ESTRO 2020 Abstract book

S1107 ESTRO 2020

PO‐1888 Dose effects of interfraction target shifts in irradiation of prostate with two partial VMAT arcs. M. Admiraal 1 , A. De la Fuente 1 , J. Van der Himst 1 , S. Virginia 1 1 Amsterdam UMC, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Full arc VMAT treatment plans for prostate cancer treatment are known to be dosimetrically robust for setup on the target using implanted gold fiducials to account for day to day variations of the prostate position [1]. This study aims to assess the dose robustness for a VMAT treatment technique consisting of two partial arcs instead of full arcs. Material and Methods 20 prostate cancer patients were included in the study. 7 patients were treated with a simultaneous integrated boost (SIB) to the CTV of 20 x 3.1 Gy, while delivering at least 20 x 3.0 Gy to the PTV. 10 patients were treated with a homogeneous PTV dose of 20 x 3.0 Gy and 3 with a PTV dose of 19 x 3.4 Gy. Normalization was applied to the PTV to achieve D98%=95%. All radiation plans consisted of two partial VMAT arcs excluding the dorsal angles between 145° and 215° to limit the dose to the rectum. Prostate and rectum motion within the pelvic region was simulated by manually shifting the prostate and rectum contours by 1 cm in either one of 4 directions (cranial, caudal, dorsal and ventral). The isocenter of the treatment plan was shifted accordingly to mimic setup on the PTV. With this, dose re-calculation of the treatment plan was performed on the initial planning CT scan to evaluate the dosimetric effect of these 4 new geometries. Dose to the target (D98% and D1cc) and hotspots in the rectum (D1cc) were evaluated and compared with the clinical treatment plan. Results For 79 out of 80 plans, the dose coverage to the prostate was robust within the clinically tolerated accuracy of 3%. In 1 plan dose coverage to the PTV volume dropped 3,4% (patient 20, figure 1). Homogeneity was affected slightly by prostate shifts, showing a systematic increase of D1cc of about 2% in the prostate for all shifts in ventral direction. For all simulated shifts, the D1cc to the PTV stayed below the plan restriction of 107%. The high dose to the rectum (D1cc) was only slightly influenced by shifts of the prostate, showing both increases and decreases with a maximum difference of 1 Gy.

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.