11.10.4 - Accuracy And Tolerance

Accuracy and Tolerance Guidelines

Linear accelerator function, patient setup and dose distributions are all subject to errors, as are the tools used to measure these factors. These errors must be taken into account when developing treatment plans, and often account for the external margin as described by the ICRU.
Given that errors are known to exist and can not be controlled easily in a complex machine or living patient, it is important to have tolerances which allow treatment to continue if errors are small and unlikely to lead to adverse outcomes. This is known as the tolerance of a measurement. Tolerances may vary between different treatments - for example, a treatment with stereotactic device has a much smaller tolerance on patient position than a palliative spine treatment.

Tolerance of mechanical and radiation parameters

A figure of 2 mm or 5% is typically used for tolerance in mechanical measurements. This goal is sometimes difficult but not impossible to achieve, and balances resources with outcomes. A 2 mm shift in the field is unlikely to have a major impact on a patients outcome, particularly given other uncertainties in treatment.

Special tolerances

Wedge angles are allowed to differ by 2 degrees from their expected value. The actual changes in dose distribution caused by the wedge are used (rather than assuming the wedge is its published value).
** Gantry, **collimator and couch angles should all be within 1 degree of expected.
Photon and electron dose distributions should differ by no more than 2% from their published values. Field symmetry should also be within 2% (comparing dose on one side of the central axis to the other side). Beam flatness should differ by no more than 90% and 103% of the central axis dose.
Gantry rotation should revolve around a 1 mm isocentre. 2 mm is typically used for other mechanical measurements.