Limitations of the point and line source approximations for the determination of geometry factors around brachytherapy sources.

Geometry factors were calculated around commercially available pellets and elongated brachytherapy sources taking into account their actual active core geometries. These calculations were compared with corresponding ones derived using the point and line source approximations commonly used for the determination of geometry factors, as proposed by AAPM Task Group 43. The point source approximation was found to be efficient for the determination of geometry factors around single active pellets, even at radial distances very close to the source. It is also valid for the determination of geometry factors at radial distances r>2L from elongated brachytherapy sources of length L. For smaller radial distances, however, this approximation introduces significant errors, >50%, around elongated source designs, thus being unacceptable for the determination of geometry factors in this case. The line source approximation was found to accurately reproduce geometry factors around elongated brachytherapy source designs. Errors greater than 3%, due to the fact that the line source approximation ignores the radial dimension d of the source, are observed only at radial distances very close to the source (r< or =L/2), at polar angles far away from their transverse bisectors. These errors depend on the ratio d/L and increase as this ratio increases.