Level Ib audits

The Level Ib audit offers a more accurate measurement of the Linac output than the Level I audit. ACDS staff visit the facility and perform on-site measurements using ACDS dosimetry equipment. The Level Ib is performed for new Linac installations, or at the facility’s request


The Level Ib audit is a more accurate independent measurement than the Level I audit. The Level Ib audit determines the absorbed dose to water per monitor unit for mega-voltage photon and electron beams under reference conditions. The ACDS visits the requesting facility and performs on-site measurements in the facility’s own water tank. All other dosimetry equipment is brought in by the ACDS, ensuring an independent audit.

The Level Ib audit consists of the following modalities:

  • Reference beams (photons and electrons)
  • FFF beams
  • Small field beams (field trial).

Audit coverage

The Level Ib audit is not part of the scheduled program and is performed at the request of the facility. The Level Ib is typically conducted for new facilities or LINACs, to assist geographically isolated facilities, or as a part of the follow up process where other audits do not fall inside the Optimal Level.

Audit outcome

The Audit results are determined by the percentage deviation of the Facility Stated Dose Output from the ACDS Determined Dose Output, for each clinical beam. All beams on the linac are defined as ‘clinical’ unless otherwise stated by the Facility. Current Result Levels are given in the following table.

Audit result levels
Level % Deviation (Client Stated/ACDS Determined)
Photons Electrons
Optimal ≤1.4 ≤2.2
Action > 1.4 and ≤ 2.1 > 2.2 and ≤ 3.3
Outside Tolerance > 2.1 > 3.3

An overall Audit Outcome for the linac is determined, which is equal to the lowest result recorded for an individual ‘clinical’ beam.

ACDS audit level Ib documentation