IJMPCERO  Vol.7 No.1 , February 2018
A Quality Assurance Approach for Linear Accelerator Mechanical Isocenters with Portal Images
Abstract: Purpose: With usually a millimeter-level PTV margin, stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) pose a stringent requirement on the isocentricity of the Linac. This requirement is partly fulfilled by routine isocenter quality assurance (QA) test to verify the size and location of the isocenter. The current common QA methods such as spoke shot were developed before SBRT/SRS became popular and when IGRT was largely absent and hence have their limitations. In this work, we describe an isocenter QA approach based on portal imaging to provide the community with a superior alternative. Methods: The proposed approach utilizes a BrainLab ball bearing (BB) phantom in conjunction with an electronic portal imaging devices (EPID) imager. The BB phantom was first aligned with a calibrated room laser system. Portal images were then acquired using 6 MV beam with a 2 × 2 cm2 open field and a 15 mm cone on a Varian TrueBeam STx machine. The gantry, collimator, and table were rotated separately at selected angles to acquire a series of portal images in order to determine the isocenter of each rotating system. The location and diameter of these isocenters were determined by calculating the relative displacement of either BB or open field edge between the acquired EPID images. The demonstration of the reproducibility and robustness of this EPID-based approach was carried out by repeating measurements 10 times independently for each rotating system and simulating clinical scenarios of asymmetric jaws and misalignment of BB phantom, respectively. Results: For our TrueBeam STx machine, the isocenter diameter derived from open-field EPID images was roughly 0.15 mm, 0.18 mm, 0.49 mm for the collimator, table, and gantry, respectively. For the collimator and gantry, images taken with the cone gave considerably smaller isocenter diameter. Results remained almost unchanged despite the presence of simulated BB misalignment and asymmetric jaws error, and between independent measurements. Isocenter location and diameter derived from images obtained at a limited number of angles (≤11) were adequately accurate to represent those derived from images of densely sampled angles. Conclusions: An EPID-based isocenter QA approach is described and demonstrated to be accurate, robust, and reproducible. This approach provides a superior alternative to conventional isocenter QA methods with no additional cost. It can be implemented with convenience for any linear accelerator with an EPID imager.
Cite this paper: Fan, Q. , Zhou, S. , Lei, Y. , Li, S. and Zhang, M. (2018) A Quality Assurance Approach for Linear Accelerator Mechanical Isocenters with Portal Images. International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 7, 100-114. doi: 10.4236/ijmpcero.2018.71009.

[1]   Timmerman, R., Paulus, R., Galvin, J., et al. (2010) Stereotactic Body Radiation Therapy for Inoperable Early Stage Lung Cancer. JAMA, 303, 1070-1076.

[2]   Andrews, D.W., Scott, C.B., Sperduto, P.W., et al. (2004) Whole Brain Radiation Therapy with or without Stereotactic Radiosurgery Boost for Patients with One to Three Brain Metastases: Phase III Results of the RTOG 9508 Randomised Trial. The Lancet, 363, 1665-1672.

[3]   Zelefsky, M.J., Kollmeier, M., Cox, B., et al. (2012) Improved Clinical Outcomes with High-Dose Image Guided Radiotherapy Compared with Non-IGRT for the Treatment of Clinically Localized Prostate Cancer. International Journal of Radiation Oncology* Biology* Physics, 84, 125-129.

[4]   Rowshanfarzad, P., Sabet, M., O’Connor, D.J. and Greer, P.B. (2011) Isocenter Verification for Linac-Based Stereotactic Radiation Therapy: Review of Principles and Techniques. Journal of Applied Clinical Medical Physics, 12, 185-195.

[5]   Gonzalez, A., Castro, I. and Martinez, J. (2004) A Procedure to Determine the Radiation Isocenter Size in a Linear Accelerator. Medical Physics, 31, 1489-1493.

[6]   Klein, E.E., Hanley, J., Bayouth, J., et al. (2009) Task Group 142 Report: Quality Assurance of Medical Accelerators. Medical Physics, 36, 4197-4212.

[7]   Kutcher, G.J., Coia, L., Gillin, M., et al. (1994) Comprehensive QA for Radiation Oncology: Report of AAPM Radiation Therapy Committee Task Group 40. Medical Physics, 21, 581-618.

[8]   Woo, M.K. (2002) A Personal-Computer-Based Method to Obtain “Star-Shots” of Mechanical and Optical Isocenters for Gantry Rotation of Linear Accelerators. Medical Physics, 29, 2753-2755.

[9]   Depuydt, T., Penne, R., Verellen, D., et al. (2012) Computer-Aided Analysis of Star Shot Films for High-Accuracy Radiation Therapy Treatment Units. Physics in Medicine & Biology, 57, 2997-3011.

[10]   Lutz, W.R., Larsen, R.D. and Bjarngard, B.E. (1981) Beam Alignment Tests for Therapy Accelerators. International Journal of Radiation Oncology* Biology* Physics, 7, 1727-1731.

[11]   Lutz, W., Winston, K.R. and Maleki, N. (1988) A System for Stereotactic Radiosurgery with a Linear Accelerator. International Journal of Radiation Oncology* Biology* Physics, 14, 373-381.

[12]   Denton, T.R., Shields, L.B., Howe, J.N. and Spalding, A.C. (2015) Quantifying Isocenter Measurements to Establish Clinically Meaningful Thresholds. Journal of Applied Clinical Medical Physics, 16, 175-188.

[13]   Imae, T., Haga, A., Saotome, N., et al. (2014) Winston-Lutz Test and Acquisition of Flexmap Using Rotational Irradiation. Japanese Journal of Radiological Technology, 70, 359-368.

[14]   Ravindran, P.B. (2016) A Study of Winston-Lutz Test on Two Different Electronic Portal Imaging Devices and with Low Energy Imaging. Australasian Physical and Engineering Science in Medicine, 39, 677-685.

[15]   Zhang, M., Driewer, J., Zhang, Y., Zhou, S. and Zhu, X. (2015) The Measurement Accuracy of Ball Bearing Center in Portal Images Using an Intensity-Weighted Centroid Method. International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 4, 273-283.

[16]   Zhang, M., Zhou, S.-M. and Qu, T. (2015) What Do We Mean When We Talk about the Linac Isocenter? International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 4, 233-242.

[17]   Sharpe, M.B., Moseley, D.J., Purdie, T.G., Islam, M., Siewerdsen, J.H. and Jaffray, D.A. (2006) The Stability of Mechanical Calibration for a kV Cone Beam Computed Tomography System Integrated with Linear Accelerator. Medical Physics, 33, 136-144.

[18]   Welzl, E. (1991) Smallest Enclosing Disks (Balls and Ellipsoids). New Results and New Trends in Computer Science, Graz, 20-21 June 1991, 359-370.

[19]   Moyers, M.F. and Lesyna, W. (2004) Isocenter Characteristics of an External Ring Proton Gantry. International Journal of Radiation Oncology Biology Physics, 60, 1622-1630.

[20]   Schiefer, H., Ingulfsen, N., Kluckert, J., Peters, S. and Plasswilm, L. (2015) Measurements of Isocenter Path Characteristics of the Gantry Rotation Axis with a Smartphone Application. Medical Physics, 42, 1184-1192.

[21]   Glide-Hurst, C., Bellon, M., Foster, R., et al. (2013) Commissioning of the Varian True Beam Linear Accelerator: A Multi-Institutional Study. Medical Physics, 40, Article ID: 031719.

[22]   Liu, G., van Doorn, T. and Bezak, E. (2004) The Linear Accelerator Mechanical and Radiation Isocentre Assessment with an Electronic Portal Imaging Device (EPID). Australasian Physics & Engineering Sciences in Medicine, 27, 111-117.

[23]   Hyer, D.E., Mart, C.J. and Nixon, E. (2012) Development and Implementation of an EPID-Based Method for Localizing Isocenter. Journal of Applied Clinical Medical Physics, 13, 72-81.

[24]   Nyflot, M.J., Cao, N., Meyer, J. and Ford, E.C. (2014) Improved Accuracy for Noncoplanar Radiotherapy: An EPID-Based Method for Submillimeter Alignment of Linear Accelerator Table Rotation with MV Isocenter. Journal of Applied Clinical Medical Physics, 15, 151-159.