Page 28 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

2.1 | Introduction
Continuous developments of more effective multimodality treatment strategies for pediatric cancer have led to a steep increase in the number of childhood cancer survivors during the last decades [1]. Inextricably, with the enhanced cancer survival, the incidence of treatment-related adverse events has become evident. Particularly, treatment with radiation therapy (RT) significantly contributes to the risk of developing adverse events [2, 3]. This underscores the need for extremely high accuracy in treatment planning and actual dose delivery to the target volume, to minimize dose to surrounding healthy tissues.
Organ motion is a well-known phenomenon that limits the maximum achievable accuracy. Organ motion is induced by respiration (intrafractional motion) or day-to-day variety of anatomical deviations (interfractional motion) related to e.g., variations in bowel- and bladder filling, tumor size, patient weight and treatment-induced tissue changes [4].
To account for treatment uncertainties, of which organ motion is the most challenging to deal with, the clinical tumor volume (CTV) is extended with an isotropic margin, thereby defining the planning target volume (PTV) [5]. Similar margin definitions are also taken into consideration for organs at risk (OAR) to define adequate planning risk volumes (PRV) [5]. The margin size is based on the systematic and random errors [6, 7] and has extensively been studied for adults [8–10]. A few studies have reported on pediatric organ motion, mainly focusing on cranial RT [11–15]. This treatment set-up allows for adequate patient immobilization. Moreover, intra-cranial organ motion is substantially smaller than that in thoracic-abdominal organs. Only two published studies quantified extra-cranial organ motion, with focus on intrafractional organ motion using 4D computed tomography (CT) and interfractional organ motion using Cone Beam CT (CBCT) [16, 17]. Panandiker et al. found a correlation between patient-specific factors including age and height and intrafractional renal motion [17]. No reports are available on relations between these factors and interfractional organ motion. To date, no clear guidelines are available for defining appropriate margins for pediatric abdominal RT. Children seem to differ from adults with respect to organ motion, and margins currently used may not be optimal in pediatric RT. Therefore, adequate data on organ motion in children is urgently needed in order to define patient-specific margins with the aim to decrease irradiated volume and minimize dose to healthy tissue thus reducing the risk of developing adverse events [2, 3].
Image-guided RT (IGRT) enables quantification of interfractional organ motion, using registration of CBCTs to the reference CT. Since the kidneys are representative for abdominal organ motion and kidneys are also considered as the primary dose-limiting organs for upper abdomen radiotherapy [18], here we focus on renal motion. To correlate interfractional renal and diaphragmatic motion we also analyzed the diaphragm.
In this study, our goal was to quantify renal and diaphragmatic interfractional motion in order to estimate the systematic and random errors and to investigate the possible correlation between interfractional motion and patient-specific factors.
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