ESTRO 36 Abstract Book

S484 ESTRO 36 _______________________________________________________________________________________________

the treatment planning system, along with a three- dimensional motion model accuracy (defined as the 75th percentile motion error in each voxel) map. The patients still undergo a commercial 4DCT protocol to provide a comparison between the current standard of care and the model-based process. Comparisons between the commercial and model-based approaches have been conducted on 19 patients to evaluate the magnitude of sorting artifacts in each process on a scale of 1-4, 1 having no artifacts and 4 having severe artifacts. The average CT noise for both protocols was described by examining a region of interest in the liver. Results Mean tumor displacement was 11.5 +/- 6.9 mm and the mean motion model error was 1.77 +/- 0.79 mm. The mean artifact severity ratings for the 4DCT and model-based CT approaches were 2.2 and 1.2, respectively. There were three instances of grade 4 artifacts and no instances of grade 3 or worse artifacts for the 4D and model-based approaches, respectively. The average CT noise was reduced from 57.7 HU to 11.6 HU. Conclusion The model-based approach provides the clinic with motion artifact free images that have lower noise and whose geometry accurately reflects the tumor and other lung tissues during the CT scanning session. We are still limited by the treatment planning system's input requirements for a series of breathing-phase defined images. Work is ongoing to develop treatment planning protocols that better match the data resulting from the model-based approach. PO-0884 Availability of MRI improves interobserver variation in CT-based pancreatic tumor delineation O.J. Gurney-Champion 1 , E. Versteijne 1 , A. Van der Horst 1 , E. Lens 1 , H. Rütten 2 , H.D. Heerkens 3 , G.M.R.M. Paardekooper 4 , M. Berbee 5 , C.R.N. Rasch 1 , J. Stoker 6 , M.R.W. Engelbrecht 6 , M. Van Herk 7 , A.J. Nederveen 6 , R. Klaassen 8 , H.W.M. Van Laarhoven 8 , G. Van Tienhoven 1 , A. Bel 1 1 Academic Medical Center, Department of Radiation Oncology, Amsterdam, The Netherlands 2 Radboud University Medical Center, Department of Radiation Oncology, Nijmegen, The Netherlands 3 University Medical Center Utrecht, Department of Radiotherapy, Utrecht, The Netherlands 4 Isala Clinics Zwolle, Department of Radiotherapy, Zwolle, The Netherlands 5 MAASTRO Clinic, Department of Radiation Oncology, Maastricht, The Netherlands 6 Academic Medical Center, Department of Radiology, Amsterdam, The Netherlands 7 University of Manchester and Christie NHS trust, Faculty of Biology- Medicine & Health- Division of Molecular & Clinical Cancer Sciences, Manchester, United Kingdom 8 Academic Medical Center, Department of Medical Oncology, Amsterdam, The Netherlands Purpose or Objective To assess whether the availability of magnetic resonance images (MRIs) alongside the planning CT scan for target volume delineation in pancreatic cancer patients decreases interobserver variation. Material and Methods Eight observers (radiation oncologists) from six institutions delineated gross tumor volume (GTV) on contrast- enhanced (CE) 3DCT and internal GTV (iGTV) on 4DCT for four pancreatic cancer patients. At least six weeks after submitting these delineations, the observers were asked to repeat the delineations, now with MRIs available in a separate window (3DCT+MRI and 4DCT+MRI). The MRI included plain and CE T1-weighted gradient echo, T2- weighted turbo spin echo, and diffusion-weighted imaging. Interobserver variation in volumes of (i)GTVs was analyzed. Also, the generalized conformity index (CI gen ), a

Results Figure 2 illustrates that proxy-free and common external breathing signal-driven PB-reconstructed 4DCT data are comparable both in terms of image quality and represented motion amount. In detail, the considered proxy-free datasets contained approximately 5% more artifacts than the PB data sets. Differences of represented tumor mass center motion as well as the amount of e.g. diaphragm motion between end-inhalation and - exhalation were negligible (max. 1 voxel).

Conclusion We presented a novel approach for proxy-free slow-pitch helical 4DCT reconstruction and illustrated its feasibility. Although the proxy-free reconstructed images contain slightly more motion artifacts, we consider the approach to be helpful especially in the case of corrupted breathing signals recordings (no need for re-scanning the patient). PO-0883 Clinical Implementation Model-Based CT to Replace 4DCT for Lung Cancer Treatment Planning D. Low 1 , D. O'Connell 1 , L. Yang 1 , J. Lewis 1 , P. Lee 1 1 UCLA Medical Center, Department of Medical Physics, Los Angeles, USA Purpose or Objective To implement motion-model based CT into clinical practice, replacing 4DCT for breathing motion management treatment planning. Material and Methods A breathing motion model that employs a mathematical motion equation, two real-time breathing surrogates, breathing amplitude and breathing rate, and employing multiple fast helical, low-dose CT scanning has been introduced into clinical practice. The imaging process uses a bellows-based system to monitor the breathin g cycle, which is defined as the amplitude and rate of the bellows signal. The fast helical CT scans are reg istered to determine the lung tissue positions, correlate d to the breathing amplitude and rate on a slice-by-s lice basis. A published motion equation is employed to characterize the motion for each voxel. The motion model is employed to reconstruct the original fast helical CT scans and the original and reconstructed scans compared to determine the overall model motion prediction accuracy. Eight amplitude-based CT images are constructed and sent to