Abstract Book

ESTRO 37

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months, 12/41 developed metastases during the observation period. Actuarial LC, PFS, and OS rates at 2/5 years were 94/94%, 69%/66%, and 82%/74%, respectively. All 4 ≥G3 toxicities occurred in this group. Conclusion Loco-regional recurrence in previously surgically treated EC patients can be successfully salvaged with modern radiation techniques including IGABT. Previous radiation does not preclude salvage therapy. If the relapse is limited to the vaginal wall, IGABT may be the therapy of choice, while in recurrences extending beyond the vaginal cuff and/or involving the pelvic lymph nodes a combination with pelvic EBRT seems appropriate. PO-1016 TRUS/CT for IGABT in cervical cancer: an interobserver study on target volume contouring S. Smet 1,2 , N. Nesvacil 2,3 , J. Knoth 2 , A. Sturdza 2 , D. Najjari-Jamal 4 , F. Jelinek 5 , G. Kronreif 5 , R. Pötter 2,3 , J. Widder 2 , C. Kirisits 2,3 , M.P. Schmid 2 1 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium 2 Medical University of Vienna, Department of Radiation Oncology- Comprehensive Cancer Center, Vienna, Austria 3 Medical University of Vienna, Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria 4 Institut Català d'Oncologia- Hospitalet de Llobregat, Department of Radiation Oncology, Barcelona, Spain 5 Austrian Center for Medical Innovation and Technology, ACMIT, Wiener Neustadt, Austria Purpose or Objective To compare the interobserver variability of combined transrectal ultrasound (TRUS) / computed tomography (CT) vs. CT only vs. MRI only based contouring of the high risk clinical target volume (CTV HR ) in image guided adaptive brachytherapy (IGABT) for locally advanced Five patients with LACC (FIGO stages IIb - IVb) treated with radiochemotherapy and IGABT were included in this prospective study. CT, TRUS and T2-weighted MRI images were performed after insertion of the BT-applicator. 3D TRUS image acquisition was done with a customized US stepper device and software (Medcom, Germany; Elekta, Sweden; Acmit, Austria). Automatic applicator reconstruction using optical tracking was performed directly in the TRUS data set and then TRUS and CT images were fused with rigid image registration with the applicator as reference structure. The CTV HR (based on the recommendations of the GYN GEC-ESTRO group) of each patient was contoured on the 3 imaging modalities independently by 5 experienced investigators, generating CTV HR _CT, CTV HR _TRUS-CT and CTV HR _MRI. A consensus reference CTV HR _MRI (MRIref) was defined for each patient. Descriptive statistics and overlap measures (generalized conformity index) were calculated using RT slicer, comparing contours of every observer with one another and with the MRIref. Results The descriptive statistics are summarized in table 1 and show a generalized conformity index (CI gen) of 0.79±0.04 for all CT contours, 0.80±0.05 for all TRUS-CT contours and 0.86±0.02 for all MRI contours. The interobserver variation in relation to the MRIref volume is larger for CT (CIgen=0.52±0.05), than for TRUS-CT (CIgen= 0.63±0.06), and smallest for MRI (CIgen=0.84±0.02). The mean CTV HR _CT volume of all observers was 71% larger than the MRIref volume (55%, cervical cancer (LACC). Material and Methods

50%, 70%, 94% and 85% for patient 1-5 respectively), whereas the mean CTV HR _TRUS-CT volume was only 15% larger than the MRIref volume (14%, 9%, 28%, 20% and 5% for patient 1-5 respectively) (figure 1).

Conclusion TRUS combined with CT as imaging modality for contouring the CTV HR in IGABT for LACC is feasible and is reproducible among multiple observers with an acceptable variation. Interobserver variability in comparison to the reference MRI volume is smaller than CT alone. Moreover, combined TRUS-CT substantially reduces the overestimation of the CTV HR volume of CT alone. A dosimetric analysis of the contouring variations is already in progress. PO-1017 Interobserver variability of vaginal dose points reporting in cervical cancer radiotherapy treatment B. Ibanez-Rosello 1 , M. Adria-Mora 1 , J. Vilanova-Ciscar 2 , V. Campo 3 , V. Carmona 1,4 , J. Gimeno-Olmos 1,4 , F. Lliso 1,4 , J. Perez-Calatayud 1,4 1 La Fe University and Polytechnic Hospital, Radiotherapy, Valencia, Spain 2 HGU Gregorio Marañón, Dosimetry and Radiation Protection, Madrid, Spain 3 Araba-Txarritxu Universitary Hospital, Medical Physics and Radiation Protection, Vitoria, Spain 4 IIS La Fe - Universitat de Valencia, Unidad Mixta de