ESTRO 2021 Abstract Book

S1490

ESTRO 2021

1).

Materials and Methods To date, 76 fractions of 10 patients (median age 57y) immobilized with an open face mask from Brainlab AG (Munich, Germany) or IT-V (Innsbruck, Austria) have been evaluated. The results of intrafractional X-ray corrections for all 3 translations and 3 rotations were compared with the mean surface/thermal motion in all 6 dimensions, recorded during the time between the start of X-ray acquisition and end of X-ray registration. X- rays were acquired for each treatment beam, at possible gantry positions 0°, 90°, 180°, and 270°. We calculated mean, standard deviation, standard error of mean, and the 95% confidence interval of all 10 patient datasets. Results A total of n=328 X-ray deviations were analyzed and compared to surface/thermal imaging positions. Table 1 shows the relatively small deviations between X-ray corrections and surface/thermal imaging monitoring, which were all in the sub-millimeter range for translations and below 0.5° for all rotations. It should be noted that the values for surface/thermal corrections were updated and set to zero after each X-ray image, and when the X-ray deviations exceeded a certain threshold, the 6 dimensional tabletop was adjusted accordingly. Therefore, a direct analysis of overall effects, such as motion drifts was not possible. The differences between surface/thermal and X-ray imaging are expected to be limited, as patient motion is restricted by the immobilization of the open masks, as can be seen in the evaluation of the X-ray shifts in table 1.

Conclusion The analysis of intrafractional motion has shown very small differences between optical/thermal surface scanning and X-ray imaging for this relatively small patient cohort. Due to the novelty of the system, a further evaluation of these datasets and an extension to other treatment sites is planned. The data already suggest that for specific patient cases, the number of X-ray images taken for monitoring purposes could be reduced (e.g. every 180°) due to the stability and accuracy of the optical surface/thermal imaging algorithm.

PO-1764 Evaluation of the setup accuracy of a surface scanning system in radiotherapy of breast cancer J. Knutsson 1 , G. Johansson 1 , L. Dagertun 2 , M. Olin 1 , A. Siegbahn 1,3