ESTRO 2020 Abstract book

S1086 ESTRO 2020

treatment is localized inaccurately. The aim of this study was to evaluate the dose sparing effect of SpaceOAR- hydrogel in the event of large posterior treatment localization error. Material and Methods Ten prostate patients treated with 5 x 7.25 Gy fractions using two-arc VMAT technique and 10 MV flattening filter free beams were included in the study. Two planning CTs, one prior to and one after the SpaceOAR-hydrogel implantation, were acquired for each patient. CTV was prostate alone and isotropic 5 mm isotropic CTV-to-PTV margins were used. Rectum started from recto-sigmoid junction and ended 2 cm below the lowest prostate-apex contour. MRI was utilized in delineation. Treatment plans with and without the gel were created for comparison. Posterior treatment localization error was simulated by shifting isocenter 5 mm posteriorly in the gel treatment plans and recalculating the dose. The dose sparing effect of the gel was evaluated by comparing rectum dose volume histograms (DVH) between the gel plans (shifted and non- shifted) and non-gel plans. Results Mean (±SD) separation between the rectum and the prostate after the gel implantation was 1.4 ± 0.3, 1.1 ± 0.5 and 1.3 ± 0.5 cm in midplane, midplane+1 cm and midplane-1 cm, respectively. Mean ± SD differences in rectum V50, V65, V90, V95 and V100 between non-shifted gel and non-gel plans were -10.4 ± 12.1, -10.2 ± 5.3, -5.2 ± 2.5, -3.7 ± 2.0 and -1.1 ± 0.9%, respectively, in favor of gel plans (fig.1). If the isocenter was shifted 5 mm in the gel plans, corresponding differences were -0.6 ± 13.0, -2.5 ± 6.7, -2.9 ± 2.4, -2.4 ± 2.0 and -0.8 ± 1.0%, respectively (fig.2). Variation in rectum volume receiving 5-20 Gy was larger in plans optimized with the gel, compared to plans optimized without it.

or individualized between patients to avoid introducing bias. Results Averaged results of planned target volume (PTV) and OARs were summarized in table 1. The typical iso-dose distribution of the 3 treatment techniques were demonstrated in figure 1. There were no significant difference in PTV coverage between 3 techniques (p>0.05). The results demonstrated that both 10MV SEP and 6MV-10MV MEP are capable to reduce radiation dose to the rectum and bladder (p<0.05), comparing with 6MV SEP. In addition, 10MV SEP was also compared favorably in PTV homogeneity and to reduce skin dose than 6MV SEP and 6MV-10MV MEP.

Conclusion Superior rectum and bladder sparing ability were observed for 10MV SEP and 6MV-10MV MEPs. 10MV SEP have also exhibited the superior result in generating the least skin dose and PTV dose homogeneity. In conclusion, the present study serves as an initial reference of dosimetric performance between SEP and MEP for patient with stage IIIB prostate cancer. PO‐1854 The effect of SpaceOAR hydrogel and posteriorly misplaced treatment on rectum dose in prostate SBRT A. Vanhanen 1 , P. Reinikainen 2 , T. Murtola 3 1 Tampere University Hospital, Department of Oncology and Department of Medical Physics, Tampere, Finland ; 2 Tampere University Hospital, Department of Oncology, Tampere, Finland ; 3 Tampere University Hospital, Department of Urology, Tampere, Finland Purpose or Objective Spacer gels such as SpaceOAR-hydrogel (Augmenix, Inc.) are used to increase the prostate-rectum separation in order to reduce rectum dose in prostate radiotherapy. Rectal dose sparing is of special interest in stereotactic body radiotherapy (SBRT) of the prostate due to few fractions with extra high fraction doses elevating the risk of rectum overdose and consequent adverse effects if the