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Title: SU-F-T-471: Simulated External Beam Delivery Errors Detection with a Large Area Ion Chamber Transmission Detector

Abstract

Purpose: The Integral Quality Monitor (IQM), developed by iRT Systems GmbH (Koblenz, Germany) is a large-area, linac-mounted ion chamber used to monitor photon fluence during patient treatment. Our previous work evaluated the change of the ion chamber’s response to deviations from static 1×1 cm2 and 10×10 cm2 photon beams and other characteristics integral to use in external beam detection. The aim of this work is to simulate two external beam radiation delivery errors, quantify the detection of simulated errors and evaluate the reduction in patient harm resulting from detection. Methods: Two well documented radiation oncology delivery errors were selected for simulation. The first error was recreated by modifying a wedged whole breast treatment, removing the physical wedge and calculating the planned dose with Pinnacle TPS (Philips Radiation Oncology Systems, Fitchburg, WI). The second error was recreated by modifying a static-gantry IMRT pharyngeal tonsil plan to be delivered in 3 unmodulated fractions. A radiation oncologist evaluated the dose for simulated errors and predicted morbidity and mortality commiserate with the original reported toxicity, indicating that reported errors were approximately simulated. The ion chamber signal of unmodified treatments was compared to the simulated error signal and evaluated in Pinnacle TPS again with radiationmore » oncologist prediction of simulated patient harm. Results: Previous work established that transmission detector system measurements are stable within 0.5% standard deviation (SD). Errors causing signal change greater than 20 SD (10%) were considered detected. The whole breast and pharyngeal tonsil IMRT simulated error increased signal by 215% and 969%, respectively, indicating error detection after the first fraction and IMRT segment, respectively. Conclusion: The transmission detector system demonstrated utility in detecting clinically significant errors and reducing patient toxicity/harm in simulated external beam delivery. Future work will evaluate detection of other smaller magnitude delivery errors.« less

Authors:
; ; ; ; ;  [1]
  1. Cancer Center, Sacramento, CA (United States)
Publication Date:
OSTI Identifier:
22649061
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; DELIVERY; DISEASE INCIDENCE; ERRORS; IONIZATION CHAMBERS; LYMPHATIC SYSTEM; MAMMARY GLANDS; MEDICAL PERSONNEL; PATIENTS; PHOTON BEAMS; RADIOTHERAPY; SIMULATION

Citation Formats

Hoffman, D, Dyer, B, Kumaran Nair, C, Stern, R, Benedict, S, and Davis, UC. SU-F-T-471: Simulated External Beam Delivery Errors Detection with a Large Area Ion Chamber Transmission Detector. United States: N. p., 2016. Web. doi:10.1118/1.4956656.
Hoffman, D, Dyer, B, Kumaran Nair, C, Stern, R, Benedict, S, & Davis, UC. SU-F-T-471: Simulated External Beam Delivery Errors Detection with a Large Area Ion Chamber Transmission Detector. United States. doi:10.1118/1.4956656.
Hoffman, D, Dyer, B, Kumaran Nair, C, Stern, R, Benedict, S, and Davis, UC. 2016. "SU-F-T-471: Simulated External Beam Delivery Errors Detection with a Large Area Ion Chamber Transmission Detector". United States. doi:10.1118/1.4956656.
@article{osti_22649061,
title = {SU-F-T-471: Simulated External Beam Delivery Errors Detection with a Large Area Ion Chamber Transmission Detector},
author = {Hoffman, D and Dyer, B and Kumaran Nair, C and Stern, R and Benedict, S and Davis, UC},
abstractNote = {Purpose: The Integral Quality Monitor (IQM), developed by iRT Systems GmbH (Koblenz, Germany) is a large-area, linac-mounted ion chamber used to monitor photon fluence during patient treatment. Our previous work evaluated the change of the ion chamber’s response to deviations from static 1×1 cm2 and 10×10 cm2 photon beams and other characteristics integral to use in external beam detection. The aim of this work is to simulate two external beam radiation delivery errors, quantify the detection of simulated errors and evaluate the reduction in patient harm resulting from detection. Methods: Two well documented radiation oncology delivery errors were selected for simulation. The first error was recreated by modifying a wedged whole breast treatment, removing the physical wedge and calculating the planned dose with Pinnacle TPS (Philips Radiation Oncology Systems, Fitchburg, WI). The second error was recreated by modifying a static-gantry IMRT pharyngeal tonsil plan to be delivered in 3 unmodulated fractions. A radiation oncologist evaluated the dose for simulated errors and predicted morbidity and mortality commiserate with the original reported toxicity, indicating that reported errors were approximately simulated. The ion chamber signal of unmodified treatments was compared to the simulated error signal and evaluated in Pinnacle TPS again with radiation oncologist prediction of simulated patient harm. Results: Previous work established that transmission detector system measurements are stable within 0.5% standard deviation (SD). Errors causing signal change greater than 20 SD (10%) were considered detected. The whole breast and pharyngeal tonsil IMRT simulated error increased signal by 215% and 969%, respectively, indicating error detection after the first fraction and IMRT segment, respectively. Conclusion: The transmission detector system demonstrated utility in detecting clinically significant errors and reducing patient toxicity/harm in simulated external beam delivery. Future work will evaluate detection of other smaller magnitude delivery errors.},
doi = {10.1118/1.4956656},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • Purpose: To examine both the IQM’s sensitivity and false positive rate to varying MLC errors. By balancing these two characteristics, an optimal tolerance value can be derived. Methods: An un-modified SBRT Liver IMRT plan containing 7 fields was randomly selected as a representative clinical case. The active MLC positions for all fields were perturbed randomly from a square distribution of varying width (±1mm to ±5mm). These unmodified and modified plans were measured multiple times each by the IQM (a large area ion chamber mounted to a TrueBeam linac head). Measurements were analyzed relative to the initial, unmodified measurement. IQM readingsmore » are analyzed as a function of control points. In order to examine sensitivity to errors along a field’s delivery, each measured field was divided into 5 groups of control points, and the maximum error in each group was recorded. Since the plans have known errors, we compared how well the IQM is able to differentiate between unmodified and error plans. ROC curves and logistic regression were used to analyze this, independent of thresholds. Results: A likelihood-ratio Chi-square test showed that the IQM could significantly predict whether a plan had MLC errors, with the exception of the beginning and ending control points. Upon further examination, we determined there was ramp-up occurring at the beginning of delivery. Once the linac AFC was tuned, the subsequent measurements (relative to a new baseline) showed significant (p <0.005) abilities to predict MLC errors. Using the area under the curve, we show the IQM’s ability to detect errors increases with increasing MLC error (Spearman’s Rho=0.8056, p<0.0001). The optimal IQM count thresholds from the ROC curves are ±3%, ±2%, and ±7% for the beginning, middle 3, and end segments, respectively. Conclusion: The IQM has proven to be able to detect not only MLC errors, but also differences in beam tuning (ramp-up). Partially supported by the Susan Scott Foundation.« less
  • 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 patientmore » 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.« less
  • Purpose: To evaluate a new transmission chamber for use as a reference chamber in measurements of beam data Methods: We assessed the performance of a new transmission detector, the Stealth Chamber, manufactured by IBA (IBA Dosimetry). The chamber has an active volume of 249 cm{sup 3}, with an attenuation equivalent of <0.5mm Al. We mounted the chamber to a TrueBeam linac (Varian Medical Systems, Palo Alto, CA) such that the active area is perpendicular to the beam direction. We performed PDD and profile measurements on field sizes from 1×1 cm{sup 2} to 10×10 cm{sup 2}, as well as for amore » 4 mm cone. The field detector was either a CC-13 chamber (IBA Dosimetry) with an active volume of 0.13 cm{sup 3} or an Edge Detector (Sun Nuclear, Melbourne, FL) with an active volume of 0.019 cm{sup 3}. For comparison we repeated all measurements using a CC-13 or CC-01 (active volume 0.01 cm{sup 3} ) chamber as reference detectors (IBA Dosimetry).All scans were acquired using a Blue Phantom2 (IBA Dosimetry) and IBA OmniPro-Accept v7.4.24 software. Results: All measurements with the Stealth Chamber were identical to those with ion reference chambers. For both regular and filter free 6MV beams there was agreement in PDDs and profiles, including the penumbra region, for all field sizes. This was true for the 4mm cone measurement, as well. The deviation between the Stealth and ion chamber measurements was on average 0.3%. Conclusion: The Stealth Chamber gives identical beam data as a conventional ion chamber for all field sizes. The advantage of the Stealth chamber over ion chambers is its efficiency. Once mounted, there is no need to reposition the chamber with varying field sizes. This translates into a huge savings in measurement time, as well as a reduction in potential errors due to reference chamber mispositioning.« less
  • Purpose: To use receiver operating characteristic (ROC) analysis to quantify the Varian Portal Dosimetry (VPD) application's ability to detect delivery errors in IMRT fields. Methods: EPID and VPD were calibrated/commissioned using vendor-recommended procedures. Five clinical plans comprising 56 modulated fields were analyzed using VPD. Treatment sites were: pelvis, prostate, brain, orbit, and base of tongue. Delivery was on a Varian Trilogy linear accelerator at 6MV using a Millenium120 multi-leaf collimator. Image pairs (VPD-predicted and measured) were exported in dicom format. Each detection test imported an image pair into Matlab, optionally inserted a simulated error (rectangular region with intensity raised ormore » lowered) into the measured image, performed 3%/3mm gamma analysis, and saved the gamma distribution. For a given error, 56 negative tests (without error) were performed, one per 56 image pairs. Also, 560 positive tests (with error) with randomly selected image pairs and randomly selected in-field error location. Images were classified as errored (or error-free) if percent pixels with γ« less