Abstract Book

S1236

ESTRO 37

Results DVIs of rectum and urethra were relatively insensitive to the reconstruction settings (range of [-0.1, 1.3]%). The D 90 and V 150 of the prostate increased when the urethra was excluded from the prostate, with medians of 1.0% respectively 1.2%. DVIs of the bladder and seminal vesicles changed up to 2.5% without interpolation, and up to 8.3% respectively 9.5% when the full slice thickness was used for the first and last contour, resulting in a decrease for the vesicles and an increase for the bladder (see figure). For the vesicles, this can be explained by the small average volume of 4.08cm 3 . For the bladder, the region of interest of the most irradiated 1cm 3 or 2cm 3 was often near the most caudally delineated contour.

increased, allowing an optimization of the occupancy time of the operating room and the mobilization of the multidisciplinary caregiver team. Mandatory quality assurance includes a priori risk analysis and risk mapping. Quality control tests of this machine are based on Canadian recommendations for quality assurance. Next development will concern applicators for rectal, skin and vaginal treatments. EP-2236 Sensitivity of dose-volume indices to organ reconstruction settings in HDR prostate brachytherapy M.C. Van der Meer 1 , P.A.N. Bosman 2 , B.R. Pieters 1 , Y. Niatsetski 3 , T. Alderliesten 1 , A. Bel 1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands 2 Centrum Wiskunde & Informatica, Life Sciences and Health, Amsterdam, The Netherlands 3 Elekta, Physics and Advanced Development, Veenendaal, The Netherlands Purpose or Objective Dose-volume indices (DVIs) play a decisive role in the evaluation of HDR prostate brachytherapy (BT) treatment plans. Calculation of DVIs is normally based on 2D delineated contours; reconstructed 3D organs may thereby vary depending on reconstruction assumptions. Therefore, the quality of a treatment plan expressed by DVIs may vary with different reconstruction settings. We investigated the sensitivity of DVIs to different organ reconstruction settings, compared to standard settings in a commercially available BT treatment planning system (TPS). Material and Methods Data of 26 prostate cancer patients consecutively treated with HDR BT were included. Contours were delineated on axial MRI scans (slice thickness: 3.3mm). The studied settings in obtaining a 3D organ from 2D contours were: • In- or excluding the urethra: it can be considered part of the prostate or not. • Smooth or stepwise interpolation: contours drawn on each slice can fill the volume spanned by the slice, or interpolation can be used between contours for a smoother organ surface. An in-house stand-alone software was developed and validated with Oncentra Brachy (OB) TPS (Elekta), which we use clinically. The impact of different settings was compared to a baseline following the default settings in OB, where the urethra is included in the prostate, interpolation is used, and the last contours span half-a- slice thickness. Studied DVIs are given in the table. The DVI calculation used a number of sample points (256.000 per organ for targets, 2.560 per cm 3 for OARs) resulting in a small variation in the DVIs; in the baseline setting, averaged over all patients, the 95% confidence interval was at most ±0.2% for targets and ±0.5% for OARs. • Full or partial top/bottom slice thickness: here, contours can span only the half-slice-thickness towards the other contours, or the full volume spanned by the slice.

Conclusion Different settings in the computation have an impact on the resulting DVIs. On average, the DVIs of a treatment plan are least favorable when the urethra is included in the prostate, and both interpolation and last slice filling is used. Reconstruction settings in a TPS should be reported and taken into account in evaluating prostate HDR BT treatment plans. EP-2237 Employing the microDiamond detector for radial dose function measurements with an 192Ir HDR source G. Rossi 1,2,3 , M. Gainey 1,2,3 , B. Thomann 1,2,3 , M. Kollefrath 1,2,3 , B. Allgaier 4 , J. Würfel 4 , D. Baltas 1,2,3 1 University Medical Center Freiburg, Department of Radiation Oncology-Division of Medical Physics, Freiburg, Germany 2 University of Freiburg, Faculty of Medicine, Freiburg, Germany 3 German Cancer Consortium DKTK, Partner Site Freiburg, Freiburg, Germany 4 PTW-Freiburg GmbH, Loerracher Strasse 7, Freiburg, Germany Purpose or Objective To investigate the suitability of the microDiamond detector (mDD) type 60019 (PTW-Freiburg, Germany) for radial dose function measurements for the HDR brachytherapy 192 Ir source model mHDR-v2r. Material and Methods The source was placed at the centre of a 3D water phantom (PTW-Freiburg, MP3) using an aluminium “bridge”, PMMA holder and a 4F plastic catheter. A microSelectron HDR afterloader (Elekta AB, Sweden) with a 192 Ir source (mHDR-v2r Elekta AB, Sweden) was used.