Comparison between continuous and discrete sources in the Paris system of implants

Since the Paris system of interstitial brachytherapy is based on iridium wires, the consequences resulting from the substitution of discrete point sources in afterloading ribbons have been analyzed. The results are presented in this article for single ribbons, single plane implants of thicknesses 0.5 and 1.0 cm, and biplanar implants of both equal and unequal areas of sizes most frequently used clinically in the thickness range 1.5-3.0 cm. These results indicate changes in the quality of implants using wires and point sources up to the maximum spacing of 1.5 cm. The quality of the implants is judged on the basis of a parameter, the uniformity index, which takes into account both the size of the treatment volume for a given target volume and the homogeneity within the treatment volume as determined from the integral dose. Other implant parameters such as total activity, treatment volume, total volume receiving greater than twice the stated dose rate, average dose rate, and variation of basal dose rate, etc., are also provided to indicate quality of the implants. The results with the customary seed spacing of 1 cm are compatible with those using wires and are found to have improved quality in many cases.