without the couch. The chamber measurements were compared to both treatment plans. Results: A maximum attenuation of 3.6% was observed when the gantrymore » angle was set to 120 and 240 degrees, passing obliquely through the couch. A uniform density of 0.6 g/cm3 for the couch wall was determined in the TPS by comparison with measured data. The VMAT ion chamber measurement/plan ratios systematically improved by 1.79% ±0.53% for all patients when the couch was included in the calculation. Conclusion: The attenuation and surface dose changes produced by the Civco couch can generate observable dose difference in VMAT plans. Including a couch model in the phantom plan used for patient specific VMAT QA can improve the ionization chamber agreement by up to ∼2%.« less

Authors:
; ;  [1]
  1. University of Iowa Hospitals & Clinics, Iowa City, IA (United States)
Publication Date:
OSTI Identifier:
22648906
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; 61 RADIATION PROTECTION AND DOSIMETRY; ATTENUATION; IONIZATION CHAMBERS; PATIENTS; PHANTOMS; PLANNING; QUALITY ASSURANCE; RADIATION DOSES; RADIOTHERAPY; SIMULATION

Citation Formats

Gelover, E, Dalhart, A, and Hyer, D. SU-F-T-298: The Impact of Modeling the Treatment Couch On Patient Specific VMAT QA. United States: N. p., 2016. Web. doi:10.1118/1.4956483.
Gelover, E, Dalhart, A, & Hyer, D. SU-F-T-298: The Impact of Modeling the Treatment Couch On Patient Specific VMAT QA. United States. doi:10.1118/1.4956483.
Gelover, E, Dalhart, A, and Hyer, D. 2016. "SU-F-T-298: The Impact of Modeling the Treatment Couch On Patient Specific VMAT QA". United States. doi:10.1118/1.4956483.
@article{osti_22648906,
title = {SU-F-T-298: The Impact of Modeling the Treatment Couch On Patient Specific VMAT QA},
author = {Gelover, E and Dalhart, A and Hyer, D},
abstractNote = {Purpose: The aim of this work is to quantify the impact of modeling the treatment couch on the passing rate of ion chamber measurements during VMAT quality assurance. Methods: For the initial characterization, attenuation and surface dose measurements were performed following the guidelines of TG-176 for the Civco Universal couch top using an Elekta VersaHD accelerator at an energy of 6 MV. A simulation CT was performed to aid in the creation of contours for representing the shape and size of the couch top in the treatment planning system (TPS). A uniform value of density for the couch wall was determined by comparing the ratios of ion chamber measurements made in a 30×30×11 cm3 water phantom with the TPS dose values of a plan with the same geometry. At our institution, patient specific quality assurance is performed using a Sun Nuclear ArcCheck with a multi-plug for chamber measurements, a 0.125cc PTW TN31010 chamber, and a Sun Nuclear 1010 electrometer. Ten VMAT plans were transferred into the phantom geometry created in the TPS with two settings: with and without the couch. The chamber measurements were compared to both treatment plans. Results: A maximum attenuation of 3.6% was observed when the gantry angle was set to 120 and 240 degrees, passing obliquely through the couch. A uniform density of 0.6 g/cm3 for the couch wall was determined in the TPS by comparison with measured data. The VMAT ion chamber measurement/plan ratios systematically improved by 1.79% ±0.53% for all patients when the couch was included in the calculation. Conclusion: The attenuation and surface dose changes produced by the Civco couch can generate observable dose difference in VMAT plans. Including a couch model in the phantom plan used for patient specific VMAT QA can improve the ionization chamber agreement by up to ∼2%.},
doi = {10.1118/1.4956483},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • Purpose: Failure to model the treatment couch during VMAT QA planar dose calculation may Result in discrepancies between measured and calculated dose. These discrepancies are due to beam attenuation by the treatment couch that is not included in dose calculation. This work evaluates effects of accounting for this attenuation on patient specific VMAT QA results using an in-house created Varian Exact couch model in Pinnacl Methods: Patient specific VMAT QA results for 13 Pinnacle SmartArc plans generated for treatment on a Varian iX accelerator were studied. These plans included 3 treatment sites (7 H'N, 5 brain, 1 prostate). A Pinnaclemore » model for Varian Exact couch was created in-house to replace the CT simulator couch. Composite arc planar doses were calculated with no couch present (NC) and with the Exact couch model (CM) in place for each plan. QA measurements were taken using IBA Matrixx Evolution ion chamber array set up in IBA MultiCube and were compared to each planar dose. Gamma passing criteria of both 3%/3mm and 2%/2mm tolerances were used. Results: Over all treatment sites, increases in gamma passing rates from NC to CM ranged from -0.4% to +27.3% at 3%/3mm and +0.1% to +30.5% at 2%/2mm. Mean increases in passing rates were +3.7% and +5.3% for 3%/3mm and 2%/2mm tolerances, respectively. Site-specific mean increases (NC to CM) in gamma passing rates were +4.4%, +3.4%, +0.4% (3%/3mm tolerance) and +6.9%, +3.7%, and +2.9% at (2%/2mm tolerance) for H'N, brain, and prostate, respectively. Conclusion: Results support use of a couch model when generating planar dose for patient specific VMAT QA analysis. The improvements were most noticeable at 2%/2mm tolerance and for the H'N and brain sites. Eliminating treatment couch beam attenuation as a source of discrepancy in QA measurements may improve the ability to recognize otherwise masked delivered dose errors.« less
  • Purpose: To present a 3D QA method and clinical results for 550 patients. Methods: Five hundred and fifty patient treatment deliveries (400 IMRT, 75 SBRT and 75 VMAT) from various treatment sites, planned on Raystation treatment planning system (TPS), were measured on three beam-matched Elekta linear accelerators using IBA’s COMPASS system. The difference between TPS computed and delivered dose was evaluated in 3D by applying three statistical parameters to each structure of interest: absolute average dose difference (AADD, 6% allowed difference), absolute dose difference greater than 6% (ADD6, 4% structure volume allowed to fail) and 3D gamma test (3%/3mm DTA,more » 4% structure volume allowed to fail). If the allowed value was not met for a given structure, manual review was performed. The review consisted of overlaying dose difference or gamma results with the patient CT, scrolling through the slices. For QA to pass, areas of high dose difference or gamma must be small and not on consecutive slices. For AADD to manually pass QA, the average dose difference in cGy must be less than 50cGy. The QA protocol also includes DVH analysis based on QUANTEC and TG-101 recommended dose constraints. Results: Figures 1–3 show the results for the three parameters per treatment modality. Manual review was performed on 67 deliveries (27 IMRT, 22 SBRT and 18 VMAT), for which all passed QA. Results show that statistical parameter AADD may be overly sensitive for structures receiving low dose, especially for the SBRT deliveries (Fig.1). The TPS computed and measured DVH values were in excellent agreement and with minimum difference. Conclusion: Applying DVH analysis and different statistical parameters to any structure of interest, as part of the 3D QA protocol, provides a comprehensive treatment plan evaluation. Author G. Gueorguiev discloses receiving travel and research funding from IBA for unrelated to this project work. Author B. Crawford discloses receiving travel funding from IBA for unrelated to this project work.« less
  • Purpose: This research investigates the use of Mult-ileaf Collimator (MLC) dynalog files to modify a Volumetric Arc Therapy (VMAT) DICOM Radiotherapy Treatment file from the Treatment Planning System (TPS) for quality assurance and treatment plan verification. Methods: Actual MLC positions and gantry angles where retrieved from the MLC Dynalog files of an approved and treated VMAT plan. The treatment machine used was a Novalis TX linac equipped with high definition MLC. The DICOM RT file of the plan was exported from the TPS (Eclipse, Varian Medical Systems) and the actual MLC leaf positions and gantry angles were inserted in placemore » of the planned positions for each control point. The modified DICOM RT file was then imported back into the TPS where dose calculations were performed. The resulting dose distributions were then exported to VeriSoft (PTW) where a 3D gamma was calculated using 3mm-3% and 2mm-2% criteria. A 2D gamma was also calculated using dose measurements on the Delta4 (Sandidose) phantom. Results: A 3D gamma was calculated in Verisoft at 3mm-3% of 99.5% and at 2mm-2% of 99.2%. The pretreatment verification on the Delta4 yielded a 2D gamma at 3mm-3% of 97.9% and at 2mm-2% of 88.5%. The dose volume histograms of the approved plan and the dynalog plan are virtually identical. Conclusion: Initial results show good agreement of the dynalog dose distribution with the approved plan. Future work on this research will aim to increase the number of patients and replace the planned fractionated dose per control point with the actual fractionated dose.« less
  • Purpose: Several promising IMRT QA tools have been developed in recent years to combat problems found in the lack of sensitivity in planar dose measurements analyzed using consensus gamma analysis criteria. The increased complexity and added information with such devices adds not only increased time, but new challenges in determining endpoints for pass/fail criteria. Using a large cohort of previously measured planar IMRT QA data, it may be possible to correlate potentially problematic plans with calculated plan metrics that can be done a priori, such that these tools can be used only in clinically relevant situations. Methods: 90 previously measured,more » clinically delivered VMAT plans were exported in DICOM RT format. Using a Matlab program, plan metrics were computed based on a previously developed set of equations (Du et al. 2014). These metrics included MU-weighted beam irregularity, which quantifies an MLC shape’s deviation from that of a circle. Machine delivery parameters such as MU delivered per degree and leaf movement in millimeters per MU were also calculated. Based on a previous analysis of 394 IMRT QA measurements, a “failing” plan was defined as one with less than 85% gamma pass rate when computed at 1% dose difference and 2 mm distance to agreement; 16 of the 90 plans were identified as failing Results: A ROC curve was generated with an AUC of 0.9409. All 16 outliers were detected with a specificity of 85.1% when using a threshold value based on a linear combination of the MU-weighted plan irregularity and the leaf speed in mm/degree (average MU delivered per degree multiplied by average leaf movement in mm per MU). Conclusion: This data supports that potentially problematic VMAT plans can possibly be predicted before measurement by assessing the MU-weighted irregularity of the MLC shapes combined with the averaged leaf speed of the MLC.« less
  • Purpose: quantification and modelling of the dosimetric impact of the treatment couch in Monaco Treatment Planning System. Methods: The attenuation characteristics of couchtop EP was evaluated for two different photon acceleration potentials (6MV and 10MV) for a field size of (10×10) cm2. Phantom positions in A-B direction: on the left half, in the center and on the right half of the couch. Dose measurements of couch attenuation were performed at gantry angles from 180° to 122°, using a 0.125cc semiflex ionization chamber isocentrically placed in the center of a homogeneous cylindric sliced RW3 phantom. Each experimental setup was first measuredmore » on the LINAC and then reproduced in the TPS. By adjusting the relative-to-water electron density (ED) values of the couch, the measured attenuation was replicated. The simulated results were evaluated by comparing the measurements and simulations. Results: Without the couch model included the maximum difference between measured and calculated dose was 5.5% (5.1%) and 6.6% (6.1%) for 2 mm and 5 mm voxel size, when the phantom was positioned on the left (center). The couch model was included in the TPS with a uniform ED of 0.18 or a 2 component model with a fiber ED= 0.6 and foam core ED= 0.1. After including the treatment couch, the mean dose attenuation was reduced from 2.8% without couch included to (0.0, 0.8, −0.2, 0.6)%. The 4 different values represent the 1 and 2 components model and 2 and 5 mm voxel grid size. Conclusion: For a uniform relative-to-water couch electron density of 0.18 a good agreement between measured and calculated dose distributions was obtained for all different energies, voxel grid spacings and gantry angles. Therefore, we conclude that the Monaco couch model accurately describes the dose perturbations due to the presence of the patient couch and should therefore be used during treatment planning. This project is supported by Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Hebei Personnel of China.« less