ESTRO 2024 - Abstract Book

S5545

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

correction. The setup isocenter was carefully chosen to ensure the clipbox includes a joint, ensuring the reliability of the match. An IGRT traffic light system was used to indicate when repositioning was necessary in case of a rotation exceeding 3 degrees and/or the target is outside the PTV. Following the patient's repositioning, a new CBCT scan was acquired for online correction. In this retrospective study, 21 fractions of 8 patients were examined. All CBCT matches resulted in translations in the left-right (LR), anterior-posterior (AP) and cranio-caudal (CC) directions and rotations over the LR (pitch), AP (yaw) and CC (roll) axes. The correction reference point (CRP) was located in the center of the CTV. Translations mentioned in this abstract are values after "Convert To Correction" since only translations were corrected online. When rotations are not corrected, the long target can rotate outside the PTV at greater distances from the CRP. The effect of the rotations was calculated, using the rotations found in the match and the distance between the CRP and the furthest boundaries of the CTV.

Results:

For 6 out of 21 fractions, repositioning of the patient was deemed necessary following the IGRT traffic light system. Therefore the rotation results are categorized into three groups: Group A was positioned once (n=15), Group B1 required repositioning (n=6) and Group B2 consists of the CBCT scans acquired after repositioning (n=6). Image 1 shows the absolute Pitch, Roll and Yaw rotations for group A, Group B1 and Group B2.

Table 1 shows the calculated effect of the rotations at the furthest point from the CRP. As shown below, Group B1 has an average LR discrepancy of 13.62 mm (± 7.16 mm) due to Yaw rotation, exceeding the 8 mm margin, therefore Yaw rotation is the main cause for repositioning. Combined, Group A and B2 have 3 outliers with discrepancies exceeding the 8 mm margin by a maximum of 1.7 mm.