ESTRO 36 Abstract Book

S198 ESTRO 36 _______________________________________________________________________________________________

vagina to a depth of 3 mm from the mucosal surface. To account for anisotropy in the longitudinal direction of the source two points (A1 and A3) were defined at 5 mm from the cranial applicator surface and additional points (A4-6) were added (fig 1). The average dose between A1 and A3 should be approximately 100%, with A1 minimal 90% and A3 maximal 110%. Central evaluation of contours and treatment plans took place and in case of deviations from the protocol, feedback was provided and necessary steps in the dummy-run repeated.

Results Fifteen centers participated, 12 centers used CT planning, 2 used MRI planning and 1 both. For 11 plans some adjustments were required, and in 6 cases a second revision was requested. Main reasons for adjustments were: delineation (N=8), dose planning (N=7), reconstruction (N=2). Three different commercially available treatment planning systems and HDR sources were used. Table 1 summarizes dose to points A1-6, CTV and OAR’s of the final submissions. Consistency with the protocol improved and interobserver differences significantly decreased with the revisions.

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

Conclusion Interobserver variation in delineation resulted in the largest dose deviations, most pronounced for bowel on postoperative CT. The use of a second point (A3) at the apex was most useful for controlling the anisotropy of the source and should be recommended for dose reporting in routine clinical practice.