ESTRO 2021 Abstract Book

S1491

ESTRO 2021

1 Södersjukhuset, Department of Oncology, Stockholm, Sweden; 2 Söderjukhuset, Department of Oncology, Stockholm, Sweden; 3 Karolinska Institutet - Södersjukhuset, Department of Clinical Science and Education, Stockholm, Sweden Purpose or Objective To evaluate the accuracy of our clinical implementation of the Catalyst (C-RAD, Uppsala, Sweden) surface- guided radiation therapy (SGRT) system in the positioning of patients undergoing radiotherapy (RT) of breast A total of 63 treatment fractions for 23 breast cancer patients who received breast RT in free breathing at our institution were included in this study. A Wing Board TM (CIVCO Radiotherapy, Iowa, USA) was used to immobilise the patients for all treatment fractions. The patients were positioned with the help of Catalyst, without the use of skin markers. To align the patient with the treatment isocenter, the live surface was matched to the reference surface, which was selected as the external contour used during treatment planning. The reference surface was cropped to include only the thoracic region together with parts of the upper arms and chin. The setup error during positioning was reduced to within +/- 3.0 mm for 3D translations and +/- 3.0° for rotations in the three orthogonal directions before proceeding with CBCT-acquisition. A CBCT image was thereafter acquired, and the necessary setup corrections were calculated by the XVI Synergy system (Elekta AB, Stockholm, Sweden), based on a bony anatomy registration between the CBCT and the planning CT images. The obtained translations and rotations were converted into translation corrections only. These were thereafter sent to the linac console and the table was moved accordingly, prior to treatment delivery. A schematic representation of the treatment flow is shown in Figure 1. To quantify the level of agreement between the Catalyst and the XVI systems, the rotational corrections for the three rotational axes (pitch, rotation and roll) calculated by the Catalyst system (prior to the CBCT-acquisition) were compared to the rotational corrections calculated in the CBCT-CT image registration. A two-sample t -test, with a significance level of 0.05 was performed. cancer in free breathing. Materials and Methods

Figure 1. Schematic representation of patient workflow from positioning to RT treatment delivery.

Results There were no significant differences between the rotational corrections indicated by the Catalyst system and the corresponding corrections obtained in the CBCT-CT image registration for pitch ( p = 0.65), rotation ( p = 0.86) and roll ( p = 0.73), see Figure 2.