Abstract Book

S1237

ESTRO 37

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 ). Dose coverage was improved in HIPO plans compared to IPSA: D90 was 100.6 vs. 96.2% (p=0.0008), COIN was also higher: 0.6 vs. 0.47 (p<0.001), while DHI was lower: 0.34 vs. 0.41 (p<0.001), respectively. The D 2ccm of rectum, sigmoid and bowels was lower with HIPO optimisation: 2.5 vs. 2.7 Gy (p=0.0059), 3.2 vs. 3.6 Gy (p<0.001) and 4.1 vs. 4.6 Gy (p=0.0039). In both HIPO and IPSA plans, higher needle number resulted improved dose coverage (p=0.0234 and 0.0034), homogeneity (p=0.0004 and p=0.0004) and COIN (p=0.002 and 0.0121). Volume of HR- CTV correlated with COIN (R 2 =0.45, p=0.0015 and R 2 =0.52, p<0.001) and D 2ccm of bladder (R 2 =0.49, p=0.0006 and R 2 =0.36, p=0.0081), rectum (R 2 =0.65, p<0.001 and R 2 =0.59, p<0.001) and sigmoid (R 2 =0.60, p<0.001 and R 2 =0.44, p=0.0058), while DHI showed correlation only with COIN (R 2 =0.31, p=0.0338 and R 2 =0.33, p=0.0245), respectively. Conclusion HIPO optimisation results more improved treatment plans in combined intracavitary-interstitial IGABT of cervical cancer than IPSA, except for dose homogeneity. The use of higher number of needles improves dose coverage, homogeneity and conformality significantly with both invers algorithms. EP-2239 Evaluation of the Advanced Collapsed Cone Engine (ACE) for Ir-192 brachytherapy treatment planning L. Eason 1 , J. Mason 1 , P. Bownes 1 1 Leeds Cancer Centre, Medical Physics and Engineering, Leeds, United Kingdom Purpose or Objective The purpose of this work is to evaluate the Oncentra Advanced Collapsed Cone Engine (ACE) dose calculation algorithm for high-dose-rate (HDR) 192 Ir brachytherapy oesophageal and surface mould treatment planning. These treatment sites present a deviation from full- scatter conditions and contain significant hetero- geneities, thus are likely to exhibit dosimetric differences

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) .