The comparison of "inverse" optimisation and reference line optimization on organs at risk doses in three-dimensional vaginal vault brachytherapy

Aim: The aim was to compare the methods of reference line (RL) and "inverse" optimization (IO) in terms of doses to organs at risk (OAR) in patients who received vaginal cuff brachytherapy (BRT). Hypothesis: Doses to OAR are lower in IO compared to RL optimization (RLO). Materials-Method: CT images of 20 patients were used who received vaginal cuff BRT using "Stump" applicator after external radiotherapy (ERT). RLO was performed to line composed of 8 symmetrical points at 0,5 cm distance from the applicator's apical and lateral surfaces. On the other hand, dose was prescribed to CTV in IO with introduction of optimization goals for Clinical Target Volume (CTV) and OARs. CTV optimization goals: D98%>85%, D90%>100%, V100%>92,5%. OAR optimization goals were defined in terms of doses to 2cc of volume (D2cc) receiving total (ERT+BRT) doses. Biologically equivalent doses in 2 Gy per fraction (EQD2): bladder<9000cGy, rectum, sigmoid, and bowel<7000cGy. Using Wilcoxon Signed-Rank test, 2 different optimization techniques were compared with respect to effects on CTV dose-volume parameters (D90%, V100%, D50%/D90%) and OAR 2cc doses. Results: When D2cc total EQD2 doses of all OARs were evaluated, it was observed that significantly (p<0,0001, p:0,004, p:0,001, and p:0,001 for bladder, rectum, sigmoid, and bowel, respectively) lower doses could be obtained with IO compared to RLO. When the parameters of D90%, V100% ve D50%/D90% were evaluated, it was seen that significantly (p<0,0001, and p<0,0001, respectively) higher doses could be obtained with RLO for D90% ve V100%. On the other hand, D%50/D%90 (which is recommended to be <1,5) is significantly (p<0,0001) lower in IO. It was detected that CTV criteria could be met in all cases where OAR criteria of <7000cGy was violated only for rectum in 1 case in IO and 3 cases in RLO. Conclusion: In both types of optimization, desired criteria are met for CTV while they may not be met constantly in all cases. The resultant significant difference in favor of IO with regard to OARs supports the routine clinical use of this optimization type in vaginal cuff BRT. Key words: brachytherapy, vaginal cuff, inverse optimization, reference line optimization, organs at risk