AAPM TG-142 Research Articles

TU-B-304-01: The Aftermath of TG-142

Although published in 2009, the AAPM TG-142 report on accelerator quality assurance still proves a challenge for full clinical implementation. The choice of methodologies to satisfy TG-142 requirements is critical to a successful application. Understanding the philosophy of TG-142 can help in creating an institution-specific QA practice that is both efficient and effective. The concept of maintaining commissioned beam profiles is still found confusing. The physicist must also consider technologies not covered by TG-142 (i.e. arc therapy techniques). On the horizon is TG-198 report on implementing TG-142. Although the community still lacks a final TG-100 report, performing a failure-mode -and-effects analysis and statistical process control analysis to determine the institution-specific clinical impact of each TG-142 test may be useful for identifying trends for pro-active surveillance. Learning Objectives: 1. To better understand the confusing and controversial aspects of TG-142. 2. To understand what is still missing from TG-142 and how to account for these tests in clinical practice 3. To describe which QA tests in TG-142 yield the largest potential clinical result if not discovered.

TU-B-304-02: Quantitative FMEA of TG-142

Although published in 2009, the AAPM TG-142 report on accelerator quality assurance still proves a challenge for full clinical implementation. The choice of methodologies to satisfy TG-142 requirements is critical to a successful application. Understanding the philosophy of TG-142 can help in creating an institution-specific QA practice that is both efficient and effective. The concept of maintaining commissioned beam profiles is still found confusing. The physicist must also consider technologies not covered by TG-142 (i.e. arc therapy techniques). On the horizon is TG-198 report on implementing TG-142. Although the community still lacks a final TG-100 report, performing a failure-mode -and-effects analysis and statistical process control analysis to determine the institution-specific clinical impact of each TG-142 test may be useful for identifying trends for pro-active surveillance. Learning Objectives: 1. To better understand the confusing and controversial aspects of TG-142. 2. To understand what is still missing from TG-142 and how to account for these tests in clinical practice 3. To describe which QA tests in TG-142 yield the largest potential clinical result if not discovered.

TU-B-304-00: The Aftermath of TG-142

Although published in 2009, the AAPM TG-142 report on accelerator quality assurance still proves a challenge for full clinical implementation. The choice of methodologies to satisfy TG-142 requirements is critical to a successful application. Understanding the philosophy of TG-142 can help in creating an institution-specific QA practice that is both efficient and effective. The concept of maintaining commissioned beam profiles is still found confusing. The physicist must also consider technologies not covered by TG-142 (i.e. arc therapy techniques). On the horizon is TG-198 report on implementing TG-142. Although the community still lacks a final TG-100 report, performing a failure-mode -and-effects analysis and statistical process control analysis to determine the institution-specific clinical impact of each TG-142 test may be useful for identifying trends for pro-active surveillance. Learning Objectives: 1. To better understand the confusing and controversial aspects of TG-142. 2. To understand what is still missing from TG-142 and how to account for these tests in clinical practice 3. To describe which QA tests in TG-142 yield the largest potential clinical result if not discovered.

SU‐E‐T‐781: Using An Electronic Portal Imaging Device (EPID) for Correlating Linac Photon Beam Energies

Purpose:Electronic portal imaging devices (EPIDs) have proven to be useful for measuring several parameters of interest in linear accelerator (linac) quality assurance (QA). The purpose of this project was to evaluate the feasibility of using EPIDs for determining linac photon beam energies.Methods:Two non‐clinical Varian TrueBeam linacs (Varian Medical Systems, Palo Alto, CA) with 6MV and 10MV photon beams were used to perform the measurements. The linacs were equipped with an amorphous silicon based EPIDs (aSi1000) that were used for the measurements. We compared the use of flatness versus percent depth dose (PDD) for predicting changes in linac photon beam energy. PDD was measured in 1D water tank (Sun Nuclear Corporation, Melbourne FL) and the profiles were measured using 2D ion‐chamber array (IC‐Profiler, Sun Nuclear) and the EPID. Energy changes were accomplished by varying the bending magnet current (BMC). The evaluated energies conformed with the AAPM TG142 tolerance of ±1% change in PDD.Results:BMC changes correlating with a ±1% change in PDD corresponded with a change in flatness of ∼1% to 2% from baseline values on the EPID. IC Profiler flatness values had the same correlation. We observed a similar trend for the 10MV beam energy changes. Our measurements indicated a strong correlation between changes in linac photon beam energy and changes in flatness. For all machines and energies, beam energy changes produced change in the uniformity (AAPM TG‐142), varying from ∼1% to 2.5%.Conclusions:EPID image analysis of beam profiles can be used to determine linac photon beam energy changes. Flatness‐based metrics or uniformity as defined by AAPM TG‐142 were found to be more sensitive to linac photon beam energy changes than PDD.Research funding provided by Varian Medical Systems. Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.

Evaluation of imaging performance of megavoltage cone-beam CT over an extended period

A linear accelerator vendor and the AAPM TG-142 report propose that quality assurance testing for image-guided devices such megavoltage cone-beam CT (MV-CBCT) be conducted on a monthly basis. In clinical settings, however, unpredictable errors such as image artifacts can occur even when quality assurance results performed at this frequency are within tolerance limits. Here, we evaluated the imaging performance of MV-CBCT on a weekly basis for ∼ 1 year using a Siemens ONCOR machine with a 6-MV X-ray and an image-quality phantom. Image acquisition was undertaken using 15 monitor units. Geometric distortion was evaluated with beads evenly distributed in the phantom, and the results were compared with the expected position in three dimensions. Image-quality characteristics of the system were measured and assessed qualitatively and quantitatively, including image noise and uniformity, low-contrast resolution, high-contrast resolution and spatial resolution. All evaluations were performed 100 times each. For geometric distortion, deviation between the measured and expected values was within the tolerance limit of 2 mm. However, a subtle systematic error was found which meant that the phantom was rotated slightly in a clockwise manner, possibly due to geometry calibration of the MV-CBCT system. Regarding image noise and uniformity, two incidents over tolerance occurred in 100 measurements. This phenomenon disappeared after dose calibration of beam output for MV-CBCT. In contrast, all results for low-contrast resolution, high-contrast resolution and spatial resolution were within their respective tolerances.

SU‐E‐T‐207: Flatness and Symmetry Threshold Detection Using Statistical Process Control

AAPM TG-142 guidelines state that beam uniformity (flatness and symmetry) should maintain a constancy of 1 % relative to baseline. The focus of this study is to determine if statistical process control (SPC) methodology using process control charts (PCC) of steering coil currents (SCC) can detect changes in beam uniformity prior to exceeding the 1% constancy criteria. SCCs for the transverse and radial planes are adjusted such that a reproducibly useful photon or electron beam is available. Transverse and radial - positioning and angle SCC are routinely documented in the Morning Check file during daily warm-up. The 6 MV beam values for our linac were analyzed using average and range (Xbar/R) PCC. Using this data as a baseline, an experiment was performed in which each SCC was changed from its mean value (steps of 0.01 or 0.02 Ampere) while holding the other SCC constant. The effect on beam uniformity was measured using a beam scanning system. These experimental SCC values were plotted in the PCC to determine if they would exceed the predetermined limits. The change in SCC required to exceed the 1% constancy criteria was detected by the PCC for 3 out of the 4 steering coils. The reliability of the result in the one coil not detected (transverse position coil) is questionable because the SCC slowly drifted during the experiment (0.05 A) regardless of the servo control setting. X-bar/R charts of SCC can detect exceptional variation prior to exceeding the beam uniformity criteria set forth in AAPM TG-142. The high level of PCC sensitivity to change may result in an alarm when in fact minimal change in beam uniformity has occurred. Further study is needed to determine if a combination of individual SCC alarms would reduce the false positive rate for beam uniformity intervention. This project was supoorted by a grant from Varian Medical Systems, Inc.

An EPID based method for performing high accuracy calibration between an optical external marker tracking device and the LINAC reference frame

With the increasing use of external 3D optical tracking cameras to guide modern radiation therapy procedures, it has become vitally important to have an accurate camera to linear accelerator (LINAC) reference frame calibration. To eliminate errors present in current calibration procedures based on the manual hand alignment of a device using the light field crosshairs and in room guidance lasers, a semiautomated quantitative calibration approach requiring only use of an electronic portal imaging device (EPID) was developed. A phantom comprised of seven highly IR reflective plastic BBs was placed on the LINAC treatment couch and imaged with both a 3D stereoscopic IR imager and the on board megavoltage (MV) EPID imager. Having knowledge of the optically determined 3D positions and projected EPID images of the BBs, simulated annealing was used to optimize the location of the BBs in the LINAC frame using four different optimization functions. Singular value decomposition was then used to calculate the transformation matrix between the camera and LINAC reference frames. Results were then compared to a traditional camera calibration method for overall accuracy. Using modeled data, the simulated annealing process was able to determine the actual locations of the BBs with a RMSE of 0.23 mm. Using projection images acquired with an MV imager, the process was able to determine locations of BBs within .26 mm. The results depend on the choice of optimization function. Results show that the method can be used to provide highly accurate spatial registration between an external 3D imaging reference frame and the LINAC frame. The experimental MV imager results, while not as precise as the simulated results, exceed 1 mm accuracy and the current accepted AAPM TG-142 standard of ≤2 mm positioning accuracy.

Quality assurance for image‐guided radiation therapy utilizing CT‐based technologies: A report of the AAPM TG‐179

Commercial CT-based image-guided radiotherapy (IGRT) systems allow widespread management of geometric variations in patient setup and internal organ motion. This document provides consensus recommendations for quality assurance protocols that ensure patient safety and patient treatment fidelity for such systems. The AAPM TG-179 reviews clinical implementation and quality assurance aspects for commercially available CT-based IGRT, each with their unique capabilities and underlying physics. The systems described are kilovolt and megavolt cone-beam CT, fan-beam MVCT, and CT-on-rails. A summary of the literature describing current clinical usage is also provided. This report proposes a generic quality assurance program for CT-based IGRT systems in an effort to provide a vendor-independent program for clinical users. Published data from long-term, repeated quality control tests form the basis of the proposed test frequencies and tolerances. A program for quality control of CT-based image-guidance systems has been produced, with focus on geometry, image quality, image dose, system operation, and safety. Agreement and clarification with respect to reports from the AAPM TG-101, TG-104, TG-142, and TG-148 has been addressed.

VMATに要求されるBackup JAWの位置精度と QA方法に関する検討

The tolerance of the Backup diaphragm (Backup JAW) setting in Elekta linac was specified as 2 mm according to the AAPM TG-142 report. However, the tolerance and the quality assurance procedure for volumetric modulated arc therapy (VMAT) was not provided. This paper describes positional accuracy and quality assurance procedure of the Backup JAWs required for VMAT. It was found that a gap-width error of the Backup JAW by a sliding window test needed to be less than 1.5 mm for prostate VMAT delivery. It was also confirmed that the gap-widths had been maintained with an error of 0.2 mm during the past one year.