Uniformity of biological effect in high-dose rate stepping source brachytherapy: an in vitro study.

In brachytherapy with a high dose rate of iridium-192, to make a flat isodose surface that covers a target volume, the dwell times of the single stepping source are computer adjusted. However, the dose rates of the irradiation vary with the positions in the volume. To examine this effect, we developed an in vitro model of stepping source brachytherapy with a 20 cm long target volume, and determined the uniformity of the biological effect in the volume on two human cancer cell lines (WiDr and A549) plated on a multi-well plate. When the source-cell distance was 10 mm, D1 (radiation dose for 1% survival) was similar in seven positions among the target volume for both cell lines. In contrast, at the 3 mm source-cell distance for one of the two cell lines (A549), D1 in the centre of the volume was significantly higher than at the periphery, suggesting a milder antitumour effect in the centre. Considering the possible decreased effect in the centre of the target volume, where most cancers clinically exhibit their maximum tumour volume, a minimum increase in radiation dose in the centre may be clinically reasonable, especially when dose reference points are set close to the source.