SU-E-T-430: Evaluation of Inverse Optimization in Brachytherapy for Locally Advanced Cervix Cancer.

PURPOSE Brachytherapy is a vital part of treating locally advanced cervix cancer. Point-based 2D treatment planning is conventionally used. New CT/MRI-compatible applicators for HDR brachytherapy make 3D conformal planning possible. GEC-ESTRO endorsed the EMBRACE (intErnational study on MRI-guided BRachytherapy in locally Advanced CErvical cancer) protocol which recommends standard-plan-based manual optimization over inverse optimization due to the concern of the uncertainty of high dose in targets and adjacent normal tissue. This study investigated the feasibility of inverse optimization planning. METHODS Eight cervix cases using tandem/ring applicators +/- parametrial needles were evaluated. Both manually and inversely optimized 3D conformal plans using Oncentra® Brachy were generated for each case following EMBRACE guidelines. Planning time and DVH parameters including D90 and V100% for HR-CTV, D90 for IR-CTV, D2cc for normal structures (bladder, rectum, sigmoid colon, and small bowel) were compared. Dose was reported in total EQD2 dose, assuming 45Gy in 25 fractions external beam and 28Gy in 4 fractions HDR treatment. Prescription dose (PD) and PD×2 isodose volumes and V100% and V200% for normal tissue (excluding all contoured OARs) were compared to evaluate the high dose volumes. RESULTS The average planning time was reduced by 55% with inverse planning. Target coverage between inverse and manual plans differed by <2% for HR-CTV (D90 and V100) and IR-CTV (D90). Inverse planning reduced the D2cc for normal structures by 4.2%-5.7% and the V100% and V200% for normal tissue by 31% and 40%, respectively. For 7 of the 8 cases, small bowel was observed near the target and received significant dose with either approach. CONCLUSIONS In this study, inverse planning improved DVH parameters over manual planning, with less planning time. Reporting normal tissue dose should alleviate the concern of undetected high dose regions. Even though small bowel is not often considered in traditional planning, dose should be reported.