Page 161 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

3.3, 1.9 mm, respectively. Our results showed that kidney position variation was the largest in the CC direction, suggesting that margins should be applied anisotropic ally rather than isotropically. Overall, interfractional position variation did not correlate with patient-specific factors and variation between patients was found to be large. This suggested that individualized margin approaches might be required.
As margins for pediatric patients are usually pragmatically translated from adult data, comparison of our results with those reported on adults in literature showed that interfractional position variation was seemingly smaller in our pediatric cohort than in adults. However, caution in interpretation of this difference was warranted concerning differences in the methodology. Therefore, to yield a straightforward comparison we pooled pediatric and adult data of patients treated at our institute in chapter 3. All patients, including 35 children and 35 adults, were analyzed with identical methodology performed by the same observer as in chapter 2. Outcomes of both groups were compared, and patient-specific factors were observed as continuous values to investigate possible correlations between age, height, weight and organ position variation. Generally, Σ and σ values in the three main directions for the children were significantly smaller than those for the adults, especially in the CC direction (mean difference 3.1 mm). However, organ position variation and patient-specific factors were only negligibly correlated. Nevertheless, these results indicate that the size of the margins should potentially be different for children than for adults.
If localization or quantification of tumor position variation is unfeasible, an anatomical structure close to the target could function as a surrogate. It is therefore crucial to have a clear understanding of the correlation of organ position variation between the tumor or organ and a particular surrogate. Therefore, to increase insight in abdominal and thoracic areas, in chapter 4, we quantified interfractional position variation of the diaphragm, liver, spleen and both kidneys in 20 children. We evaluated possible correlations between abdominal organs and determined whether organ position variation is location-dependent, and if this had possible consequences for margin values. No (strong) correlations between interfractional position variations of abdominal organs in children were observed. We concluded that diaphragm dome position variations could be more representative for superiorly located abdominal (liver, spleen) organ position variations than for inferiorly located (kidneys) organ position variations. Differences of Σ and σ between abdominal organs were small (<2 mm), suggesting that for margin definitions, there was insufficient evidence of a dependence of organ position variation on anatomical location.
Besides interfractional position variation, intrafractional motion is an important component of the CTV-to-PTV margin and is the focus of the second part of this thesis. Especially, since in children, motion management techniques to minimize breathing motion, such as breath holding, are not widely used. Additionally, respiratory-induced organ motion is often not known prior to treatment in children and therefore needs to be accounted for in the CTV-to-PTV margin. In chapter 5 we described a retrospective study in which we analyzed respiratory-induced diaphragm motion in 45 children using daily or weekly acquired cone-beam CT (CBCT) scans. Variabilities within and between fractions were analyzed and possible correlations between respiratory-induced diaphragm motion and age, height, and weight, were investigated. Over all patients, interfractional variability was smaller than intrafractional variability. We found large ranges of respiratory-induced diaphragm amplitude motion (range 4.1-17.4 mm) and cycle time (2.1-3.9 s), indicating substantial differences between patients.
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