Investigation of X-ray focal spot alignment using a jig of novel design

  • Lockie YuenEmail author
  • Cameron McLucas
Technical Paper


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.


Focal spot Quality assurance Alignment Commissioning 



The authors would like to acknowledge Mr Steve Cripps from Varian Medical Systems Australasia for his valuable time and contribution to this investigation.

Compliance with ethical standards

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.


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Copyright information

© Australasian College of Physical Scientists and Engineers in Medicine 2017

Authors and Affiliations

  1. 1.W.P. Holman ClinicLaunceston General HospitalLauncestonAustralia

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