ESTRO 37 Abstract book

S1237

ESTRO 37

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. The experimental set-up was simulated using MCNP6 (Los Alamos National Laboratory, USA) to evaluate possible effects of phantom and set-up geometry, detector volume effect and detector response with distance due to photon spectrum changes in water. All resulted correction factors were normalized to the reference radial distance of 1.0cm. Profiles in X, Y and Z directions were then measured to determine the exact position of the source. Repeated measurements were taken at radial distances from 0.5 to 15cm with constant source position and repeated source drive out. Ten repeated measurements were taken in the aforementioned range (Y-axis). The centre of the mDD sensitive volume was considered for the determination of the radial distances. After applying all MC-based corrections (detector volume k v and detector response k E ) the radial dose function g(r) was calculated, as defined in TG-43 formalism from the mean values of detector readings at the radial distances. The resulting experimental g(r) values were compared with the consensus g(r) (ESTRO and AAPM) and with the values derived from our own MC simulation. Results The results of repeated measurements demonstrated good reproducibility, both with constant source position and with repeated source drives (maximum standard deviation of ±1.1% and ±1.3% respectively). The MC-based g(r) values for the MP3 water phantom including the experimental set-up are in very good agreement with the consensus g(r) values for a spherical water phantom with 40cm radius. The percentage difference varied in the range -0.9 to +0.1%. This indicates that there is no significant effect of our experimental set-up on the results at least up to radial distances of 10cm. The MC- based correction factors for the mDD were normalized to 1cm and are summarized in table 1. When compared to the consensus g(r) , the measured g(r) showed a percentage difference between -2.6% (10cm), and +0.6% (2cm) (figure 1). Our experimental and MC g(r) are also in good agreement, with percentage difference in the range

-2.2 to +1%. For radial distances above 8cm, the mDD tends to underestimate g(r) .

Conclusion Using the mDD we could reproduce the consensus data and our MC-based results for g(r) with very good agreement in the range of 0.5 to 15cm, when proper correction factors are determined and applied. The underestimation at large radial distances may be attributed to a sensitivity variation with dose rate of the mDD. Employing the mDD to measure the anisotropy function is currently under development. EP-2238 Comparative analysis of HIPO vs IPSA optimisation algorithms in interstitial IGABT of cervix cancer G. Fröhlich 1 , J. Vízkeleti 1 , N. Anhhong 1 , N. Mészáros 1 , T. Major 1 , C. Polgár 1 1 National Institute of Oncology, Centre of Radiotherapy, Budapest, Hungary Purpose or Objective Detailed dosimetric evaluation and comparison of HIPO and IPSA inverse optimisation algorithms in combined intracavitary-interstitial high-dose-rate image-guided adaptive brachytherapy (IGABT) of cervical cancer. Material and Methods 54 treatment plans of combined intracavitary-interstitial cervix IGABT were analysed. HIPO and IPSA optimised plans were created with standard presets. Dose-volume parameters were compared using Wilcoxon matched pairs test, the effect of needle number were analysed with Kruskal-Wallis ANOVA and median test, and Spearman rank order correlation were also calculated in both HIPO and IPSA plans. Results Median number of implanted needles was 3 (range: 1-6), mean volume of HR-CTV was 35.6 cm 3 (8.3-100.2 cm 3 ).