Dosimetric properties of the new 125I BrachySeed model LS-1 source

The BrachySeed model LS-1 is one of the latest in a series of new brachytherapy 125I seeds that have recently become available commercially for interstitial implants. The dosimetric properties of the seed were investigated analytically, experimentally, and by Monte Carlo simulation. Following the AAPM Task Group 43 formalism, the radial dose function, dose rate constant, and anisotropy parameters were determined. Experimental measurements were made in solid water-equivalent phantoms using GafChromic MD-55-2 films, with correction for the low energy film response. Analyses were carried out from absolute measurements, as well as relative measurements against the Nycomed Amersham OncoSeed Model 6711, which also served to validate our experimental methodology. A small, but systematic difference in the absolute measurements was observed depending on the duration of the irradiation. Monte Carlo simulation was performed using the Integrated Tiger Series CYLTRAN code. We benchmarked the code by comparing the dose parameters of Model 6702 with published values. The radial dose function, g(r), of the Model LS-1 seed was computed at distances from 0.25 to 10 cm by analytical and Monte Carlo calculations with reasonably good agreement. The suggested dose rate constant, A, based on the Monte Carlo simulation is 0.90+/-0.03 cGy h(-1) U(-1). This value is smaller than, but overlaps the experimental determination of 0.98+/-0.06 cGy h(-1) U(-1). The anisotropy function, F(r, theta), and anisotropy factor, phi(an)(r), compared favorably with those of the Model 6711.