Abstract Book

S1122

ESTRO 37

photon and particle treatments. Moreover, the generated 4DCTs can be directly used as uncertainty scenarios to feed robust optimizers in commercial TPS. EP-2050 Geometrical uncertainties of humerus and breast in breast cancer radiotherapy. J.T.J. Honkanen 1 , M. Loukkola 1 , J. Seppälä 1 1 Kuopio University Hospital, Cancer Center, Kuopio, Finland Purpose or Objective This study aimed to determine the geometrical uncertainties of humerus and breast during radiotherapy treatment course of breast cancer (BCa) patients and compare two patient fixation devices. Material and Methods Changes in the daily positioning of BCa patients were investigated by comparing low-dose CBCT images with planning CT images with two patient fixation devices. Patients ( n =40) treated with whole breast irradiation including axillary lymph nodes were aligned using either C-Qual breastboard with arm and wrist support ( n =21) or C-Qual M breastboard with Monarch overhead arm positioner ( n =19) (CIVCO). The patients were treated with conventional fractionation (50 Gy/25 fr) using a VMAT technique and daily CBCT image guidance (Elekta Infinity/XVI). CBCT images were co-registered with the respective planning CT in six degrees of freedom with a box shaped region-of-interest (ROI) using maximization mutual information (MMI) or Chamfer registration algorithm (Mosaiq v2.62, Elekta AB). For each patient, the co- registrations were computed for 4-9 treatment fractions (182 and 160 fractions in total for C-Qual and C-Qual M patients, respectively) using two different ROIs covering 1) the PTV and 2) proximal humerus (Fig. 1). The orthogonal shifts and rotations (direction of the rotation was omitted) were recorded and compared between the two fixation devices.

However, when the co-registration was conducted based on the proximal humerus, transversal rotations were significantly larger with C-Qual (2.2±2.0°) than with C- Qual M (1.7±1.5°) breastboard. 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).

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

Fig 1. Planning CT (magenta) and CBCT (green) images were co-registered based on the mutual information using two different ROIs (delineated with yellow line) covering ( A ) the target volume and ( B ) the proximal humerus. Results The average orthogonal shifts were -3.0±4.0 mm, - 1.2±3.9 mm and -1.6±4.5 mm for PTV and -1.3±5.1 mm, - 1.6±6.3 mm and -2.1±6.3 mm for humerus in longitudinal, lateral and vertical directions, respectively. Significant differences in orthogonal shifts between the two breastboards were found in lateral and vertical directions with both ROIs (Table 1). Moreover, the lateral shifts were larger with C-Qual M breastboard but the vertical shifts were larger with C-Qual breastboard. The average rotations were 0.9±0.7°, 1.0±1.0° and 1.0±0.9° for PTV and 2.0±1.7°, 2.1±1.7° and 2.0±1.8° for humerus in coronal, sagittal and transversal planes, respectively. Transversal rotations were significantly larger with C-Qual M when the co-registration was conducted using ROI covering the target volume.