Page 17 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

Population-based margins are derived from these errors, which are usually determined for specific treatment sites and patient groups. For each patient, a mean error and standard deviation (SD) is measured over a number of fractions. The CTV-to-PTV margin recipe is typically formulated as 2.5 Σ +0.7σ, where the systematic error (denoted as Σ) is the square root of the quadratic sum of standards deviations (SD) of the individual means of the errors and the random error (denoted as σ) is the square root of the quadratic sum of the root mean squares of the individual SDs of the errors [57, 59]. Similar margin definitions are also formulated for OARs to define adequate planning risk volumes (PRV) [31].
In adults, many studies focus on quantification of the organ motion in order to define accurate population-based margins for specific tumor sites [61]. Since childhood cancer is a rare disease, the small population makes it difficult to derive population-based margins for children. More importantly, childhood cancer patients are a highly diverse group, varying from infants to adolescents with different heights and weights. Therefore, margins for children should be defined with more distinction. Additionally, achievements in radiotherapy are mainly focused on adult patients and pragmatically translated and implemented into a pediatric setting [62]. Data on appropriate margin sizes in pediatric radiotherapy is lacking, and clinically used margins are mainly based on experience of the radiotherapist or knowledge based on available adult data. Also, since children are smaller than adults and body compositions differ, it is expected that organ motion also differs. With the introduction of IGRT in children, imaging data has become available to quantify organ motion in children and thus a first step towards being able to derive adequate margins for children. Besides, the benefit of IGRT includes pre-treatment position verification and possibly reduces PTV expansions and dose to OARs [63, 64]. However, in institutes where IGRT is frequently used in children, there was notably variability in PTV expansions for different tumor sites [46]. This stressed the need for a consensus regarding appropriate margin definitions in children, as was also recommended by the PROS [35].
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