Consequences of optimization in PDR brachytherapy--is a routine geometrical optimization recommendable?

PURPOSE

On the basis of clinical examples for interstitial volume implants and surface molds the benefit and disadvantage of optimization in pulsed dose rate (PDR) brachytherapy using a stepping source was investigated. Geometrically optimized PDR dose distributions were compared with nonoptimized ones as produced by Ir wires.

METHODS AND MATERIALS

In 25 patients who were treated with a double-plane interstitial breast implant with flexible catheters the first smoothly surrounding isodose, the reference volume and the uniformity and quality index derived from natural dose-volume histograms (Anderson) were considered. The effect of geometrical optimization on surface molds, which were used to irradiate chest wall relapses from breast cancer, was investigated by analyzing the reference surface, dose profiles, and depth-dose curves of a source arrangement covering an area of 10 x 10 cm2.

RESULTS

Only in 3 of 25 patients the dose distribution of the volume implant was worsened by geometrical volume optimization regarding the homogeneity or the first smoothly surrounding isodose. Predominantly geometrical volume optimization reduced underdosage at the edges of the implants and improved the dose uniformity. The reference volume was increased. The geometrical distance optimization of surface mold dose distributions resulted in an enhancement of the reference surface and an improvement of the dose homogeneity on the skin surface.

CONCLUSIONS

Geometrical optimization substantially changes the dose distribution. Only in a minority of cases the dose distributions of volume implants were worsened by geometrical volume optimization. Nevertheless, a close look at the results has to be recommended, especially if an overdosage in the lateral regions of the dose distribution and a dose falloff near the middle catheters is observed. The enhancement of the reference volume by geometrical volume optimization must be considered when choosing the active lengths. In surface mold treatment planning geometrical distance optimization is necessary to achieve homogeneous dose distributions for irradiation fields of different size and shape.