Optimization in high dose rate brachytherapy for utero-vaginal applications

Background and purpose: High dose rate (HDR) remote afterloading intracavit ary brachytherapy is an effective treatment modality which has some advanta ges over low dose rate (LDR) techniques for gynaecological cancer. Optimiza tion is one of the possibilities of modem brachytherapy techniques, especia lly the stepping source technology. The use of the term 'optimization' impl ies achieving the desired optimum dose distribution by changing some parame ters 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. Materials and methods: Working on a virtual utero-vaginal model, the dose d istribution variations in the rectum, bladder, mean point B reference point s and volume parameters were investigated whilst giving a standard dose to point A in the Manchester system. In this model, the intrauterine tandem co nsisted 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 Plate treatment planning system and a Microse lectron Ir-192 HDR stepping source unit, the dwell times of the intrauterin e (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. Results: All evaluated parameters decreased with increasing dwell time rati os of uterine tandem to Vaginal colpostats, with the greatest fail in the p ercentage of rectum reference dose (D-R %), 23 and 28% for 2.5 and 5 mm dwe ll positions respectively; in addition, the reference isodose volume decrea sed by 14 and 17% for 2.5 and 5 mm dwell positions, respectively. All evalu ated parameters increased with decreasing dwell time ratios of uterine tand em 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 f rom 49.2 to 67.5 (37%) for 5 mm dwell positions. The volume was increased b y 27 and 37% for 2.5 and 5 mm dwell positions respectively. Conclusions: Modern brachytherapy techniques enable the individualization o f treatments by optimization procedures in gynaecological brachytherapy app lications. By altering the dwell time and position, some important changes in reference points, volume and treatment time can be achieved, whilst main taining a standard dose to point A. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.