Page 125 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

whether diaphragm positions on CBCT significantly differed from the baseline diaphragm position on the averaged 4DCT.
Additionally, six patients received multiple CBCTs during one fraction. For above mentioned analysis, only the first CBCT was included in the analysis. To validate if respiratory-induced diaphragm motion measurements on CBCTs could be predictive for respiratory-induced motion that continues post-acquisition (i.e., the actual respiratory motion during dose delivery), we compared the amplitude measured on the first and second CBCT (ACBCT(1) to ACBCT(2); paired t-test, significance level p<0.05). These are acquired within 4-10 minutes, which is a representative time interval between CBCT acquisition and start of dose delivery.
R Software package version 3.2.1. (R foundation for statistical Computing, Austria) was used for all statistical analysis.
7.3 | Results
Over all patients, the mean A4DCT was 10.4 mm (SD = 4.3 mm) and the mean ACBCT was 11.6 mm (SD = 5.7 mm). For 9 out of 12 patients, A4DCT differed statistically significantly (p<0.05) from ACBCT (Figure 7.2). Underestimation of A4DCT compared to ACBCT was found in 76% of the measurements (95/125 CBCTs), and was observed in 11 out of 12 patients. Hence, overestimation was found in 24% of the measurements (30/125 CBCTs), and was observed in 3 out of 12 patients. Differences >3 mm were found in 69 of the 125 CBCTs (55%).
For each patient, we plotted ACBCT over time of the treatment course (Supplementary Figure 7.1) where day 0 is the day of 4DCT acquisition. We found that 8 of the 12 trend lines had a negative slope. Absolute slopes larger than 0.1 mm/day were observed in 4 patients with only few data points (4 to 6 points for patients 4, 7, 8, and 11). For the other patients, with more data points, we observed by both visual inspection and linear fits, no obvious time trend (absolute slopes ranged from 0.00-0.09 mm/day).
Overall, interfractional variability of ACBCT was 2.2 mm (range 0.7-4.4 mm; individual values shown in Figure 7.3). For 7 out of 12 patients, averaged diaphragm positions in CC direction observed on CBCTs differed statistically significantly (mean 7.4 mm, SD = 5.9 mm; p<0.05) from the baseline diaphragm position as measured on the averaged 4DCT (Figure 7.3).
Patients 3 and 4 had on multiple days additional CBCT scans. Patients 5, 6, 7, and 11 had only on one day an additional CBCT. The subsequent CBCTs were acquired within a time interval of 4-10 minutes. Over all six patients, ACBCT(2) was significantly different from ACBCT(1) (mean difference 2.9 mm, SD = 2.5 mm, p=0.002) (Figure 7.4). However, patient 4 (in Figure 7.4 indicated by the green cross symbol) showed significant deviations from the group measurements. We performed a sensitivity analysis by excluding this patient from the analysis. Although the average difference was now 1.7 mm (SD = 1.4 mm), ACBCT(2) remained significantly different from ACBCT(1) (p=0.033).
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