ESTRO 37 Abstract book

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ESTRO 37

according to tattoos on patient’s skin and external lasers as common practice. Again, IGRT was used for final isocenter correction. Based on orthogonal IGRT images the remaining pitch and rotation after each fraction were calculated and monitored for the two patient groups. Results Our investigation shows no significant difference between the two methods regarding the remaining rotational deviations, MLM performs slightly better for chest and inferior to MBM for abdomen, although not significantly (see image (1)). As both methods presume correlation between patient’s surface and bony anatomy the final accuracy is still strongly patient-dependent. However, with treatment workflow and patient comfort in mind the MLM includes several advantages: 1. no skin markers required, 2. higher patient safety due to independent and consistent support device positioning, 3. deformations can be corrected efficiently using information of the surface scanner and 4. after a training period the workflow is fast and straightforward.

treatment a video-based 3D optical surface monitoring system with 3 ceiling-mounted camera pods was used to monitor the real-time motion of a region of interest. This motion-monitoring was performed in 6 dimensions, namely lateral, longitudinal, vertical, rotational, pitch and roll. A tolerance of 0.3 cm for linear directions and 0.5° degrees for rotational directions was set, in accordance with a PTV margin of 3mm. If the optical surface monitoring system detected a violation of the set tolerance, treatment automatically stopped. If deemed necessary, the treating physician decided to acquire an extra CBCT for setup verification. Intrafraction motion was derived by registering CBCT2 and CBCT3 to the planning CT scan. A registration method in two phases was used. First, the bony anatomy of the CBCT was registered to the planning CT followed by registration of the tumor. The intrafractional shifts were calculated and compared with the set tolerances of the optical surface monitoring system (OSMS). For the shifts in each direction, mean, median and standard deviation were calculated. Results From June 2017 to September 2017, the records of 17 patients and 54 fractions were analysed. In 5 sessions treatment stopped because of violation of the set OSMS tolerance and the treating physician decided to acquire an extra CBCT for setup verification. For these sessions, the linear or rotational tumor shift detected on the extra CBCT was larger than the set tolerance of the OSMS, confirming the accuracy of the OSMS for detecting intrafractional tumor movement.

For all the other sessions, the linear and rotational shifts detected on CBCT were within the tolerance level of the OSMS, except for one session were the difference in one rotational direction is 0.6°.

Conclusion The presented new method (MLM) allows positioning of patients at least as accurate as the common used practice with skin marks. Our clinic is applying this method since 01/2016. [1] Martens R et al. The workflow and benefits of patient positioning based on absolute table coordinates. ESTRO33 2014, Poster RTT track OC-0194 Evaluation of an optical surface monitoring system for intrafractional movement during SABR C. Mercier 1 , A. Sprangers 1 , D. Verellen 1 1 GZA- Ziekenhuizen - St. Augustinus, Radiotherapy, Wilrijk, Belgium Purpose or Objective To evaluate the accuracy of a real time 3D optical surface imaging system for monitoring intrafraction motion during frameless stereotactic ablative treatment of spine, lymph node and non-spine bone metastases. Material and Methods Patients treated with SABR for spine, lymph node and non-spine bone metastases were immobilized in a comfortable and appropriate position to irradiate the metastatic lesion(s). Support devices were used to increase patient comfort and to ensure set-up reproducibility, but no thermoplastic body mask or dual vacuum system was used. During each fraction, three CBCT scans were acquired; CBCT1 before treatment, CBCT2 after correction for tumor misalignment, and CBCT3 after treatment. During

This results in an accuracy of the OSMS of 99.7%. Conclusion Our evaluation showed that real-time surface imaging provides sufficient accuracy for monitoring intrafraction motion in frameless SABR of nodal, spine and non-spine bone metastases. OC-0195 Reproducibility of head and neck setup in MR Simulator environment K.F. Cheng 1 , P.H. Fok 1 , G. Chiu 1 1 Hong Kong Sanatorium & Hospital, Division of Radiotherapy, Happy Valley, Hong Kong SAR China Purpose or Objective To investigate the reproducibility of head and neck setup in MR Simulator environment. Material and Methods Fifteen volunteers were recruited. Each volunteer was immobilized by head and neck thermoplastic cast. They were firstly setup and positioned with two Flex- 4 coils and one Body-18 coil covering the head and neck regions in MRI-Simulator (Siemens MAGNETOM RT Pro edition based on 1.5T Aera MRI). A volumetric T1w MR images (3D SPACE, TR/TE=420/7.2ms, voxel size=1.4x1.4x1.4mm 3 , scan time = 1.5mins) was then performed. The procedures were repeated for 10-20