Comparison of two methods of therapy level calibration at⁶⁰Co gamma beams

Abstract

The accuracy and traceability of the calibration of radiotherapy dosimeters is of great concern to those involved in the delivery of radiotherapy. It has been proposed that calibration should be carried out directly in terms of absorbed dose to water, instead of using the conventional and widely applied quantity of air kerma. In this study, the faithfulness in disseminating standards of both air kerma and absorbed dose to water were evaluated, through comparison of both types of calibration for three types of commonly used radiotherapy dosimeters at 60Co gamma beams at a few secondary and primary standard dosimetry laboratories (SSDLs and PSDLs). A supplementary aim was to demonstrate the impact which the change in the method of calibration would have on clinical dose measurements at the reference point. Within the estimated uncertainties, both the air kerma and absorbed dose to water calibration factors obtained at different laboratories were regarded as consistent. As might be expected, between the SSDLs traceable to the same PSDL the observed differences were smaller (less than 0.5%) than between PSDLs or SSDLs traceable to different PSDLs (up to 1.5%). This can mainly be attributed to the reported differences between the primary standards. The calibration factors obtained by the two methods differed by up to about 1.5% depending on the primary standards involved and on the parameters of calculation used for 60Co gamma radiation. It is concluded that this discrepancy should be settled before the new method of calibration at 60Co gamma beams in terms of absorbed dose to water is taken into routine use.

A comparison between calculated and experimentalkQphoton beam quality correction factors

“…Previous direct or indirect comparisons at the BIPM (cf Boutillon et al 1994, Boutillon andAndreo 1997) had shown differences of up to 1% between some laboratories, even for those of a given geographical region where a common dosimetry protocol is used. The differences between chamber calibrations provided by some laboratories, reported by Bjerke et al (1998), may be of relevance in the present context only for the few k Q values measured by clinical users (Vatnitsky et al 1995, Palm 2000. Many experimental k Q values have been obtained using Fricke dosimetry.…”

Absorbed dose beam quality correction factors for the NE2571 chamber in a 5 MV and a 10 MV photon beam

“…Although the air kerma calibration network is intrinsically more sensitive to systematic errors (see for example Bielajew and Rogers 1992) as in all standard dosimetry laboratories it is determined using graphite walled ionization chambers, it has not yet been proven that an absorbed dose calibration network can reach the same level of consistency as the air kerma calibration network. A recent experimental study by Bjerke et al (1998) showed that direct calibration in terms of absorbed dose to water using calculated k Q values could reduce the total uncertainty on absorbed dose determination in the user beam by 1-1.5%. However, they found that, depending on the standard laboratory, discrepancies of up to 1.5% could show up due to the change from air kerma based dosimetry to absorbed dose based dosimetry.…”

Comparação entre fatores de calibração em termos de dose absorvida no ar para uma câmara de ionização de placas paralelas

“…Results show that the N D,ar values for the parallel plate chambers determined in 60 Co beams are 1.2% higher than the value obtained in high energy electron beams. This difference in measurements series may be related, like in some published studies (12,13,17) , but this hypothesis is promptly discarded, since, in the charge measurement, the maximum uncertainty between measurements is ±0.15% for each voltage. So, this discrepancy is assigned to parallel plate chambers walls attenuation correction factors supplied by the protocol, which should not be coherent when calibration in photon beams is performed.…”