3.2 - Regular Quality Assurance Procedures

Quality assurance is a program designed to ensure radiotherapy devices are functioning as specified. This is vital to the accurate and safe delivery of radiation. Quality assurance checks are performed regularly, with different time intervals depending on:

  • The importance of the check
  • The time involved in performing the check
  • The likelihood that recorded values will vary

QA checks are arranged into daily, monthly and yearly frequencies. Dosimetric checks are typically performed by the medical physics team; mechanical checks may be performed by the biomedical engineering team instead.

Daily checks

Before the linear accelerator is used in the morning, the following checks must be completed:

  • Measurement of x-ray and electron output constancy for each beam (within 3% of recorded values)
  • Measurement of laser beam concordance with isocentre (within 2 mm of recorded values)
  • Accuracy of optical distance indicator (within 2 mm)
  • Functionality of door interlock and AV equipment

Monthly Checks

Some centres perform fortnightly checks; this allows QA to take a shorter time each session.

  • Measurement of x-ray and electron output constancy for each beam (within 2% of recorded values)
  • Central axis depth dose measurements (photons/electrons)
  • Beam flatness and symmetry
  • Functionality of safety switches and tray interlocks
  • Light / radiation field coincidence
  • Gantry rotation accuracy
  • Tray position
  • Jaw symmetry

Yearly Checks

Yearly checks take several days to perform:

  • As for monthly checks, and
  • Measurement of output versus gantry/collimator angle
  • Mechanical isocentre
  • Radiation isocentre
  • Coincidence of isocentres
  • Table top sag