Page 42 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

3.1 | Introduction
As a result of improved treatment modalities, long-term childhood cancer survival has increased remarkably with five year survival rates approaching 80% [1]. Concurrently, increased survival is accompanied by late treatment-related adverse events, especially when radiotherapy has been part of the treatment [2–4]. Developing the optimal radiation treatment plan involves a delicate tradeoff between tumor coverage on the one hand and sparing surrounding healthy tissues (organs at risk; OARs) on the other hand. The radiation treatment volume is determined by the planning target volume (PTV), which is an expansion of the clinical target volume (CTV), thus accounting for geometrical uncertainties due to daily anatomical variations (interfractional organ position variation), breathing (intrafraction motion) and delineation errors [5]. All these components are considered when calculating PTV safety margins using the group systematic (Σ) and the group random error (σ) [6]. Likewise, planning-organ-at-risk volume (PRV) margins can be calculated to avoid OARs [7].
Since pediatric cancer is a rare disease, and data on organ motion in children is scarce [8], safety margins as currently used in pediatric radiation treatment are mainly based on margins as defined for adults. Abdominal organ position variation and its impact on margin size for different tumor sites have so far only been reported for adult patients [9–14] and pediatric patients separately [15– 17]. In our previous study, we quantified renal and diaphragmatic interfractional motion in children [17]. Comparison of our results with those reported in literature [12] showed that organ motion was seemingly smaller in our pediatric cohort than in adults [17]. This suggests that adult-based safety margins might be too large for pediatric patients. However, to date no comparative studies have been performed in a cohort including both children and adults. In this study, we aim to quantify interfractional renal and diaphragmatic position variation in children and adults, thus enabling a straightforward comparison of results.
3.2 | Methods and Materials
Patient population
For this retrospective study, we included children (<18 years) and adults (≥18 years) who had been treated at the AMC radiation oncology department between October 2010 and December 2014. Patients were included if a pretreatment CT scan and at least five cone beam CT scans (CBCTs) of the upper abdomen and/or thorax were available for registration; five registrations were considered to enhance the reliability of the statistical analyses [18]. Thirty-five children who met these criteria had been had been diagnosed with a variety of pediatric tumors (Table 3.1); a part of these data (19/35 children) have been analyzed previously [17]. Based on the inclusion criteria, we randomly selected 35 adults who had been treated in the same time period for esophageal, gastric and pancreatic tumors. We collected information on general anesthesia, and patient characteristics including age at the first radiation treatment fraction, height, weight, primary cancer diagnosis and radiation site.
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