ESTRO 2020 Abstract book

S411 ESTRO 2020

demand. Finally, post-treatment checks are performed by RO and MP for each fraction. Results During the first 14 months 63 patients with LNO have been treated on the MRL so far with a total of 315 SBRT fractions having been delivered. Overall clinical production increased from 1 treatment slot per day to currently 5-7 slots with on average 56% being used for LNO. Currently 5 RTTs have full competence for the contouring part and 4 for treatment planning. Teams of 3 competent RTTs perform all necessary steps for the total treatment and control the individual actions before going to the next step in the workflow. Post-fraction evaluations did not show deviations beyond the predefined thresholds. Currently the 5 RTTs competent for LNO delineation having also gained or are in the process of gaining competence for other sites such as rectum and prostate. Furthermore, 7 additional RTTs are starting the training program for LNO. Conclusion LNO SBRT in 5 fractions using the ATS workflow on the 1.5 T MRL can be performed by RTTs alone after adequate training. Supervision on demand and post-fraction evaluation have to be in place. The role of the RTTs for MRL treatments with higher complexity and further hypo- fractionation needs to be explored. PD-0675 Evaluation of transbronchial implanted fiducials for lung tumor tracking in Cyberknife Y.Y. Chan 1 , K.M. Ku 1 1 Hong Kong Sanatorium & Hospital, Department of Radiotherapy, Happy Valley, Hong Kong SAR China Purpose or Objective Stereotactic body radiotherapy (SBRT) for lung tumor is a high-dose precision technique requiring accurate target tracking. Fiducials as surrogates of tumor position can be used for tracking in Cyberknife (Accuray, Inc.) but fiducial migration may affect the feasibility of the treatment. There are two common approaches to implant fiducials for lung cancer patients: transthoracic and transbronchial. The transthoracic approach is associated with higher risk of pneumothorax. In our hospital, fiducials were implanted transbronchial under fluoroscopic guidance. The purpose of this study was to evaluate the use of transbronchial implanted fiducials for tracking in lung cancer patients who were planned to have SBRT in Cyberknife in terms of safety, stability and traceabilty. Material and Methods This retrospective study included 13 lung cancer patients. Fiducials measured 0.8 x 5mm (PointCoil MTCTXPC08) were implanted transbronchial by specialist in respiratory medicine.CT at exhalation was done the next day after implantation of fiducials. This set of CT was used as reference location of the fiducials. Another set of CT at exhalation was done for radiotherapy planning purpose and to ensure the stability of fiducials on the 5th to 7th days after implantation. The CT scans of post-implantation day and planning day were used to compare the locations of fiducials to assess their relative migration and displacement. As a measurement of stability, average center distances (ACD) of the fiducials as given were computed. The ACD were defined as the average of all Euclidian distances of the fiducials to their center. The center was the centroid of a fiducial configuration. Results 85% patients inserted with fiducials treated successfully with fiducial tracking in Cyberknife. No patient suffered from pneumothorax after fiducial implantation. The

retention rate of implanted fiducials is 79.2%. By comparing the percentage difference of ACDs between post-implantation day and planning day of each patient, relative migration of retained fiducials is found to be 0 to 12.2%. The average fiducial usage rate (tracked/planned) is 75%. 6 degrees of freedom (DOF) correction tracking in Cyberknife requires at least three fiducials to be tracked. 71.4% patients treated with successful 6DOF correction tracking were implanted with more than 4 fiducials. 25% patients treated with successful 6DOF correction tracking were implanted with 4 fiducials. Conclusion Our results showed that 85% lung cancer patients implanted with fiducials successfully completed SBRT with fiducial tracking in Cyberknife using this workflow. The implantation of fiducials using transbronchial approach was safe and free from complication of pneumothroax. The retention rate of fiducials is comparable to other international studies. The results suggested the successful rate of using 6DOF correction tracking is higher with implantation of more than 4 fiducials. PD-0676 A pilot study on evaluating the breast reproducibility of Chabner® XRT Radiation Bra using MRI S.T. Chiu 1 , S.Y. Man 1 , G. Chiu 1 1 Hong Kong Sanatorium & Hospital, Department of Radiotherapy, Happy Valley, Hong Kong SAR China Purpose or Objective Adjuvant RT is a main treatment regime for breast cancer patient. Immobilisation of breast is difficult because its softness and deformity. Chabner® XRT radiation bra (“the bra”), a FDA-approved immobilization support for breast cancer patient during radiotherapy has been recently introduced into the market. The bra has 13 sizes and is compatible to all radiologic procedures, e.g.CT and MRI. This pilot study aims to evaluate the breast reproducibility A total of 10 female subjects, with healthy breast tissues, were recruited in this study. All subjects first underwent a bra fitting session. A fitting bra was selected based on the measurement of the upper chest, bust and band. The shoulder straps and band were adjusted in order to support like a normal bra. The straps’ position and closure of the band were marked on the bra. All subjects underwent T2W 3D TSE MRI scans on the 1.5T Philips Ingenia with the bra. All subjects were scanned initially in a supine position with both arms supported by the breastboard to mimic the treatment position as patients. The scan was then repeated after refitting the bra. All images were sent to MIM® software for analysis. Left breast tissue, with reference to the RTOG Breast Cancer Atlas, was delineated for each volunteer. The two sets of MRI images were superimposed manually by rotation and translation to best match the breast tissue. The translational and rotational shifts were recorded. The volumetric overlap was evaluated using DSC with the equation (V1nV2)/½(V1+V2), where V1 is the left breast tissue in the initial MRI and V2 is the volume of the refit MRI. A DSC value of 1 represents a perfect overlap of the left breast tissue between the MRI images, hence a perfect reproducibility. Pearson correlation coefficient (ρ) was calculated to analyse the correlation between the breast volume and its associated shift. Results of the bra using MRI. Material and Methods