Direct megavoltage photon calibration service in Australia

Abstract

The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) maintains the Australian primary standard of absorbed dose. Until recently, the standard was used to calibrate ionisation chambers only in 60Co gamma rays. These chambers are then used by radiotherapy clinics to determine linac output, using a correction factor (kQ) to take into account the different spectra of 60Co and the linac. Over the period 2010–2013, ARPANSA adapted the primary standard to work in megavoltage linac beams, and has developed a calibration service at three photon beams (6, 10 and 18 MV) from an Elekta Synergy linac. We describe the details of the new calibration service, the method validation and the use of the new calibration factors with the International Atomic Energy Agency’s TRS-398 dosimetry Code of Practice. The expected changes in absorbed dose measurements in the clinic when shifting from 60Co to the direct calibration are determined. For a Farmer chamber (model 2571), the measured chamber calibration coefficient is expected to be reduced by 0.4, 1.0 and 1.1 % respectively for these three beams when compared to the factor derived from 60Co. These results are in overall agreement with international absorbed dose standards and calculations by Muir and Rogers in 2010 of kQ factors using Monte Carlo techniques. The reasons for and against moving to the new service are discussed in the light of the requirements of clinical dosimetry.

Characterisation of a synthetic diamond detector for end-to-end dosimetry in stereotactic body radiotherapy and radiosurgery

“…The seven detectors were cross-calibrated using a normalised dose to water (N Dw ) approach [20] against a Farmer type secondary standard ionisation chamber (PTW 30013), traceable to the Australian Primary Standard [21][22][23]. The detectors were setup under reference conditions in water at 90 cm SSD, 10 cm depth, 10x10 cm 2 field.…”

Calibration of radiotherapy ionization chambers using Co‐60 is outdated and should be replaced by direct calibration in linear accelerator beams

“…To use a measured k Q only requires that the chamber response be reasonably insensitive to photon energy, so that users can interpolate between beam quality indicies. 6 Relative standard uncertainties for the experimental measurements of k Q factors for the most commonly used chambers have been reported as 0.3%. 7 Although similar relative standard uncertainties of <0.5% have been achieved recently for Monte Carlo calculated k Q values, 8 theoretical calculations are limited by the energydependent uncertainties of W/e and stopping power values.…”