Optimization in high dose rate brachytherapy for utero-vaginal applications

High dose rate (HDR) remote afterloading intracavitary brachytherapy is an effective treatment modality which has some advantages over low dose rate (LDR) techniques for gynaecological cancer. Optimization is one of the possibilities of modern brachytherapy techniques, especially the stepping source technology. The use of the term 'optimization' implies achieving the desired optimum dose distribution by changing some parameters of the treatment. The aim of this study was to theoretically evaluate the optimization possibilities by modifying dwell times and dwell positions of the uterine and vaginal sources.

Working on a virtual utero-vaginal model, the dose distribution variations in the rectum, bladder, mean point B reference points and volume parameters were investigated whilst giving a standard dose to point A in the Manchester system. In this model, the intrauterine tandem consisted of 27 dwell positions for 2.5 mm steps and 14 dwell positions for 5 mm steps. Vaginal colpostats consisted of five dwell positions each for 2.5 mm steps. Using a Nucletron Plato treatment planning system and a Microselectron Ir-192 HDR stepping source unit, the dwell times of the intrauterine (T(u)) and vaginal sources (T(v)) were modified at the ratios of (T(u)/T(v)) 1:1; 1:2; 1:3; 1:4; 1:0.50; 1:0.33; and 1:0.25 for the two different dwell positions, 2.5 and 5 mm steps, of the intrauterine tandem.

All evaluated parameters decreased with increasing dwell time ratios of uterine tandem to vaginal colpostats, with the greatest fall in the percentage of rectum reference dose (D(R) %), 23 and 28% for 2.5 and 5 mm dwell positions respectively; in addition, the reference isodose volume decreased by 14 and 17% for 2.5 and 5 mm dwell positions, respectively. All evaluated parameters increased with decreasing dwell time ratios of uterine tandem to vaginal colpostats for both dwell positions. The DR% of 1:1-1:4 (T(u)/T(v)) weightings showed an increase from 40.6 to 58.3 (44%) for 2.5 mm and from 49.2 to 67.5 (37%) for 5 mm dwell positions. The volume was increased by 27 and 37% for 2.5 and 5 mm dwell positions respectively.

Modern brachytherapy techniques enable the individualization of treatments by optimization procedures in gynaecological brachytherapy applications. By altering the dwell time and position, some important changes in reference points, volume and treatment time can be achieved, whilst maintaining a standard dose to point A.