Radiotherapy calibrations

ARPANSA provides calibration services for radiotherapy dosemeters. These dosemeters are used by radiotherapy providers to calibrate the output of linear accelerators and kilovoltage X-ray tubes for patient treatment.

Most of these services are accredited to ISO 17025 by NATA. ARPANSA’s accreditation number is 14442 and the current scope of services can be viewed at www.nata.com.au.

There are several beam qualities available for calibration. The most common calibration is for absorbed dose to water in Cobalt-60. This calibration may be used with a dosimetry protocol such as the IAEA TRS-398 Code of Practice to determine the output of a medical linear accelerator. From July 2014 radiotherapy providers can choose to get their ionisation chambers calibrated at ARPANSA directly in linear accelerator beams. The new services result in more accurate dosimetry in the clinic because the chamber is calibrated at similar energies, reducing the emphasis on correction factors.

The new services are fully compatible with the IAEA’s TRS-398 dosimetry code of practice and are intended to complement the existing Cobalt-60 calibration service.

Table 1: Common calibration combinations of a therapy dosemeter (consisting of an electrometer and one or more ionisation chambers).

Device Beam quality Nominal energy Quantity Calibration coefficient
Electrometera - - Charge or current kelec
Thimble chamber Cobalt-60 Cobalt-60 Absorbed dose to waterb ND,w
Thimble chamber Cobalt-60 Cobalt-60 Air kerma NK
Thimble chamber Cobalt-60 + MV photons 1 – 18 MV Absorbed dose to water ND,w,Q
Electron chamber MV electrons 18 MeVc Absorbed dose to water ND,w,Q
Thimble chamber MEXd 30 – 300 kVp Air kerma NK
Thin window chamber LEXe 20 – 100 kVp Air kerma NK

aElectrometers are calibrated separately from ionisation chambers. A comparison of the product of the separate calibration coefficients and the combined calibration coefficient is however performed in Cobalt-60.
bCalibrations in Cobalt-60 include a calibration for air kerma, which is always performed for QA purposes.
cThe cross-calibration is performed at 18 MeV and calculated correction factors from TRS-398 can be used to determine the calibration coefficient at other qualities.
dMEX = Medium Energy kilovoltage X-rays
eLEX = Low Energy kilovoltage X-rays

Past events

For more information contact: radiotherapyatarpansa.gov.au