Characterization of a Co-60 unit at a secondary standard dosimetry laboratory: Monte Carlo simulations compared to measurements and results from the literature

• Purpose: To compare a Monte Carlo (MC) characterization of a Co-60 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 air K-air and absorbed dose to water D-w in 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 of Co-60 beams are of interest since Co-60 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. Methods: The BEAMnrc MC code was used with a detailed geometrical model of the treatment head and two models of the Co-60 source representing the sources used before and after source exchange, respectively. The active diameters of the Co-60 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. Results: 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 of K-air/D-w before 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 a Co-60 source with an active diameter of 2.0 cm inside a different treatment head. Conclusions: The reduction in the value of K-air/D-w following source exchange was explained by the spectral differences between the two sources that were larger in the free in-air geometry used for K-air calibrations than at 5 g/cm(2) depth in the water phantom used for D-w calibrations. Literature review revealed differences between published in-air Co-60 spectra derived for sources of different active diameters, and investigators in need of an accurately determined Co-60 in-air spectrum should be aware of differences due to source active diameter.

Nyckelord

Co-60; radiation quality; spectra; experimentally verified Monte Carlo

MEDICINE

MEDICIN

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