ESTRO 2020 Abstract Book

S1054 ESTRO 2020

Results Due to minor anatomical inconsistency between pMR and pCT mostly due to bowel and breathing motion, the mean dose of bowel D max among all the 10 subjects resulted in a significant difference of 1.1 Gy (Z=-2.07, p=0.038). The mean PTV target dose difference between the plans was 0.2 Gy (Z=-0.65, p=0.515) in D 95 coverage, 3.7% (Z=-0.39, p=0.694) in V 105 and 0.2 Gy (Z=-0.35, p=0.730) in D max , showing no statistically significant change between the two plans. The paired samples t-test between CT plan and CT actual showed that there was significant average PTV dose difference of 1.2 Gy in D 95 (t 40 =8.84, p<0.001), 4.6% in V 105 (t 40 =4.83, p<0.001) and 0.4 Gy (t 40 =3.28, p=0.002) in D max . In comparison with MR-MR matching, MR actual resulted in a target delivered dose that was better in agreement with its planned dose compared with kVCT-MVCT matching. The mean PTV dose difference of D 95 , V 105 and D max were 0.6 Gy (t 40 =5.55, p<0.001), 3.6% (t 40 =3.40, p=0.002) and 0.4 Gy (t 40 =3.540, p=0.001) respectively. In terms of OAR sparing, CT actual had a reduced average dose of 0.3 Gy (t 9 =2.31, p=0.046) from CT plan in bowel D mean . Whereas in MR actual delivered dose, rectum D max and D mean showed a slightly reduced dose difference of 0.7 Gy (t 8 =-3.29, p=0.011) and 0.6 Gy (t 8 =-3.32, p=0.011) respectively.

Conclusion Our planning technique with two partial VMAT arcs with daily online setup on the prostate was shown to be robust for prostate motion of 1cm. Dose homogeneity to the prostate is affected slightly, most prominent when the prostate motion is directed ventrally. Dose differences are small (less than 3%) and no hotspots >107% were found. Changes to the high dose to the rectum (D1cc) were limited to about 1 Gy and were not systematically dependent on the prostate motion direction. These simulations reflect the dose effect in case the prostate shift is the same over all treatment sessions. In clinical practice the dose effect of prostate shifts will therefore be even smaller. PO-1889 To evaluate the feasibility of MR-only pelvis TomoTherapy during treatment planning and verification S.Y. MAN 1 , R. Appleyard 2 , G. Chiu 1 1 Hong Kong Sanatorium & Hospital, Department of Radiotherapy, Hong Kong, Hong Kong SAR China ; 2 Sheffield Hallam University, Faculty of Health and Wellbeing, Sheffield, United Kingdom Purpose or Objective The primary purpose of this study was to investigate the dose accuracy of the bulk density assigned planning MR in Helical TomoTherapy (HT) of the pelvis. The secondary aim of the study was to investigate accumulated dosimetric difference of using MR-only image guidance as opposed to using CT images in HT for online patient setup verification. Since current research in MR-guided RT (MRgRT) using HT remains unexplored, the research of this paper aims to prepare for a new potential workflow and practice. Material and Methods Ten previously treated pelvis patients using HT were included in this study. Both their planning CT (pCT) and RT-MRIs were retrieved to create a bulk density assigned planning MR (pMR). The pMR of each patient was used to re-plan in HT. Using the patient’s re-planned pCT as gold standard, the plans were compared. Using deformable image registration (DIR) based method, daily MR verification image (vMR) was created from the daily MVCT image from HT. The pre-existing 3D CT-CT and the new 3D MR-MR daily set-up shifts were compared by comparing the accumulated actual dose delivered to their respective clinical treatment planned dose in terms of target coverage and avoidance of OAR. The results were analysed using paired t-tests and non-parametric Wilcoxon test.

Conclusion In this study, MR-based planning and MR-based verification have been proven to be feasible where CT was not needed for electron density information and image verification procedure for pelvis HT. The planned dose effect has shown to be insignificant. In terms of daily positional verification, we can facilitate more precise delivery with better target coverage if MR images could be used. PO-1890 Comparative assessment of the verification performance, ART and imaging dose between MVCT and kVCBCT Y.S. Tang 1 , A.M. Chow 2 1 Hong Kong Baptist Hospital, Charles Cheng Oncology Centre, Hong Kong, Hong Kong SAR China ; 2 The University of Hong Kong, Department of Diagnostic Radiology, Hong Kong, Hong Kong SAR China Purpose or Objective Megavoltage computed tomography (MVCT) and kilovoltage cone-beam computed tomography (kV CBCT) are two commonly used volumetric verification modalities across radiation therapy centres in Hong Kong. MVCT and