Page 84 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

relationships might be beneficial for developing a more individualised approach and precise margin definitions.
Daily or weekly cone-beam (CB)CT scans acquired during IGRT for position verification enable quantification of inter- and intrafractional organ motion. However, methodologies differ in large extent. For the quantification of respiratory motion the 2D fluoroscopic projection images of the CBCT scans are used. Several methods using 2D projection images for quantification of respiratory motion have been investigated in multiple adult-based studies [30–33]. The Amsterdam Shroud (AS) method is a frequently used and reliable method for detection of respiratory-induced diaphragm motion [32, 34, 35]. Our study is the first to extract the respiratory signal of children from 2D projection images using an adapted version of the AS method, thus enabling the assessment of the variability of respiratory motion within and between fractions throughout a paediatric treatment course.
The purpose of this study was to quantify and analyse respiratory-induced diaphragm motion, since the diaphragm is well visible in CBCT images and the respiratory-induced motion of the diaphragm is assumed to correlate with upper abdominal and thoracic target volumes and OARs. Additionally, our relatively large patient number (n=45) enabled investigation of possible correlations between respiratory-induced diaphragm motion and aforementioned patient-specific factors.
5.2 | Methods
Patient data
We retrospectively analysed the CBCT data of 45 children who received IGRT between December 2010 and May 2016. Patients were included when the diaphragm was visible on upper abdominal or thoracic CBCTs. Mediastinal surgery, causing diaphragmatic dysfunction, was an exclusion criterion because the remaining diaphragmatic motion was minimal and therefore not representative for normal free breathing. Median age at the start of RT was 11 years (range 2–18 years). Median height was 148 cm (range 90–186 cm). Most patients (43/45) were treated in supine position and 7 children (age range 2–11 years) were treated under general anaesthesia (GA). Immobilization devices are not used in our institute for abdominal and thoracic RT. A full overview of patient characteristics and treatment details, including tumour indication and RT location is listed in Table 5.1.
CBCT acquisition
For each patient, CBCTs (Synergy, Elekta Oncology systems, Crawly, UK) for position verification were acquired for the first three treatment fractions, followed by daily or weekly CBCTs according to an accustomed extended no-action level (eNAL) protocol [36], and corresponding to tumour type and patient-specific needs. The acquisition parameters for all CBCTs were 120 kV, 10 mA and 10 or 40 ms exposure time per projection. The circumferential rotation varied from 200 to 360 degrees and the timeframe varied between 35 s and 120 s, resulting in a variation in number of projection images per CBCT (180 to 760). A single projection was acquired in 180 ms. In this study, we included a total of 480 CBCTs (median 7, range 4-32 CBCTs per patient).
81