Refinements to the geometry factor used in the AAPM Task Group Report No. 43 necessary for brachytherapy dosimetry calculations

Determination of the geometry factor is necessary for brachytherapy dosimet ry calculations as recommended by the AAPM Task Group No. 43 (TG-43). The e quivalence and errors associated with use of a point source approximation f or an extended line segment source are examined. For all angles, the error using the point source approximation is less than 2% for distances in which the ratio of radius to active source length, (r/L), exceed about 3.6. A no vel approach to determining the geometry factor using Monte Carlo methods i s discussed in which the particle flux emanates from the active source and streams with no interactions occurring within the source or phantom. This m ethod was performed for determining the geometry factor along the transvers e axis for six brachytherapy sources. Differences in the geometry factor ex ceeding 2% between the point source approximation and that obtained using M onte Carlo methods occurred at distances ranging from 0.5 to 5 mm from the source center along the transverse plane. The merits of the Monte Carlo app roach for solving the geometry factor are discussed in light of using a poi nt or line source approximation for calculating additional brachytherapy do simetry parameters. (C) 1999 American Association of Physicists in Medicine . [S0094-2405(99)02611-5].