Abstract Book

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ESTRO 37

parallel with the leaves’ motion axis. On continuously acquired EPID images (approx. 10Hz), positions of both ball bearing and tracked circular aperture were detected. Sinusoidal fits yielded the an estimated tracking latency of the entire tracking system, including MR-imaging, image processing and MLC motion.

randomly selected for inclusion, each undergoing 15-25 fractions. For all patients, after initial set up, electronic portal images (EPIs) were matched to planning CT Digitally Reconstructed Radiographs (DRRs). This generated vertical, longitudinal and lateral displacements that were converted into an absolute displacement. The mean absolute displacement using all fractions for all patients in each cohort were compared using an unpaired t-test (p <0.05). Results The mean [SD] absolute displacement from the planned isocentre was 0.39cm [±0.11cm] for Cohort 1, 0.28cm [±0.09cm] for Cohort 2 and 0.27cm [±0.08cm] for Cohort 3, as summarised in Figure 1.

Results

Figure 1 shows the excellent object visibility in both MRI and EPID, which enabling the system latency determination. Tracking experiments with different MRI- frequencies (Figure 2) showed the strong dependency of the system latency on MRI-sampling frequency (92ms for 8Hz, 213 ms for 4Hz). Due to coarse temporal sampling and spurious MRI signals, we observed oscillatory tracking In this work we successfully demonstrate the first real- time MRI-guided MLC-tracking on the 1.5T MR-linac system. The performance of the tracking system was determined and validated using the independent portal imager. Ongoing work focuses on optimizing the tracking latency with respect to MRI-sequence types. Furthermore, fast and more stable image processing are under investigation, in order to suppress oscillatory effects and improve system latency. responses. Conclusion OC-0190 Surface guided radiation therapy for breast cancer improves accuracy without the need for skin marks C. Russell 1 , H. Mack 1 , S. Paul 1 , S. Senthi 1 1 The Alfred, Alfred Health Radiation Oncology, Melbourne- Victoria, Australia Purpose or Objective Patient positioning for breast radiation therapy has traditionally relied upon permanent or temporary skin marks. However, permanent tattoos increase the incidence of body image dissatisfaction and psychological distress following treatment, while temporary pen marks may fade and be inaccurately redrawn. Surface guided radiation therapy (SGRT) using an optical system can provide a 3D coordinate reconstruction for patient setup without requiring skin marks, and has the additional benefit of intra-fraction monitoring. We compared accuracy of SGRT setup with setup utilising skin marks. Material and Methods Patients undergoing radiation therapy following breast conservation and not needing nodal irradiation or deep inspiration breath hold technique were included in this retrospective study. Prior to acquiring the AlignRT SGRT system (VisionRT, London, UK), patients were set up using temporary skin marks (Cohort 1). Following its acquisition, patients were set up using temporary skin marks for initial gross positioning, followed by SGRT (Cohort 2) and later, SGRT without any skin marks (Cohort 3). Fifteen patients from each cohort were Proffered Papers: RTT 2: Reproducibility optimisation in clinical practice

When compared to Cohort 1, SGRT resulted in significantly improved patient displacement for both Cohort 2 and Cohort 3 (p=0.009 and p=0.003 respectively). There was no significant difference between Cohort 2 and Cohort 3 (p=0.66). Conclusion SGRT significantly improved patient setup accu racy when compared to using skin marks alone. T he standard deviation was also reduced, suggesting greater setup consistency and reliability. There was no reduction in accuracy when skin markings were not present, indicating that a skin mark-free setup for breast radiation therapy is feasible. As SGRT additionally allows intra- fraction motion management, it may also allow target volume margin reduction and reduce doses to at risk organs. OC-0191 Improved set-up accuracy for adjuv ant whole breast irradiation in the prone-crawl position P. Deseyne 1 , G. Post 2 , A. Van Greveling 1 , B. Speleers 2 , K. Vandecasteele 1 , L. Paelinck 1 , B. Boute 2 , H. Depypere 3 , C. Mbah 2 , W. De Neve 1 , L. Veldeman 1 1 Ghent University Hospital, Radiation Oncology Department, Gent, Belgium 2 Ghent University, Radiotherapy and Experimental Cancer Research, Ghent, Belgium 3 Ghent University Hospital, Breast and Menopause Clinic, Gent, Belgium Purpose or Objective Prone positioning for whole-breast irradiation (WBI) reduces dose to organs at risk (OAR). Nevertheless, the prone position for WBI is not generally implemented, in part because of reduced set-up speed and accuracy. We aimed to improve the set-up accuracy of prone positioning by placing patient on a new type of breast couch, which adds more support points for the patient, and allows for lymph node irradiation in prone position. As the patient’s position resembles a phase of crawl swimming, the device was called the crawl couch.