ESTRO 36 Abstract Book

S196 ESTRO 36 2017 _______________________________________________________________________________________________

2 Hospital Universitario de Gran Canaria Dr. Negrín, Radiación Physics, Las Palmas de Gran Canaria- Ca, Spain Purpose or Objective Vaginal stenosis as consequence of cervical cancer BT treatment severely impact quality of life. No dose constraints have been published so far. Aim of this study is to identify a threshold level for volume packing and a dose response curve for vaginal stenosis. Material and Methods 211 consecutive cervical cancer patients treated between 2008-16 (median FU time 42.4 months) with a median age at BT of 52.6 years (range 23.7- 88.5) were considered. All pts received 3DRCT (45-50 Gy with weekly concomitant CDDP 40 mg/m2 when feasible) and tandem ovoids HDR BT (or intracavitary-interstitial cylinder application when needed). Patients received simulation CT scan with radiopaque vaginal tube in place in order to delineate vagina from a plane tangential to lower border of pubic bone up to fornix. At BT vaginal packing (VP) was contoured from a plane tangential to lower border of pubic bone up above ovoids surface. Vaginal walls were delineated as a 2 mm expansion of packing subtracted of packing volume. 85 pts. (group A) received CT based BT (5 fractions of 5,5 Gy), 126 patients (group B) received MR based BT (4 fractions of 7 Gy). Group A pts had a treatment slightly optimized to OARs. Group B pts had a treatment optimized to OAR and HRCTV according GEC ESTRO recommendations. All patients entered prospective follow up. Morbidity was scored according CTCAE 4.0 vaginal volume was also measured with appropriate vaginal cylinders (diameters 10 to 45 mm).To assess the relationship between vaginal stenosis, VP and vaginal dose a median VP volume (VPm) among the 5 (group A) or 4 (group B) application each patient received was calculated. Moreover the cumulative EBRT+BT EQD2 dose to vagina was calculated. A Logistic model (LM) was used to analyze data. Results Results are summarized in Tab1. In 929 applications a double exponential fit was noticed between vaginal dose and VP volume, with a fast growing exponential part (minimal variations in VP volume corresponding to huge variations in vaginal dose), and a slow growing exponential part (variation in VP volume have modest impact on dose). VP volume cut off values dividing the two parts of the curve for all considered vagina DVH parameters were encompassed between 75 and 80 cc. LM showed good correlation (R 2 =0.97 and 0.96 respectively) between VPm and G3 or G2-3 vaginal stenosis (Fig1 A_B). Risk of vaginal stenosis G3 or G2-3 was less than 10% when a VPm volume >82 or 105cc was obtained. A dose response curve was found for G3 or G2-3 stenosis and vaginal EBRT+BT EQD2 D80 (R 2 0.99 and 0.98 respectively) with a risk of G3 or G2-3 stenosis lower of 10% when EQD2 dose parameters was lower than 63 and 44Gy EQD2 respectively (Fig1 C-D).

Conclusion At our knowledge this is the first report finding a correlation between vaginal stenosis, VP volume and vaginal dose on a relatively large serie. Further studies on larger dataset are needed to confirm such data OC-0368 Postoperative vaginal brachytherapy: a quality assurance dummy-run procedure in the PORTEC-4 trial R. Nout 1 , E. Astreinidou 1 , M. Laman 1 , C. Creutzberg 1 1 Leiden University Medical Center LUMC, Department of Radiotherapy, Leiden, The Netherlands Purpose or Objective As part of the quality assurance program in the ongoing randomized multicenter PORTEC-4 trial a ‘dummy-run’ procedure for vaginal brachytherapy was mandatory before centers could participate. The aim was to evaluate whether the CT- or MRI based clinical target volume (CTV) and organ at risk (OAR) delineations and the standard treatment plans were according to the trial protocol, especially since for many centres this involved introduction of CT-based delineation and planning for Pelvic CT and MRI scan of a postoperative endometrial cancer patient with a cylinder in situ were made available to participating Dutch centers. Centers were asked to use their own treatment planning and delineation software and follow the study protocol in order to: 1) delineate CTV and OAR’s: bladder, rectum, sigmoid and small bowel; 2) reconstruct the single line source path; 3) create a treatment plan prescribing 7 Gy at 5 mm from the surface of the applicator (point A2, fig 1); 4) perform DVH analysis. The CTV consisted of the proximal 4 cm of the vaginal brachytherapy. Material and Methods