206. Critical issues conformity index definition for endometrial cancer in Simultaneous Integrated Boost

Purpose To evaluate Target Volumes (TVs) and Organs At Risk (OARs) irradiation in endometrial adenocarcinoma (EADK) Volumetric-Modulated-Arc-Therapy Simultaneous Integrated Boost (SIB-VMAT) plans using Conformity Indices (CIs). Methods 10 EADK patients treated with SIB-VMAT (45 Gy (TV45) on pelvic lymph-node and 50 Gy (TV50) on vaginal vault were selected. Median TV45 and TV50 were 1106.1 cc (774.1–1699.3 cc) and 504.5 cc (346.1–643.5 cc) respectively. Bones, rectum, bladder, small bowel and femoral heads were the considered OARs. The assessed indices, reported in Table 1 , were calculated separately for both TVs (TV4595%, TV5095%). Two critical aspects emerged evaluating CNs and COINs. One problem was related to the choice between TV4595% and TV5095% as a good parameter for CN and COIN calculation; therefore we compared CNs and COINs with a parameter (pconstr) defined as ratio between the respected constrains and total OARs constraints in plan (Spearman’s and Kendall’s test). The other issue was that COIN tends to zero if VRI largely includes at least one of OARs, leading to wrong conclusion that dose is not conformed. Wherefore, we introduced in COIN formula a weight factor (fw) as reported in Table 1 . Results Both TV45 and TV50 had good conformed dose as demonstrated by CIs, HTCIs and CNs values ( Table 2 ). As previously underlined, COINs wrongly indicated no protection of OARs, while wCOINs provided values in accordance with the other indices. Regarding the comparison with pconstr, we found a better correlation for CNs and wCOINs evaluated with TV4595% than with TV5095% ( Table 2 ). Conclusions CIs in SIB are related to anatomic site and dimension of TV; in this study, it seems more correct refering to larger volume of minor dose for the evaluation of CN and wCOIN. Moreover, we suggest to correct COIN relatively to the weight that each organ had respect to all OARs.