ESTRO 2021 Abstract Book

S1263

ESTRO 2021

approach in the case of online adaptive MR-guided radiotherapy for Nasopharyngeal Carcinoma (NPC) patients.

Materials and Methods The original CT and MRI T2 were acquired for 10 NPC patients and targets, organs at risk (OARs), bone and air regions were delineated on original CT in Monaco system (v5.40.02). Each patient’s MRI/CT image pair was aligned using a rigid registration algorithm. Then the contours were copied to MRI from original CT. The sCT was then generated in MRI using bulk rED assignment approach, based on the rED values of the delineated region of interests (ROIs). Three different types of sCT images were generated: The sCT ICRU uses the rED values recommended by ICRU46, the sCT tailor uses the patient-specific mean rED values, and the sCT Homogeneity was created using water equivalent value of 1 g/cm 3 for entire patient data set. The same treatment plan was calculated on three sCTs and original CT. The dose calculation accuracy was investigated in terms of gamma analysis, point dose comparison and dose volume histogram (DVH) parameters.

Results For both criteria, 1%/1mm and 2%/2mm gamma passing rate results of plans on sCT tailor

were always higher

than plans on sCT ICRU on point dose in PTV nx calculated on sCT ICRU

and sCT Homogeneity

when compared with original CT based plans (p = 0.005). The difference

for each case calculated on sCT tailor (-8.77% ± 2.33%) and sCT Homogeneity

(-0.14% ± 1.44%) was smaller if compared to those (1.65% ± 2.57%). The DVH differences for plan based on

sCT tailor

were much smaller than sCT ICRU

and sCT Homogeneity .

Conclusion The sCT generated with bulk rED assignment approach guarantees an acceptable level of dose accuracy for NPC patients with 1.5 T MR-Linac, making this approach suitable to online adaptive MR-guided radiotherapy. The accuracy can be improved by using patient-specific rED values. PO-1538 Inter- and intrafraction organ motion for cervical cancer patients treated on a 1.5T MR-Linac S. Ding 1 , H. Liu 1 , B. Wang 1 , Y. Li 1 , B. Liu 1 , Y. Ouyang 1 , X. Huang 1 1 Sun Yat-sen University Cancer Center, Department of Radiation Oncology, Guangzhou, China Purpose or Objective Internal bladder and rectum motion occur during radiotherapy for cervix cancer. IMRT planning studies have shown that the quality of radiation delivery is influenced by these anatomical changes. This MR-Linac system allows for daily MR image guidance and real-time imaging throughout the treatment fraction, which is ideal for managing and monitoring both inter-fraction and intrafraction motion, without incurring additional radiation exposure. The study is to evaluate bladder and rectum inter- and intrafraction motion using MRI during the full course of MR-Linac radiotherapy fractions, in preparation of MR-guided online adaptive radiotherapy. Materials and Methods Five stage Ⅱ A - Ⅲ B cervix cancer patients were treated with 25x1.8 Gy fractions on a 1.5T MR-Linac. Each fraction, pre-treatment MRI scans were obtained at the start of every treatment session, and post-treatment MRI scans were obtained at the end of every treatment session. The bladder and rectum were delineated on each MRI by the same radiation oncologist. Inter-fraction and intrafraction variability were evaluated with the Dice similarity coefficient (DSC), the Hausdorff distance (HD), mean distance to agreement (MDA), volume (V) and dose volume parameters. Results Large inter-fraction motion was found for bladder and rectum due to their filling variation (Bladder: DSC: 0.68±0.17, HD: 30.58±15.32, MDA: 7.77±4.65, V: -23.02%±13.13%, D mean : 2.23%±7.10; Rectum: DSC: 0.69±0.04, HD: 22.98±4.02, MDA: 3.45±0.63, V: 13.57%±18.09%, D mean : 0.4%±0.7%). Median in-room treatment time pre fraction was 35 min which was much longer than conventional treatment time. The intrafraction motion for bladder was obvious that was explained by the bladder filling variation due to prolonged in-room treatment time per fraction (DSC: 0.88±0.07, HD: 14.66±8.48, MDA: 1.89±1.15, V: 13.06%±15.66%, D mean : -0.89%±2.45%). The intrafraction rectum motion was much smaller. Conclusion Considerable inter-fraction bladder and rectum motion is observed and intrafraction bladder motion which caused by longer treatment time is also obvious in cervical cancer patients on 1.5T MR-Linac. Intrafraction MR- guided online adaptive radiotherapy may potentially further correct this motion. PO-1539 Daily Plan Verification Before and After Treatment in Prostate Stereotactic Body Radiotherapy E. Kucukmorkoc 1 , N. Kucuk 1 , E. Sanli 1 , S. Celebi 1 , D. Canoglu 1 , R. Rzazade 1 , M. Turna 1 , H.B. Caglar 1 1 anadolu Medical Center, Radiation Oncology, Kocaeli, Turkey Purpose or Objective To compare the dosimetric differences and volumes of organs at risk (OARs) and planning target volumes (PTVs) in patients who were treated with prostate SBRT. Materials and Methods Patients with low and intermediate-risk prostate cancer were included. All the patients had proper rectal and bladder preparation before the simulation. The acquired planning CT images were transferred to the Precision treatment planning system (TPS). Before the contouring, MR images were fused with planning CT images. Gross tumor volumes (GTVs), PTVs, and OARs were contoured and robotic-based SBRT plans were generated using the same dose objectives for each patient. The prescribed doses were 36,25Gy or 37Gy/5F to PTV. All the patients are treated with robotic radiosurgery machine and fiducial tracking algorithm. The same rectum and bladder preparation protocol were applied before each fraction and a new CT scan (PreCT) was obtained. PreCT images were sent to Precision TPS. Original plans and structures were transferred to PreCT using