ESTRO 2021 Abstract Book

S692

ESTRO 2021

translations for pre-CBCT, and translations >1.0mm for mid-CBCT. Pre- and mid-CBCT were repeated if correction exceeded 3.0mm. Translational systematic and random error and rotation data was compared between TC and CC groups. Results A total of 108 fractions and 261 CBCTs were analysed between March and December 2020. Repeated CBCT, related to correction >3mm, was performed in 35 out of 108 fractions (TC 21/53, CC 14/55) for pre-CBCT and in 0/16 for mid-CBCT. Absolute translation per fraction and mean translation per treatment plan were comparable in all directions in both TC and CC groups. The mean differences in systematic error between TC and CC were <0.4mm. Similarly, in all directions, no significant differences were found in random error (describing day-to-day variation). The observed absolute rotational intrafraction motion was <0.5 degrees in both groups. Differences between groups for both rotational motion per fraction and mean rotation per treatment plan did not exceed 0.07° (p>0.5). (Table 1) Mean time interval between pre- and post-fraction CBCT was 8 minutes, and performing a mid-fraction CBCT prolonged this interval to 14 minutes. In both groups, with prolongation of fraction duration, no increase in translational intrafraction motion was observed. (Figure 1)