Page 68 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

Statistical Analysis
For each patient, organ specific mean and standard deviation (SD) of the interfractional position variation relative to the bony anatomy were determined in the three orthogonal directions, and in the CC direction only for the right and left diaphragm domes. Furthermore, over all patients, we estimated per organ the group mean (i.e., mean of the individual means), the group systematic error (Σ; the SD of the individual means of all patients), and the group random error (σ; the root mean square of the individual SDs of all patients).
To evaluate whether organ position variation is location-dependent, we compared contralateral and superiorly and inferiorly located abdominal organs separately (indicated in Figure 4.1). Since not all data fitted a normal distribution (tested with the Shapiro-Wilk’s test), differences between contralateral organs’ systematic and random errors, were separately tested (i.e., right diaphragm dome vs. left diaphragm dome, liver vs. spleen, right kidney vs. left kidney) with the Levene’s test (for Σ) and Mann-Whitney U-test (for σ). Also, differences in Σ and σ between superiorly and inferiorly located abdominal organs were tested (i.e., liver vs. right kidney, spleen vs. left kidney). Since differences were tested in fourteen combinations (i.e., LR, CC, AP for four organs, and CC only for both diaphragm domes), we adjusted p values according to the Bonferroni correction. Differences were considered to be significant if test outcomes showed a p value<0.004 (i.e., 0.05/14).
We used the Spearman’s correlation test (significance level p<0.05) to investigate the possible correlations in position variations between contralateral organs.
Additionally, to test if right- and left-sided organ position variations in the CC direction are related to the position variations of the superiorly located right- and left-sided diaphragm dome respectively, we used linear regression analysis (significance level p<0.05). Both tests were also performed for each individual patient.
All statistical analyses were done using R version 3.2.1. (R Foundation for Statistical Computing, USA).
Table 4.2 | The group systematic (Σ) and group random errors (σ) in mm in the orthogonal directions for the right kidney, left kidney, liver, and spleen and in CC direction for the diaphragm.
(mm)
Group mean
Σ 1.4 2.8 0.9 1.1 3.3 1.3 2.1 3.4 2.7 2.2 3.5 2.7 3.0 3.4 σ 1.6 2.2 2.4 1.3 2.9 1.4 1.8 2.8 1.9 2.8 3.0 2.7 3.6 3.4
Abbreviations: LR = left–right; CC = cranial–caudal; AP = anterior–posterior
Right Kidney Left Kidney Liver Spleen
LR CC AP LR CC AP LR CC AP LR CC AP CC CC
Right Left
Diaphragm
Diaphragm
-0.6 0.7 -0.4 0.4 0.4 -0.4 0.4 -0.1 1.0 0.8 0.8 0.0 1.2 1.8
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