ESTRO 36 Abstract Book

S148 ESTRO 36 2017 _______________________________________________________________________________________________

(Figure1) .

Conclusion Three dimensional performance analysis showed that Cyberknife Synchrony® RMTS is successful in tumor tracking regardless of the amplitude of movement. This study is supported by TUBITAK 3001 project, project number 115S446 PV-0285 Using a surface scanner for positioning of pelvic patients - can X-ray images be omitted? J.B. Thomsen 1 , S. B.N. Biancardo 1 , S.H. Hattel 1 , L. Søndergaard Vinther Merkelsen 1 , B. Roche 1 , J.P. Bangsgaard 1 1 Rigshospitalet, Finsenscentret- Radiotherapy, Copenhagen, Denmark Purpose or Objective Reproducing the correct treatment position prior to radiotherapy is crucial for accurate dose delivery. The golden standard for positioning is X-ray based imaging with the drawback of exposing the patient to ionising radiation. More recently surface scanners using infrared light has been introduced to monitor the patient surface. We investigate a surface monitor system ”AlignRT” for positioning pelvic patients prior radiotherapy [Vision RT, www.visionrt.com ]. This is attractive in terms of saving time and reducing imaging dose to the patient. Even when acquiring daily X-ray images routinely, the ability to correct rotations using AlignRT is of value to limit Patients undergoing pelvic irradiation were positioned using the surface scanner. The body surface was extracted from the CT therapy scan acquired before radiotherapy and imported in the surface scanner software. With the patient on the couch it is possible to monitor the surface in the treatment region and the system displays the deviations from the CT therapy scan translational and rotational. We chose a region of interest (ROI) around the treatment region of about 20 cm in cranio-caudal direction and extending on both sides of the patient. Following positioning using the surface scanner, a cone-beam CT scan (CBCT) was acquired which makes a comparison between the positioning using the surface scanner and the CBCT possible. For the CBCT an automatic bone match was applied using commercial software (Varian inc., offline review) and visually inspected. In total, 105 fractions from 6 patients were analyzed and a paired T-test was applied to detect any significant differences between the two systems. Results In 50 of 105 fractions (48%) the difference between the two positioning procedures was larger than 5 mm in at least one direction. In 39 of 78 fractions (37%) the difference in rotations was larger than 3 degrees . In 71 of 105 fractions (68%) either a difference in translations was above 5mm or rotation above 3 degrees. In fig. 1 and fig. 2 the difference in translations and rotations are shown for all 105 fractions originating from 6 patients. For the translation there was a significant difference for vertical and longitudinal directions (P<0,001 for vertical, P<0,001 for longitudinal, P<0,9 for lateral). The difference in rotations were all significant (P<0,001 for pitch, P<0,05 for roll, P<0,01 for rotation around the vertical axis). repeated X-ray images. Material and Methods

Conclusion We did show significant deviations between positioning using the surface scanner and CBCT with the chosen ROI. In this small patient cohort, 68% of the fractions would have been out of tolerance using a threshold of 5mm and 3 degrees if positioned solely based on the surface scanner. Therefore a surface scanner does not replace the usual X-ray image guidance procedure. Furthermore, for pelvic patients it does not seem possible to use the surface scanner for reliable estimations of rotational deviations which could have limited repeated x-ray imaging. PV-0286 Quantifying registration uncertainties in image-based data mining E.M. Vasquez Osorio 1 , A. McWilliam 1,2 , J. Kennedy 3 , C. Faivre-Finn 1,4 , M. Van Herk 1,2 1 The University of Manchester, Division of Molecular & Clinical Cancer Studies- School of Medical Sciences- Faculty of Biology- Medicine and Health, Manchester, United Kingdom 2 The Christie NHS Foundation Trust, Christie Medical Physics and Engineering, Manchester, United Kingdom 3 The Christie NHS Foundation Trust, Informatics, Manchester, United Kingdom 4 The Christie NHS Foundation Trust, Clinical Oncology, Manchester, United Kingdom Purpose or Objective Image based data mining relies on non-rigid registration to bring image data on a common frame of reference. Registration uncertainties will affect the analysis and must be quantified and incorporated. We have developed a method to quantify global and local random registration uncertainties. Additionally, we evaluated the impact of accounting for global random registration uncertainties on the results of a recent lung data mining study that identified the base of the heart as a dose sensitive region affecting survival in lung cancer patients [1]. Material and Methods