11.10.6 - Types Of Errors

Random errors occur due to unpredictable variations in measurements. Measurements may fluctuate around a mean value. A precise instrument minimises random errors but may be subject to systematic error.
Systematic errors are when the recorded measurement is constantly inconsistent with the actual situation. They occur when measurement devices are calibrated incorrectly, or when environmental factors are not taken into account.
In radiotherapy, random errors occur due to random variations in patient movement, production of x-rays from the linear accelerator, calculation of monitor units and so on. They can be reduced by using more precise set up procedures or equipment.
Systematic errors occur due to a wide variety of issues, including:

  • Human error - Incorrect data entry, incorrect learning of procedures
  • Computer error - 'Bugs' in computer software, or poor dose calculations around inhomogeneity can lead to systematic errors in dose.
  • Measurement error - Incorrect calibration of measurement tools (dosimeters, rulers)
  • Organ motion errors - Most treatments are planned with 3D CT scans, which do not take into account organ motion. This can be corrected for by the use of an appropriate internal margin
  • Setup error - Skin marks are used to position patients, but the skin is not in a fixed position relative to deep tissues. If skin marks (or other immobilisation techniques) are performed incorrectly to begin with, a systematic error may be introduced.