ESTRO 36 Abstract Book

S991 ESTRO 36 2017 _______________________________________________________________________________________________

Material and Methods Three dimensional conformal radiotherapy plans (3D-CRT) for patients treated in our hospital were analysed. The analysis and assessment of differences in calculations obtained with two TPS were performed for 160 and 130 patients treated with 6 MV and 15 MV X-rays, respectively. 3D-CRT treatment plans were prepared in the Eclipse TPS (Analytical Anisotropic Algorithm, version 10). Subsequently the DICOM RT data were transferred to the Oncentra MasterPlan TPS (Collapsed Cone Convolution, version 3.3) and dose distribution was calculated. The same number of monitor units were kept. The differences between mean dose in the PTV and dose in the isocenter were compared. For statistical analyses, the Wilcoxon matched-pair signed rank test was used. In addition, the calculations of both TPS were compared with measurements performed in the inhomogeneous phantom. Results The statistically significant (p < 0.001) systematic shift between systems, both in the mean dose in the PTV and dose in isocenter was observed. The doses calculated in the Oncentra MasterPlan TPS were always lower than doses calculated in the Eclipse TPS. The average difference of mean dose delivered to the PTV was 1.4% ± 1.0% for 6 MV X-rays and 2.5% ± 0.6% for 15 MV X-rays. The average difference in dose in the isocenter was 1.2% ± 3.5% and 2.4% ± 2.7% for X6 MV and X15 MV, respectively. In most cases the largest differences were caused by air cavities and bone structures. The significant influence of inhomogeneities on dose calculations was confirmed by measurements. Conclusion Estimated differences of dose calculations in most cases did not exceed the action level of 3% recommended by the Knöös et al. publications (2006 Phys. Med. Biol., 51: 5785– 5807). Based on this study the independent verification of the Eclispe TPS calculations was introduced in our hospital. EP-1836 Study of changes in bowel gas in pelvic radiotherapy A. Horne 1 , M. Zahra 1 , W. Keough 1 1 NHS Lothian, Clinical Oncology, Edinburgh, United Kingdom Purpose or Objective To assess volumetric changes in pelvic bowel gas using serial CT scans in patients receiving pelvic radiotherapy. Material and Methods We performed a retrospective analysis of bowel gas volume and location using planning CT scans for 22 patients. All patients had an initial plan scan (CT1) with a full bladder . In group 1 (n=9) this was compared with a planning scan(CT2) done after about 30 minutes of CT1 with an empty bladder . In group 2 (n=15) CT1 was compared with a CT scan (CT3) done for a phase 2 boost 4 weeks after CT1. Eclipse TM planning software was used to contour the luminal bowel gas volume within a conventional pelvic radiotherapy field (see figure 1). For each of the 2 groups the total volume and the volume in each quadrant was measured and compared. Significance testing was carried out using a paired t-test with two-tailed significance levels

at 0.05. Results The results have been summarised in the table below:

Conclusion In group 1 no significant difference was identified due to changes in bladder volume over a short period of time. In group 2 there was a tendency towards an increase in bowel volume over the 4 week period of radiotherapy more so in the upper quadrants reflecting changes mainly in the colon rather than in the rectum. This is possibly due to the radiotherapy toxicity and associated diet changes. The volume of bowel gas does not appear to be consistent during a course of pelvic radiotherapy and this should be taken into consideration when dosimetry changes are made to the initial planning scan to compensate for bowel gas. EP-1837 Dosimetric effect by rotational error in VMAT on Brain tumor patients H. Park 1 1 Yonsei University, Radiation Oncology, SEOUL, Korea Republic of Purpose or Objective This study is to evaluate the dose effects on whether Hexapod Couch is applied for patient positioning errors to be corrected in the course of VMAT on brain tumor patients. Material and Methods For this study, a total of 1129 cases of CBCT acquired from 46 brain tumor patients with VMAT and Hexapod used are comparatively analyzed and difference on dose effects is evaluated by dosimetric quality assurance(DQA) and Electronic Poster: RTT track: Image guided radiotherapy and verification protocols