An evaluation of the AAPM-TG43 dosimetry protocol for I-125 brachytherapy seed.

The EGSnrc Monte Carlo system has been used to calculate the dose distributions from 125I radioactive seeds (model 6711). The results showed that the agreement between EGSnrc and the American Association of Physicists in Medicine Task Group Report 43 (AAPM-TG43) dosimetry protocol is generally within +/-15% for radial distances less than 1.0 cm in both the transverse axis and longitudinal axis of the source. For radial distances between 1.0 and 2.5 cm the agreement between Monte Carlo simulations and the AAPM-TG43 dosimetry protocol is within +/-20%. In the longitudinal axis of the source the difference between Monte Carlo simulations and the AAPM-TG43 dosimetry is up to 40% for radial distances greater than 2.5 cm. The agreement between the EGSnrc/Monte Carlo simulation and the AAPM-TG43 dosimetry protocol improved significantly when recently published data of the anisotropic function were implemented (Weaver 1998 Med. Phys. 25 2271-8). The difference between Monte Carlo simulations and the AAPM-TG43 dosimetry protocol is not more than +/-10% in the transverse axis of the source up to a radius of 2.5 cm. The EGSnrc Monte Carlo simulation and the AAPM-TG43 with the Weaver anisotropic data were also used to investigate the differences in the dose distribution caused by small differences in the construction of individual seeds (Sloboda and Menon 2000 Med. Phys. 27 1789-99). The results show that a change in length of the silver rod containing the 125I radioactive material of 0.14 mm does not affect the dose distribution significantly in the transverse and longitudinal axes but a change of 0.13 mm in the thickness of the welded end of the encapsulation affected the dose significantly in the longitudinal axis of the source.