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
- Practical Reference Dosimetry Course - 11-14 April 2016
- The ARPANSA Small-Field Comparison Day - 15 April 2016
For more information contact: psdlarpansa.gov.au