ESTRO 2021 Abstract Book

S1458

ESTRO 2021

For pediatric patients, no standardized margins are defined since studies on these geometrical uncertainties are relatively scarce. Therefore, we aim to present a literature overview of organ motion, variations in applied margin sizes, and delineation variability in patients <18 years. Materials and Methods A systematic search in MEDLINE, Embase, Web of Science, ClinicalTrials.gov and the International Clinical Trials Registry Platform from 2000 to 2020 resulted in 5102 studies. Using predefined in- and exclusion criteria, study selection based on title/abstract and full text followed by data extraction was performed independently by three authors. Overall, 107 studies were included reporting on organ motion (N=16), margin sizes (N=89), and delineation variability (N=2). Besides reported motion, margin sizes, and delineation results, data on variables such as age, tumor types, use of general anesthesia (GA), and image (guidance) modalities were extracted. Results Studies were heterogeneous concerning age, tumor types, GA, and imaging modalities. Use of image guidance techniques (CBCT, orthogonal kV port films) was reported in 16% of the studies. Overall, cranio-caudal inter- and intrafractional motion as reported for different organs ranged from -9.1 to 10.0 mm and -4.4 to 19.5 mm, respectively (Table 1). Motion quantification methodologies differed between studies concerning measures of displacement (e.g. center of mass, border of organs) and definitions of motion direction. GTV-CTV and CTV-PTV margin sizes were reported for thoracic, abdominal, spinal and pelvic sites for different primary tumor types and metastases, treated with different radiation techniques (e.g. CRT, IMRT, VMAT, protons). GTV-CTV margins ranged from 5 ‒ 20 mm for thoracic and abdominal targets, and from 1 ‒ 10 mm and 5 ‒ 15 mm for spinal and pelvic targets, respectively (Figure 1). CTV-PTV margins for thoracic and abdominal targets varied from 3 ‒ 20 mm, and for spinal and pelvic from 3 ‒ 15 mm and 3 ‒ 10 mm, respectively. Most studies specified one margin size, however, in 24% of the studies a range of margin sizes was reported. Delineation studies assessed inter-observer variability of target volumes and showed that variability in delineated target volumes affected dose distributions to both target volumes and organs at risk.