Characterization of a60Co unit at a secondary standard dosimetry laboratory: Monte Carlo simulations compared to measurements and results from the literature.
To compare a Monte Carlo (MC) characterization of a60Co unit at the Swedish Secondary Standard Dosimetry Laboratory (SSDL) with the results of both measurements and literature with the aims of (1) resolving a change in the ratio of air-kerma free in airKairand absorbed dose to waterDwin a water phantom noted experimentally after a source exchange in the laboratory and (2) reviewing results from the literature on similar MC simulations. Although their use in radiotherapy is decreasing, the characteristics of60Co beams are of interest since60Co beams are utilized in calibrating ionization chambers for the absolute dosimetry of radiotherapy beams and as reference radiation quality in evaluating the energy dependence of radiation detectors and in studies on radiobiological effectiveness.



The BEAMnrc MC code was used with a detailed geometrical model of the treatment head and two models of the60Co source representing the sources used before and after source exchange, respectively. The active diameters of the60Co sources were 1.5 cm in pellet form and 2.0 cm in sintered form. Measurements were performed on the actual unit at the Swedish SSDL.



Agreement was obtained between the MC and the measured results within the estimated uncertainties for beam profiles, water depth-dose curve, relative air-kerma output factors, and for the ratios ofKair/Dwbefore and after source exchange. The on-axis energy distribution of the photon fluence free in air for the unit loaded with its present (1.5 cm in diameter) source agreed closely with the results from the literature in which a source of the same make and active diameter, inside a different treatment head, was simulated. The spectrum for the larger (2.0 cm in diameter) source was in close agreement with another published spectrum, also modeling a60Co source with an active diameter of 2.0 cm inside a different treatment head.



The reduction in the value ofKair/Dwfollowing source exchange was explained by the spectral differences between the two sources that were larger in the free in-air geometry used forKaircalibrations than at 5 g/cm2depth in the water phantom used forDwcalibrations. Literature review revealed differences between published in-air60Co spectra derived for sources of different active diameters, and investigators in need of an accurately determined60Co in-air spectrum should be aware of differences due to source active diameter.
Tedgren
Åsa Carlsson
ÅC
Department of Medical and Health Sciences (IMH), Radiation Physics, Faculty of Health Sciences, Linköping University, SE-581 85 Linköping, Sweden and Swedish Radiation Safety Authority, SE-171 16 Stockholm, Sweden.
de Luelmo
Sandro
S
Swedish Radiation Safety Authority, SE-171 16 Stockholm, Sweden.
Grindborg
Jan-Erik
JE
Swedish Radiation Safety Authority, SE-171 16 Stockholm, Sweden.
D003037:Cobalt Radioisotopes / Q000032:analysis* / Q000592:standards*; D038622:Internationality; D009010:Monte Carlo Method; D011874:Radiometry / Q000592:standards*; D012015:Reference Standards; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
D003037:Cobalt Radioisotopes