NONUNIFORM DWELL TIMES IN-LINE SOURCE HIGH-DOSE-RATE BRACHYTHERAPY - PHYSICAL AND RADIOBIOLOGICAL CONSIDERATIONS

The ability to vary source dwell times in high dose rate (HDR) brachyt herapy allows for the use of non-uniform dwell times along a line sour ce. This may have advantages in the radical treatment of tumours depen ding on individual tumour geometry. This study investigates the potent ial improvements in local tumour control relative to adjacent normal t issue isoeffects when intratumour source dwell times are increased alo ng the central portion of a line source (technique A) in radiotherapy schedules which include a relatively small component of HDR brachyther apy. Such a technique is predicted to increase the local control for t umours of diameters ranging between 2 cm and 4 cm by up to 11% compare d with a technique in which there are uniform dwell times along the li ne source (technique B). There is no difference in the local control r ates for the two techniques when used to treat smaller tumours. Normal tissue doses are also modified by the technique used. Technique A pro duces higher normal tissue doses at points perpendicular to the centre of the line source and lower doses at points nearer the ends of the l ine source if the prescription point is not in the central plane of th e line source. Alternatively, if the dose is prescribed at a point in the central plane of the line source, the dose at all the normal tissu e points are lower when technique A is used.