Page 19 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

1.4 | Objective and outline of this thesis
Since treatment with radiotherapy significantly contributes to the risk of developing adverse events later in life of this vulnerable group of childhood cancer survivors, extremely high accuracy in radiotherapy is essential. However, geometrical uncertainties, due to interfractional position variation (i.e., day-to-day variations of the anatomy) and intrafractional motion (mainly caused by respiration), are present and limit the accuracy. To account for these uncertainties, a safety margin is added to ensure target coverage. However, in children, data on these uncertainties is lacking. Margins in pediatric radiotherapy are based on clinical experience from the radiotherapist or pragmatically translated from adult data and settings. Therefore, the main focus of this thesis is the quantification of interfractional position variation and intrafractional motion in children in order to define appropriate child-specific based margins.
Children are treated with abdominal and thoracic radiotherapy for a wide range of primary cancer diagnosis. Especially in the abdominal and thoracic area, tumors and organs are prone to motion. Moreover, the tumor can be in very close proximity to radiosensitive OARs. Therefore, in the first part of the thesis (chapters 2-4) we focus on interfractional position variation of abdominal organs. First, in chapter 2, we quantify interfractional position variation of the kidneys and the diaphragm in children in order to estimate the interfractional component of the safety margin. For comparison with adults, in chapter 3, we analyze interfractional position variation of the kidneys and diaphragm in adults and compare both groups. Additionally, we investigate the possible correlations of continuous values of age, height and weight with interfractional position variation. In chapter 4, we quantify interfractional position variation of several abdominal organs in children. To increase insight on abdominal organ position variation and in order to evaluate if close located organs could function as a surrogate for organ motion, we investigate the possible correlations of position variations between the kidneys, spleen, liver and diaphragm.
In the second part of this thesis, intrafractional motion will be investigated. The magnitude and variability of respiratory-induced diaphragm motion in children is described in chapter 5, and a pooled analysis of pediatric and adult data is performed in chapter 6. In order to assess respiratory motion prior to treatment, pre-treatment 4DCT was introduced in pediatric radiotherapy. However, studies on adult patients have indicated that respiratory motion, as measured on 4DCT, is not always representative for respiratory motion during the subsequent treatment course. Therefore, in chapter 7, we analyze whether, in children, respiratory-induced diaphragm motion on a single 4DCT can accurately predict daily respiratory motion.
From the interfractional position variation and intrafractional motion, the systematic and random errors can be calculated for the abdominal and thoracic areas, leading towards a first step in assessing appropriate pediatric-data driven margins. Moreover, for techniques like proton and carbon ion therapy, where, compared to photon therapy, the very sharp dose fall-off of protons and carbon ions are even less forgiving for anatomical variations during treatment, accounting for organ motion is paramount [65, 66]. Therefore, quantification and an extensive understanding of organ motion in children is essential for high-accuracy image-guided radiotherapy in children.
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