ESTRO 2021 Abstract Book

S1610

ESTRO 2021

Caption: DVH comparison for HA- (blue) and MBM-results (red) of all ten multiple metastases cases. In favor of better overview, only whole brain and targets are plotted. Conclusion We conclude that both solutions are able to generate acceptable SRS plan quality without significant planner interactions. Still, there are differences. HA achieves better conformity, with MBM providing the better gradient, which is also reflected in better V12. Due to the fact that the planning technique of MBM has fewer degrees of freedom than HA, this must be compensated for by the number of arcs and MUs, which is at the expense of the V5. PO-1891 Dosimetric impact of personalized vs PORTEC-4a vagina contouring on postop endometrial brachytherapy J.P. Rochera Alba 1 , H.M. Antonio 1 , B. José 2 , R.C. Angeles 1 , B.M. Clara 1 1 Hospital Clínic de Barcelona, Radiation oncology department, Barcelona, Spain; 2 Universitat Autònoma de Barcelona, Departament de Matemàtiques, Cerdanyola del Vallès, Spain Purpose or Objective In PORTEC-4a QA, non-personalized vaginas are considered in endometrial cancer. Some institutions use a personalized contouring methodology, but this approach requires a lot of resources for delimitation. Does this technique have significant dosimetric differences from the PORTEC-4a method? The aim of this study is to determine retrospectively if there are dosimetric significant differences between these two procedures. Materials and Methods Vaginas of 24 treated patients have been delineated retrospectively following the procedure described in PORTEC-4a. In both delimitation modes we consider an automatic contouring of the vaginal applicator based on Hounsfield units in CT images followed by a 3mm isotropic expansion of the vaginal vault for the same plan. Then, we modify the contouring in the personalized one adapting to the real vagina. Due to both sample size and non normality, a permutation test has been applied to test the significance of the differences of the values of the dose-volume histogram: V100, D90 and D98. Bootstrap was used to obtain 95% confidence intervals. Significance level was set at 0.05. Results The vaginal volumes delimited according to PORTEC-4a are on average 26% greater than those obtained by a personalized method. Table 1 shows mean differences (custom-PORTEC-4a), 95% confidence intervals, and the p -value for the analyzed metrics. According to p -values, the null hypothesis of equal means was rejected in all cases.

Metric Mean differences personalized-PORTEC 4a (CI 95%) (%) p -value V100 -0.30 (0.07,0.56) 0.0230 D90 4.38 (2.90,5.77) 0.0000 D98 4.79 (2.78,6.89) 0.0001

Table 1: Mean differences (personalized-PORTEC 4a) obtained under both contouring methods. The second column shows the estimated mean and the 95% confidence interval and the third, the p -value of the permutations test (n = 24). Conclusion The dose values in the uniform expansion method are significantly lower than in the personalized contour because the volume obtained is greater. The personalized mode allows a significant reduction in the dose. There are dosimetric significant differences between these two delimitations. Thus, we are able to conclude than we have the chance to reduce prescribed dose in our treatments, so it could reduce toxicity in patients.

PO-1892 Prediction of PTV coverage for LINAC SABR Prostate 36.25Gy/5# from OARs overlap with PTV M. Kroiss 1 1 Mount Vernon Cancer Centre, Radiotherapy Physics, London, United Kingdom

Purpose or Objective SABR Prostate plan statistics need to comply with UK SABR Guidelines. The dose tolerances are: V PTV36.25Gy

> 95%, V CTV40Gy

>

< 1cm 3 , V Rectum 29Gy

< 10cm 3 , V Bladder 18.125Gy

< 1cm 3 , V Bowel

95%, V Rectum 36Gy

< 20%, V Rectum 18.125Gy

< 50%, V Bladder 37Gy

< 40%, V Bowel 30Gy

18.125Gy < 5%, testicles must be out of the treatment field. Planned in ECLIPSE, the doses achieved in the individual plans might easily start failing the allowed tolerances, depending on the involvement of rectum, bladder and bowels within PTV. It is important that the planner is confident enough about the best possible distribution achieved for the defined anatomy. In order to recognise the quality of the plan, we create a prediction model < 5cm 3 , V PenileBulb 29.5Gy < 50%, V Femoral Joints 14.5Gy