Page 29 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

2.2 | Methods
Patient population
This retrospective multicenter cohort consisted of 39 patients, treated between 2006 and 2014, for whom a reference CT and at least 5 CBCT images of the upper abdomen and/or thorax were available. The median age at diagnosis was 8 years (range 1.6-17.8 years). The median height at diagnosis was 124 cm (range 77-184 cm). The most frequent cancer diagnoses among children younger than 8 years were neuroblastoma (n=11) and Wilms’ tumor (n=5). In children older than 8 years, Ewing sarcoma (n=7) was the most prevalent malignancy. Most (37/39) patients were treated in the supine position and 2 patients were treated under general anesthesia. A full overview of patient characteristics and treatment details, including tumor indication, number of CBCTs, and treatment location is listed in Table 2.1.
Reference CT and CBCT imaging
For each patient, a pretreatment CT scan (120 kV, 2.5- or 5 mm slice thickness) was obtained according to institution-based standard protocols for planning purposes. This CT scan was considered as the reference for organ position (refCT). According to institution-based protocols, all patients received half-fan width CBCT imaging during their treatment (Synergy, Elekta Oncology systems, Crawly, UK). The timeframe varied between approximately 35-60 seconds and the degree of circumferential rotation of the CBCT image was 200 or 360 degrees. For each patient, a CBCT was acquired at the first three treatment fractions, followed by daily or weekly CBCTs throughout the treatment depending on the tumor type and its corresponding, institution dependent, treatment protocol. CBCTs were either used for offline verification protocols or enabled direct set-up verification through online registration with the refCT. In this study, we retrospectively analyzed the imaging data, including a total of 39 refCTs and 527 CBCTs, ranging from 5 to 33 CBCTs per patient. For some patients one or both kidneys or diaphragm were not evaluable (i.e., organ not included in image, due to nephrectomy, or insufficiently visible due to artifacts) on the CBCT, resulting in 25 right kidneys (291 CBCTs), 28 left kidneys (331 CBCTs), and 32 diaphragms (422 CBCTs) included in our analyses (Table 2.1).
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