11.10.8 - Errors Due To Computer Control

This topic is mainly on errors due to computer control and setup of patient setup and treatment delivery.
Computer errors in other aspects of planning can occur but they are not covered in this particular topic.

Computers are just machines, and may be unable to recongise errors that would seem obvious to a human planner. They may also be fooled by errors that would *not* be obvious as well!
Errors related to computer control include:

  • Inability to recognise incorrect data: If the computer is provided with incorrect data by a human operator it may not recognise this, and continue to plan the radiation dose regardless. This is a systematic error, which could be wide ranging if the errors are in the absolute dosimetry of the linear accelerator.
    • Human staff may enter data that is incorrect but suits the computer's requirements, eg. recording the SSD as within tolerance when the patient is not in the correct position at all. This may occur due to inadequate measuring equipment, inadequate training of staff or other factors (such as pressure to quickly treat patients in a busy centre).
    • These problems can be avoided by ensuring that accurate measurements are taken at all time, by not overloading staff with work and by having two staff to check all positioning data.
  • Inability to recognise patient or positioning: The computer relies on staff selecting the correct patient for a treatment. It is not able to detect whether Mr Joe Blogs is receiving Mrs Jane Doe's cervical cancer treatment!
    • Strategies to overcome this could include additional verification of the patient (such as having photos of the patient in their treatment documentation or on the computer screen), or alternatively the patient could be provided with an ID card that provides an additional 'interlock' - if the incorrect card is inserted then the treatment can not proceed!
  • Software errors: A computer could have a software 'bug'. This could lead to an incorrect calculation, incorrect position of beams, incorrect monitor unit delivery or (in severe cases) death or maiming of a patient / staff. Computers may display the correct positions but the machine may be set up incorrectly. See this article on the Therac incidents in the 1980s.
    • This can be potentially avoided by additional interlocks. However the computer has to recognise that these interlocks are malfunctioning. For example, in the Therac incident there should have been an interlock which prevented the beam from activating unless the flattening filter was in place.