ESTRO 37 Abstract book

S1125

ESTRO 37

Table 1. Orthogonal shifts and rotations (mean±SD) for C- Qual and C-Qual M breastboards when the co-registration of planning CT and daily CBCT was conducted using two different ROIs (PTV and proximal humerus).

current setup. Monitoring the patient in the bore is theoretically feasible according to our results, but we suspect inclusion of a minimal angle between the surface ROI and a camera ray will prohibit monitoring during treatment. Further refinement of the camera-model, such as the inclusion of two stereovision sensors, will be Based on a geometric simulation study and ceiling camera position optimization, surface tracking systems shows promise for positioning of thorax patients for O-ring gantry linac systems. performed. Conclusion

Conclusion Non-systematic deviations of several millimeters were found in the orthogonal shifts with both breastboards and co-registration ROIs. Hence, daily image guidance for patient positioning is suggested to minimize positioning errors. EP-2051 Surface scanner camera position optimization on the Varian Halcyon™ O-ring gantry linac system L. Delombaerde 1,2 , S. Petillion 1 , T. Depuydt 1,2 1 University Hospital Gasthuisberg, Department of Radiation Oncology, Leuven, Belgium 2 KU Leuven - University of Leuven, Department of Oncology, Leuven, Belgium Purpose or Objective The access to the patient for setup inside the bore of O- ring gantry linear accelerators, such as Varian’s Halcyon, is limited. 3D surface scanning technology can provide a solution for patient setup in a pseudo-isocenter some distance in front of the gantry but requires a direct view of the patient. In this study we investigated the viability of using ceiling mounted surface scanners for patient positioning at the bore entry of a Halcyon linac. Material and Methods At our institution the patient surface is monitored using AlignRT (VisionRT) which uses stereophotogrammetry from 3 ceiling mounted camera pods. Currently, the AlignRT isocenter is located 1 m from the radiation isocenter. To determine if monitoring at the Halcyon laser pseudo-isocenter at 57.5 cm is possible, a simulation environment was created including geometric models of the gantry and patients. The Halcyon was modelled using building plans and manual measurements to accurately represent the curvature at the front of the bore. Five patients (thorax and breast indications) were selected on basis of the longitudinal range of the planning CT. The body contours were segmented in a 3D modelling environment. For every patient a region of interest (ROI) surface was determined in which the thorax, the upper arms, chin and lateral sides were included. The ROI was positioned at 3 locations: at 1m from the radiation isocenter (current setup of AlignRT), at Halcyon laser positions (pseudo-isocenter at 57.5cm from true isocenter) and at the radiation isocenter in the bore (Fig.1). For every position of the ROI, optimal camera positions were calculated maximizing the ROI coverage while penalizing high overlap between 2 or 3 cameras using a ray tracing algorithm. Results were averaged over all patients and optimized locations were compared to the currently installed configuration. Results The current positions are adequate for patient positioning at 1 m from the isocenter, however suboptimal for surface monitoring at the Halcyon pseudo-isocenter. Due to the close proximity of the camera pods to the linear accelerator O-ring gantry the upper part of the ROI (upperarms, neck and chin) can no longer be monitored by the system (Fig. 2). The simulated optimal positions are for pod 1 and 2: lat -/+174cm, vert +150cm, long +142cm and for pod 3: lat 0cm, vert +190cm, long +230cm from the radiation isocenter. The optimal position shifts the pods ±46cm longitudinally from the

EP-2052 On-line dose-guided proton therapy to account for inter-fractional motion: a proof of concept K. Busch 1 , L. Muren 1 , A. Andersen 1 , J. Pedersen 1 , L. Dong 2 , S. Thörnqvist 3 , J. Petersen 1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark