ESTRO 38 Abstract book

S579 ESTRO 38

brachytherapy (BT) of cervical cancer using ring applicator, if supplementary interstitial needles are used? Material and Methods 36 BT fractions from 9 patients were studied retrospectively. Target volumes (CTV HR ) and OARs (bladder, rectum and sigmoid colon) were delineated in MR image sets acquired before each fraction. The BT dose was prescribed to provide a total CTV HR dose of 90-95Gy (EQD 2 , α/β=10) in terms of D 90% when combined with external radiation of 1.8Gyx25. The maxdose (D 2cc ) to bladder, rectum and sigmoid colon was restricted to 80, 65 and 70Gy (EQD 2 , α/β=3), respectively. The Hybrid Inverse Planning Optimization (HIPO) algorithm (Oncentra Brachy, Elekta version 4.3) was used. The 1 st set of 36 plans was generated to fulfil CTV HR dose prescriptions without needles, whereas the OAR doses had secondary priority. These plans revealed the most challenging OARs regarding violating of dose limits. The 2 nd set of 36 plans was generated with the primary goal to fulfill the OAR dose limits. These plans revealed which parts of the CTV HR that were difficult to cover with adequate dose without needles. The unsuccessful fractions of the 2 nd set were re-optimized with use of interstitial needles for the 3 rd set of plans. The needles were thoughtfully placed where possible, with appropriate distances to the OARs. Results In the 1 st and 2 nd set of plans, the dose constraints for OARs and CTV HR were fulfilled in 19 of 36 fractions (53%). The remaining 17 fractions (47%) were regarded as failed. The plans that failed in the 1 st set of plans in terms of OAR dose constraints also failed in the 2 nd set of plans in terms of not fulfilling CTV HR dose. The OAR most frequently exceeding the dose constraint in the 1 st set of plans was sigmoid colon (11 fractions, mean excess dose (MED): 0.37Gy), followed by bladder (9 fractions, MED: 0.32Gy) and rectum (4 fractions, MED: 0.38Gy). For 10 of the 17 failed BT fractions, the dose constraints became fulfilled when interstitial needles were added (see table). The mean volume of CTV HR was 31.8±8.9cc for the primary successful plans, 38.2±7.7cc for the plans fulfilled using needles and 37.1±10.1cc for the not fulfilled plans.

Conclusion When utilizing needles as supplement to ring applicators, the total number of BT fractions which fulfilled the prescribed CTV HR and OAR doses increased from 53% to 81%. Although not all plans became successful using needles, the CTV HR D 90% was always improved. Sigmoid colon and bladder were most frequently surpassing the dose limits in the failed plans. For each patient, it was not possible to predict whether the dose plan would be successful or not for the consecutive fractions. The CTV HR tended to be smaller for the plans that were primary fulfilled. The result implies that interstitial needles are a helpful tool for optimizing the dose distribution; however, they do not ensure complete success for every BT fraction. PO-1042 Toxicity results after Electronic Brachytherapy treatment in endometrial cancer patients A. Miranda Burgos 1 , A. Méndez Villamón 1 , M. Gascón Ferrer 1 , A. Campos Bonel 1 , B. Garcia Gimeno 1 , S. Lozares Cordero 2 , J.A. Font Gómez 2 , R. Ibáñez Carreras 1 , M. Tejedor Gutiérrez 1 1 Hospital Universitario Miguel Servet, Radiation Oncology, Zaragoza, Spain ; 2 Hospital Universitario Miguel Servet, Medical Physics and Radiation Protection, Zaragoza, Spain Purpose or Objective To analyse the toxicity outcomes after treatment with Electronic Brachytherapy (XB) in postsurgical endometrial cancer patients treated at our medical centre. Material and Methods Prospective study in which we selected 118 patients, between September/2015 and May/2018, that received treatment with XB (Axxent Xoft device) administered twice a week after endometrial cancer surgery, with IMRT planification. The patients were divided in two groups: Group 1 (73/118) considered high risk received external beam radiotherapy (46Gy) followed by XB (15Gy in 5Gy fractions) and group 2 (45/118) considered intermediate risk received exclusive XB (25Gy in 5Gy fractions). We analysed the median dose in bladder, rectum and sigmoid D2cc, V50, V35 with XB comparing the doses with Ir192. The vaginal mucosa, gastrointestinal (GI) and genitourinary (GU) toxicities were analysed with the Common Terminology Criteria for Adverse Events (CTCAE 4.0) scale.