Page 83 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

5.1 | Introduction
High-precision image-guided radiotherapy (IGRT) is extremely important in children [1–9], since anatomical deviations during the RT course can yield a miss of the tumour. A large dose deposition in the surrounding healthy tissues induces the risk of (late) side effects [10–12]. To ensure adequate tumour coverage while minimizing dose to healthy surrounding tissues, it is crucial to comprehensively quantify the geometric uncertainties.
In clinical practice, to account for these geometric uncertainties, the clinical target volume (CTV) and organs at risk (OARs) are expanded with a safety margin, defining the planning target volume (PTV) and planning OARs volumes (PRVs), respectively [13]. Solely accounting for the internal organ motion leads to the internal target volume (ITV), which includes the CTV plus an internal margin (IM), covering the entire tumour motion range. However, this leads to large margins and increased dose to healthy surrounding tissues. An alternative approach, the mid-ventilation based PTV planning, leads to smaller margins, simultaneously accounting for respiratory motion and other geometrical uncertainties [14–16].
The optimal safety margin to account for respiratory motion in children is yet unknown, since quantitative studies in children are scarce. A few studies have reported on accuracy of RT for cranial and head and neck tumours in children [1–5], indicating that appropriate patient immobilization and a safety margin <3.5 mm are sufficient to account for target motion. However, literature on extra- cranial RT is limited [6–8]. Recently, we quantified day-to-day position variation of the kidneys and diaphragm in children [7] and compared this to adults [17]. This interfractional renal and diaphragm position variation in children was notably smaller than in adults [7, 17]. However, mainly due to respiration during the treatment, abdominal and thoracic tumours are more prone to intrafractional organ motion, which limits the accuracy of RT.
Motion compensating techniques, such as breath-hold, beam gating, tracking, and abdominal compression are well studied and applied in adults [18]. Some of these techniques are not patient- friendly, require intensive training, and imply an increased workload in the clinic. Although, paediatric RT could potentially benefit from these techniques as well, children experience RT already as a stressful procedure [19–21] and these techniques may cause further distress and anxiety. Additionally, it is questionable if the youngest children (e.g. ≤ 8 years) would be able to follow a breath-hold procedure. Therefore, these techniques are currently not frequently used in paediatric RT.
Also, due to the ALARA principle (keeping doses as low as reasonable achievable), and previously reported radiation risks in children from computed tomography (CT) [22–25], four- dimensional (4D) CT is not commonly used in children. Additionally, respiratory patterns vary from day- to-day and a single 4DCT might not be a good representation for daily respiratory motion during the whole treatment course [26]. Therefore, to account for respiratory motion in children the entire range of motion should be considered when defining the safety margins. Knowledge on variability in respiratory motion is essential for adequate margin definitions, which is even more important in proton therapy than in photon therapy [27–29].
The only study on respiratory motion in children quantified renal and diaphragmatic intrafractional motion using 4DCTs and showed a strong correlation between age and diaphragm motion [6]. However, since the measurements were performed in a single 4DCT, no data were available on the variability of respiratory motion during and between multiple fractions throughout the treatment course. Also, no data were available about possible relationships between respiratory motion variabilities and patient-specific factors, such as age, height, and weight. Knowledge about such
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