We recommend that radiotherapy providers purchase chambers which are already listed in the relevant dosimetry protocol. International recommendations regarding the behaviour of chambers suitable for reference dosimetry should also be consulted (see reference 1 below).
The most common chamber types currently in use in Australia include the 2571 Farmer chamber, the IBA FC65‑G and the PTW 30013 [2]. Similar designs are offered by several other manufacturers.
Please note that the older ground-referenced dosimetry systems can no longer be calibrated by ARPANSA (namely the 2561 or 2611A chambers with flying leads). Only connection systems of type BNT (bayonet triaxial connector) or TNC (threaded triaxial connector) (Figure 1) can be calibrated unless adaptors are supplied. Chambers older than 10 years should not be submitted for calibration unless the user has a continuous set of data establishing that the performance of the chamber is still acceptable for reference dosimetry.
Figure 1: Triaxial adaptors for TNC (screw type, left top) to BNT (left bottom) and BNT (bayonet type, right) to TNC (right bottom).
The calibration services provided by ARPANSA are intended for conventional reference dosimetry in large fields (i.e. 10 cm x 10 cm). Strictly speaking, the calibration coefficient is only valid at the field size and distance used for the calibration. The uncertainty in a dose measurement increases as the field size departs from the calibration field size. We note that in particular, calibration coefficients determined in 10 cm x 10 cm fields cannot be used for very small fields. As a rule of thumb, Farmer-type chambers cannot be used below about 4 cm x 4 cm.
References
[1] McEwen M R, Measurement of ionization chamber absorbed dose kQ factors in megavoltage photon beams Med. Phys. 37 2179-93, 2010
[2] R L Brown and D J Butler, Results of the 2009 dosimetry survey of Australian radiotherapy centres, Australas. Phys. Eng. Sci. Med., 33 285-297, 2010
