XYZ Pointers for Oncology LINACs

One of the most important tasks in Medical Physics is ensuring the quality of treatments delivered to cancer patients undergoing radiotherapy. The main piece of treatment equipment we use for this is a medical electron linear accelerator or “linac”. The linac produces electrons that are traveling close to the speed of light and directs them towards a target that converts them into high energy photons. All of this requires that the beam producing and shaping elements in the linac are aligned with millimetre precision. A linac weighs several tons and rotates about a point in space that is called the “isocentre”. One of the tests we make in ensuring high quality treatment delivery is monitoring how well the linac aligns with its isocentre.

With this in mind we tasked the Bioenginnering workshop with creating a device we can mount on the linac to monitor the alignment with isocentre. The device had to be easy to align as a linac can rotate about two axes and we need to translate about the three orthogonal dimensions of space to determine isocentre.  Also, the device had to be able to display deviations from isocentre less than a millimetre.

This required the construction of a rigid, tiltable platform that mounts without slop onto the linac. A pole with a ruler inscribed for determining distance to isocentre is mounted on this platform with micrometer screw gauges controlling the translation. The challenge in making this was accuracy - each and every component of the device had to be made to extremely tight tolerances, provide very precise adjustment and yet be as rigid as possible.  By using nitride P20 steel, micrometre barrels celebrated to 0.01 mm, and some precise hand fitting we were able to achieve this. The end result allows us to determine isocentre to within 0.5 mm, which means that treatment delivery is as accurate as possible for our patients.

Page last reviewed: 06 October 2014