Positional uncertainties

Figure 2 shows the definitions of positional uncertainties of the CoIM and external markers. Positional repeatability was determined by the difference between the reference and zero positions in positional verification and each BH session. The reference position of the CoIM was represented as the position from zero which was equivalent to the planned isocenter. The reference position of the external marker was represented as a position from zero which was defined as the most posterior position in the first respiratory cycle of the RPM signals under free breathing reacquired after the CBCT scan.

Positional uncertainties of the centroid of the internal marker and external marker. The zero position of the external marker was defined as the most posterior position in the first respiratory cycle in the Real-time Position Management signals under free breathing, reacquired after cone-beam computed tomography.

Positional variation was defined as the absolute displacement from the reference marker position during each BH session under MV beam delivery.