Abstract
The X-ray focal spot and the centre of the flattening filter should be placed as close as possible to the central axis (CAX) on a linear accelerator (linac) to produce a radially symmetric beam. The aim of this study is to devise a method to easily optimise the focal spot position of Varian linac photon beams generated with a flattening filter. A simple and robust jig was designed and built to be inserted into the largest electron applicator. Accessory and jaw position interlocks were overridden to enable photon beam operation. The jig was made from aluminium and consists of a plate permanently fixed inside a low melting point alloy (LMPA) insert frame and a block machined to suspend an ion chamber below the plate, such that the axis of the chamber is at the linac isocentre. The jig was used to optimise the position of the X-ray beam focal spot with respect to the device central axis (CAX). This was achieved by minimising the percentage change in ionisation chamber signal between collimator rotations from 90° to 270° as position steering was changed. As part of the investigation, in-plane (radial) and cross-plane (transverse) profiles obtained from water phantom scans were used to quantify how large the percentage ionisation change can be before profiles are distorted in the region of the CAX.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Podgorsak EB (2005) Radiation oncology physics: a handbook for teachers and students. IAEA, Vienna
Greene D, Williams PC (1997) Linear accelerators for radiation therapy. 2nd edn. Institute of Physics Pub, Bristol
Kirkness B (2006) C-series Clinac® accelerator system basics. Revision AG. Varian Medical Systems, Palo Alto
ICRU (1976) ICRU Report 24. Determination of Absorbed dose in a Patient Irradiated by beams of X or Gamma Rays in Radiotherapy Procedures International Commission on Radiation Units and Measurements, Washington, D.C
Van Dyk J (2013) The modern technology of radiation oncology. A compendium for medical physicists and radiation oncologists. Volume 3 Medical Physics Publishing, Madison
Thwaites D (2013) Accuracy required and achievable in radiotherapy dosimetry: have modern technology and techniques changed our views? J Phys 444:012006. doi:10.1088/1742-6596/444/1/012006
International Electrotechnical Commission (2014) IEC 60601-2-1:2009+AMD1:2014 Medical electrical equipment- Part 2-1: Particular requirements for the basic safety and essential performance of electron accelerators in the range 1 MeV to 50 MeV
Gao S, Balter P, Rose M, Simon W (2013) Measurement of changes in linear accelerator photon energy through flatness variation using an ion chamber array. Med Phys 40:042101. doi:10.1118/1.4791641
Goodall S, Harding N, Simpson J, Alexander L, Morgan S (2015) Clinical implementation of photon beam flatness measurements to verify beam quality. J Appl Clin Med Phys 16(6):5752
Nyiri BJ, Smale JR, Gerig LH (2012) Two self-referencing methods for the measurement of beam spot position. Med Phys 39(12):7635–7643. doi:10.1118/1.4766270
International Electrotechnical Commission (2011) IEC 61217:2011 Radiotherapy equipment- Coordinates, movements and scales
Klein E, Hanley J, Bayouth J, Yin F, Simon W, Dresser S, Serago C, Aguirre F, Ma L, Arjomandy B, Liu C, Sandin C, Holmes T (2009) Task group 142 report: quality assurance of medical acceleratorsa). Med Phys 36:4197–4212. doi:10.1118/1.3190392
Rohrig N (2006) Structural shielding design and evaluation for megavoltage x- and gamma-ray radiotherapy facilities, NCRP Report No. 151. Health Phys 91:270. doi:10.1097/01.hp.0000205229.89321.ce
Jaffray D, Battista J, Fenster A, Munro P (1993) X-ray sources of medical linear accelerators: Focal and extra-focal radiation. Med Phys 20:1417–1427. doi:10.1118/1.597106
Caprile P, Hartmann G (2009) Development and validation of a beam model applicable to small fields. Phys Med Biol 54:3257–3268. doi:10.1088/0031-9155/54/10/020
International Organization for Standardization (1995), ISO/ IEC Guide 98–3: 2008 Uncertainty of measurement—Part 3: guide to the expression of uncertainty in measurement (GUM), ISO, Geneva
Das I, Cheng C, Watts R, Ahnesjö A, Gibbons J, Li X, Lowenstein J, Mitra R, Simon W, Zhu T (2008) Accelerator beam data commissioning equipment and procedures: report of the TG-106 of the therapy physics committee of the AAPM. Med Phys 35:4186–4215. doi:10.1118/1.2969070
Wang LLW, Leszczynski K (2007) Estimation of the focal spot size and shape for a medical linear accelerator by Monte Carlo simulation. Med Phys 34:485. doi:10.1118/1.2426407
Acknowledgements
The authors would like to acknowledge Mr Steve Cripps from Varian Medical Systems Australasia for his valuable time and contribution to this investigation.
Author information
Authors and Affiliations
Corresponding author
Ethics declarations
Conflicts of interest
The authors declare that they have no conflict of interest.
Ethical approval
This article does not contain any studies with human participants or animals performed by any of the authors.
Rights and permissions
About this article
Cite this article
Yuen, L., McLucas, C. Investigation of X-ray focal spot alignment using a jig of novel design. Australas Phys Eng Sci Med 40, 455–461 (2017). https://doi.org/10.1007/s13246-017-0549-z
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s13246-017-0549-z