Page 63 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

4.1 | Introduction
Continuous developments in pediatric cancer treatment using multimodality strategies, including surgery, chemotherapy, and radiotherapy have led to increasing numbers of childhood cancer survivors [1]. Inevitably, the occurrence of treatment associated adverse events has also increased. Treatments including radiotherapy significantly contribute to the risk of developing adverse events.
Children are treated with abdominal and thoracic radiotherapy for a wide range of primary cancer diagnoses, including Wilms’ tumor, neuroblastoma, and Ewing sarcoma. Moreover, treatment of the craniospinal axis and lung metastasis involve irradiation of the abdominal and thoracic region. The anatomical locations of these tumors and adjacent organs at risk (OARs) vary; target volumes can be in very close proximity to the lungs, diaphragm, liver, spleen, and kidneys. As a result, healthy tissues and OARs are unavoidably exposed to radiation when irradiating the tumor [2, 3]. Although adequate tumor dose coverage is the primary goal in radiotherapy, sparing the vital and long-term functions of adjacent organs is also of great concern. Especially in children, who have a relative long life expectancy when surviving cancer, organs are still growing and have low tolerance to radiation [4, 5]. To ensure adequate tumor dose coverage while minimizing radiation dose to surrounding healthy tissues, knowledge about the extent of target and organ motion, particularly present in the abdominal and thoracic area, is needed. Thus, quantifying the motion of vital and sensitive organs such as the liver, spleen, and kidneys is essential.
These abdominal organs move with every breathing cycle (intrafraction motion) and from day- to-day (interfraction motion). Intra- and interfractional motion of the tumor and OARs are incorporated by expanding the clinical target volume and OARs volumes to the planning target volume (PTV) and planning risk volumes (PRVs), respectively [6]. In adults, many studies have quantified motion of various organs, enabling to define accurate margins for PTVs and PRVs. Despite the increasing number of publications on pediatric organ motion [7–15], data is still limited and no consensus has been reached in pediatric radiotherapy to define PTV or PRV margins for abdominal tumors or OARs. Therefore, PTV margins for children are currently pragmatically based on available adult data and PRV margins are often not used in pediatric radiotherapy. Due to different anatomical locations (e.g., right vs. left side of the abdomen, (retro)peritoneum, adjacent to the vertebrae), or abdominal processes (e.g., intestinal peristaltic or air pockets), abdominal organ motion might be location-dependent, as was discussed before in Van Dijk et al. [14]. This could lead to differences in PTV and PRV margins depending on the anatomical location.
The most commonly used PTV margin recipe is from van Herk et al. (2.5 ∑ + 0.7 σ), where the systematic (∑) and random (σ) component are based on quadratically adding the systematic/random
𝟐𝟐𝟐𝟐𝟐𝟐𝟐𝟐
errors that occur during treatment (√∑ + ∑ and √𝝈𝝈 + 𝝈𝝈 ) [16]. Previous studies
𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊 𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊 𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊
𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊
mainly reported on intrafractional organ motion, focusing on respiratory-induced abdominal organ
motion through various phases of the breathing cycle as measured on a single four-dimensional
computed tomography (4DCT) [9, 11, 15] or 4D magnetic resonance imaging (4DMRI) [12, 17].
Although organ motion seems to be more prone to respiratory motion than to day-to-day position
abdominal organ motion was larger than intrafraction motion (∑ 𝑎𝑎𝑎𝑎𝑎𝑎 𝜎𝜎 > ∑ 𝑎𝑎𝑎𝑎𝑎𝑎 𝜎𝜎 ) 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖
variations, Guerreiro et al. showed that in a homogenous group of 15 children, interfractional
the systematic error was found to be smaller than the random error (∑
to indicate that the systematic component of the PTV and PRV margins is predominated by the day-
[15]. In addition, Huijskens et al. showed that for respiratory-induced diaphragm motion in children
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< 𝜎𝜎 ) [8]. This seems 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖