Page 101 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

6.1 | Introduction
Precise knowledge of organ motion is extremely important for high-precision image-guided radiotherapy, aiming for an optimal balance between accurate target coverage and minimizing dose to surrounding healthy tissues. As the field of radiotherapy is expanding rapidly, with proton and carbon ion therapies, the need for high accuracy is of increasing importance [1]. This holds especially in pediatric radiotherapy, where dose to healthy surrounding tissues is associated with a highly unfavorable increased risk of developing adverse events later in life [2]. Particularly, respiratory- induced organ motion is one of the main challenges to deal with during abdominal radiotherapy. Continuous developments and research have focused on accounting for respiratory-induced organ motion in radiotherapy [3–5].
Typically, safety margins surrounding the tumor and organs at risk are determined to account for inter- and intrafractional organ motion, setup variations, and delineation errors [6–8]. In adults, pre-treatment four-dimensional computed tomography (4DCT) is often acquired to quantify respiratory-induced organ motion in order to assess the intrafractional component of the safety margin. With the 4DCT technique, the image acquisition is related to the patient’s respiration and is binned in a number of uniform respiratory phases [9]. This results in a series of reconstructed 3DCT scans representing the entire respiratory cycle, thereby encompassing the full range of respiratory- induced organ motion. However, day-to-day (interfractional) variability and irregular respiration (i.e., intrafractional variability) have shown to be limiting factors of the 4DCT technique and application for treatment planning in adults [10, 11]. First of all, 4DCT acquisition captures a single time-point while there might be variability of respiratory motion during different treatment days. Adult studies have investigated the predictive value of a single 4DCT for a variety of treatment sites and have reported both positive and negative on it [12–15]. Besides, the 4DCT images are often subject to motion artifacts mostly resulting from irregular respiration, which causes misidentification of the respiratory cycles. Although these limitations are present, 4DCT provides useful information for planning purposes and is routinely applied for highly mobile tumors in adults. However, in pediatric radiotherapy a 4DCT is not commonly applied, since the 4DCT acquisition requires extra patient training and treatment time. Additionally, a 4DCT involves a slightly higher imaging dose and due to the ALARA (keeping doses As Low As Reasonable Achievable) principle, reluctance remains to use 4DCT in the pediatric population. Nevertheless, for mobile targets in the thoracic and abdominal region, such as neuroblastomas, Wilms’ tumors and lung metastases, imaging with 4DCT might yield a more precise treatment. This lowers the risk of adverse effects, but the additional imaging should be weighed against increased imaging dose to the patient.
We recently quantified respiratory-induced diaphragm motion, as a surrogate for motion of upper abdominal and thoracic target volumes and organs at risk, in 45 children [16] and concluded that this respiratory-induced diaphragm motion was smaller and more regular in children than previously reported by another group in adults [17], indicating that a pre-treatment 4DCT could also be promising in pediatric radiotherapy. However, respiratory-induced diaphragm motion in adults was quantified using a partly different methodology in lung cancer patients [17]. Since these patients suffered from lung pathologies, it is likely that respiration was affected and comparison with our pediatric data [16] was inconclusive. A solid comparison of respiratory characteristics in children and adults that could affect 4DCT image quality and its effectiveness requires the same clinical image- guided practice, analysis and similar tumor sites, i.e. excluding adults with lung tumors.
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