Page 104 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

Diaphragm Tracking
Identical to the methodology used for the pediatric group [16] an adapted version of the Amsterdam Shroud (AS) method [17, 18] was used to track diaphragm motion from CBCT imaging. For each CBCT, a two-dimensional AS image was created. Along the horizontal axis of this image, representing the projection images, we manually selected the projection images corresponding to end-exhale and end- inhale positions of the diaphragm. In each of those selected projection images, we then manually determined the cranial-caudal (CC) position of the top of the diaphragm (i.e., corresponding to the peak-to-peak position variation). Subsequently, the pixel coordinate corresponding to the position of the top of the diaphragm was translated to millimeters relative to the patients’ planned isocentre, by including a magnification correction to account for the difference in scale between the imaging panel and the isocentre [19]. Additionally, we corrected for the geometry of the CBCT scanner [19]. This resulted in a respiratory pattern describing the CC position of the diaphragm in end-exhale and end- inhale phases over the course of CBCT acquisition (detailed overview shown in Supplementary Figure 6.1).
Respiratory analysis
The amplitude was defined as the difference in CC position of the diaphragm between end-exhale and end-inhale phases. The cycle time described the time between two consecutive end-inhale positions. Day-to-day variation was expressed as interfractional variability (i.e., the SD over mean amplitudes from each fraction), and irregular breathing was expressed as intrafractional variability (root mean square of the SDs from each fraction).
For the adult patients, we calculated the same parameters as in our pediatric study; mean amplitude, interfractional variability, and intrafractional variability (see schematic overview; Supplementary Figure 6.1). For the whole patient group, including both children and adults, we calculated the group mean amplitude by averaging the patients’ mean amplitude, the group interfractional variability by averaging the patients’ interfractional variabilities, and the group intrafractional variability by averaging the patients’ intrafractional variabilities. Calculations of these respiratory parameters were also computed for the cycle time.
Statistical Analysis
Since not all data fitted the normal distribution (tested with the Shapiro-Wilks test), differences in mean amplitude, mean cycle time, and inter- and intrafractional variabilities in children and adults were tested for significance with the Mann-Whitney U test, considering p<0.05 significant. This comparison also provides insight into possible explanations on respiratory-induced motion based on continuous values of age, height and weight. Therefore, we used the Spearman’s correlation test (significance level p<0.05) to test for possible relationships between respiratory-induced diaphragm motion parameters and patient-specific factors (age, height, and weight). For the pediatric group separately, we tested with the Mann-Whitney U-test (significance level p<0.05) whether respiratory parameters of children treated under GA (n=7, age range 2-11 years) differed from children treated without GA in a similar age range (n=12, age range 3-10).
101