The guide gutter or loop techniques of interstitial implantation and the Paris system of dosimetry

The predictive dosimetry system for implants known as the Paris system can be used with either loops or hairpins. When using the guide gutter technique, implant geometry is predetermined by the inherent spacing and parallelism of the branches of the hairpins. When using loops, their branches should not be spaced too widely apart and should be parallel over an adequate distance to obtain a fairly regular dose distribution between them. The basic principles of implantation are the same as for rectilinear sources. Branches must be rectilinear, parallel, arranged so that their centers are located in the same plane (central plane). Adjacent branches must be equidistant from each other and the reference linear kerma rate (or the linear activity) must be uniform and identical for all sources. When these conditions are met, the dimensions of the treated volume (volume encompassed by the reference isodose surface with a value equal to 85% of the basal dose rate) can be estimated at the time of the implantation procedure. In practice, only a few relationships presented in this paper, with examples of application, must be known. Although, the Paris system permits forecasting the final dosimetry, the geometry of the implant must be verified and the dose calculated according to the implantation as actually achieved. The best method of checking the exact position of radioactive sources in an implant and determining the dose rate at any desired point is a reconstruction by computer program although alternative methods are occasionally appropriate.(ABSTRACT TRUNCATED AT 250 WORDS)