ESTRO 2021 Abstract Book

S1584

ESTRO 2021

evaluate the coverage error of PTVs due to residual distortion uncertainties, the 80% isodose was defined on PTVs and then applied to iPTVs. Data analysis is based on descriptive statistics.

Results Over the whole cohort, PTVs varied from 0.3 cm 3 to 37.1 cm 3 , with a mean volume of 9.3 cm 3 . The mean radial distance from the lesion center and the isocenter was 63.5 mm. The closest lesion from the isocenter was at 22.6 mm and the furthest was at 104.5 mm. System residual distortions ranged from 0.07 to 1 mm, with a mean value of 0.41 mm. The magnetic susceptibility distortion was between 0 and 0.4 mm. Depending on the radial lesion-isocentre distance, the total distortion ranged between 0.15 and 1.3 mm, with a mean total distortion of 0.55 mm. In line with these results and a possible margin increment of 1 mm on TPS, the margin chosen for iPTV was 1 mm. The coverage error on PTV (80% isodose) was 3.5% for the largest lesion and 19% for the smallest.

Conclusion The definition of an iPTV appears like a solution to take into account the uncertainties in the PTV definition due to geometric distortions in MRI. It should be specified that this margin must be defined according to the MRI sequence, the on-line corrections of geometric distortions and the type of cancer (size of the explored volume). PO-1860 Physical vs. biological based VMAT optimization for prostate cancer radiotherapy D. Teixeira 1 , A. Gulyban 1 , S. Poeta 2 , Y. Jourani 2 , R. Van den Begin 3 , F. Otte 3 , D. Van Gestel 3 , N. Reynaert 1 1 Institut Jules Bordet - Université Libre de Bruxelles, Medical Physics department, Brussels, Belgium; 2 Institut Jules Bordet - Université Libre de Bruxelles, Medical Physics department, , Brussels, Belgium; 3 Institut Jules Bordet - Université Libre de Bruxelles, Radiation Oncology department, Brussels, Belgium Purpose or Objective Application of equivalent uniform dose (EUD) for cost functions during IMRT/VMAT optimization can improve plan quality in a dynamic manner making it attractive to establish class solutions [Kopanitsa et al. JAMP 2014, Dirscherl et al. ZMP 2011, Mihaylov et al. IJROBP 2012]. The aim of this work is to compare physical, dose volume histogram (DVH)-based, and biological, generalized EUD (gEUD)-based, approaches using VMAT for intermediate risk prostate cancer radiotherapy. Additional objective was to reduce the number of optimization volumes and objectives for the planning process, without

decreasing plan quality. Materials and Methods

Nine consecutive patients irradiated in our department were selected for the study. For each case, two planning target volumes, i.e. high dose (prostate+ 6mm=PTV-HD) and low dose (prostate+seminal vesicles+6 mm=PTV-LD); and organs at risk such as rectum, bladder and femoral heads were delineated. Two different plans were created a physical DVH based and a biological optimization based. For each approach, a dual 6MV 360° VMAT arc was used within the Monaco TPS