Dosimetry rules for brachytherapy using high dose rate remote afterloading implants.

The increasing availability of high dose rate (HDR) afterloading units has highlighted the potential use of this form of brachytherapy in an ever wider number of clinical sites. Alongside this, commercial planning systems for brachytherapy offer almost infinite possibilities for the differential loading of applicators in an attempt to produce an ideal dose distribution. Dosimetry in the UK has been based largely on the Manchester and Paris systems; modifications and optimization of the Paris system in HDR brachytherapy have been proposed, but the Manchester dosimetry system, with its emphasis on dose homogeneity throughout the specified treatment plane or specified treatment volume, can also be successfully applied, unmodified, in HDR dosimetry. A comparison of Manchester, Paris and optimized distributions for an HDR implant is presented, illustrating how subjectively and on the basis of dose-volume histograms, optimization has in this clinical case failed to improve on the uniformity of the Manchester rules distribution. It is proposed that optimization systems should have an option to apply these rules with further individualized clinical optimization if required. This would permit uniform reporting of implant parameters where, by describing the clinical target volume, the total reference air kerma and initial distribution of dwell times will be defined.