SU‐F‐T‐29: The Important of Each Fraction Image‐Guided Planning for Postoperative HDR‐Brachytherapy in Endometrial Carcinoma

Purpose:Cylindrical applicators are often used for postoperative HDRbrachytherapy in endometrial carcinoma. It has been considered that dosimetric variation between fractions for this treatment is minimal and might not be necessary to perform treatment planning for every fractions. At our institute, it is traditional to perform treatment planning with CT simulation on the first fraction and uses this plan for the rest of treatment. This study was aim to evaluate the errors of critical structure doses between the fractions when simulation and planning were done for first fraction only.Methods:Treatment plans of 10 endometrial carcinoma patients who received postoperative HDR‐brachytherapy and underwent CT‐simulation for every HDR‐fractions at our department were retrospectively reviewed. All of these patients were treated with cylindrical applicator and prescribed dose 15Gy in 3 fractions to 0.5cm from vaginal surface. The treatment plan from the first fraction was used to simulate in second and third CT‐simulation. Radiation dose for critical structures in term of Dose‐to‐2cc (D2cc) were evaluated and compared between planning CT.Results:The D2cc for bladder and rectum were evaluated. For bladder, the mean error of D2cc estimation for second and third fractions was 7.6% (0.1–20.1%, SD=5.7). And the mean error for D2cc of rectum was 8.5% (0.1–29.4%, SD=8.5).Conclusion:The critical structure doses could be significant difference between fractions which may affects treatment outcomes or toxicities. From our data, image‐guided brachytherapy at least with CT‐Simulation should be done for every treatment fractions.