Physics of pulsed dose-rate brachytherapy

Pulsed Dose-rate (PDR) brachytherapy is a highly fractionated treatment with a single stepping source which intends to replace Continuous Low Dose-rate (CLDR) therapy with radioactive wires or seeds. The advantages of the stepping source are it's better possibilities of optimizing the shape and the homogeneity of the dose distribution by modulating the dwell times. Radiobiological calculations predict conditions concerning fraction ("pulse") dose and inter-fraction intervals under which PDR should yield an equivalent biological effectiveness to a CLDR treatment. PDR afterloaders operate with source activities around 1 Ci of Ir-192 leading to irradiations at high dose-rate. The safety issues of high dose-rate treatments have so far kept the number of PDR users small. One of the important issues is the question whether the presence of a physicist is required at all times during the treatment, or if an availability within an as yet unspecified response time is sufficient. Other important issues are staff training, and quality assurance of applications suited for long overall times of automatically controlled treatment.