Page 162 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

Moreover, no clinically significant correlations were found between amplitude and cycle time, and patient-specific factors, confirming that respiratory-induced diaphragm motion is patient-specific and requires an individualized approach to account for. Since interfractional variability was small, we suggested that a pre-treatment 4DCT in children could be sufficiently predictive to quantify respiratory-induced organ motion.
However, only few studies on 4DCT in children are available in literature, implying that 4DCT is not widely used in pediatric radiotherapy. In adults, 4DCT is extensively used. However, day-to-day (interfractional) variability and irregular respiration (intrafractional variability) have shown to be limiting factors of 4DCT effectiveness. In order to evaluate the applicability of a 4DCT in children, in chapter 6, we retrospectively included 90 patients (45 children and 45 adults) and quantified inter- and intrafractional variability of respiratory motion. Using daily/weekly acquired CBCT imaging, identical methodology, as used in chapter 5, was used to extract respiratory-induced diaphragm motion and calculate the inter- and intrafractional variabilities. This pooled analysis enabled a solid comparison to reveal the effectiveness of 4DCT application for planning purposes in pediatric radiotherapy. The mean amplitude was slightly smaller in children than in adults (10.7 mm vs. 12.3 mm). Overall variability was smaller in children than in adults, indicating that respiratory motion is more regular in children than in adults. This implied that a single pre-treatment 4DCT could be a good representation of daily respiratory-induced organ motion in children and could be at least equally beneficial for planning purposes as it is in adults.
However, studies on adult patients have indicated that respiratory motion, as measured on 4DCT, is not always representative for respiratory motion during the subsequent treatment course. Therefore, a single pre-treatment measurement for planning purposes might be a misrepresentation and could lead to under- or overestimating respiratory motion, yielding insufficient target coverage or undesired dose to OARs. Therefore, in chapter 7, we investigated the predictive value of 4DCT in 12 children. We analyzed whether respiratory-induced diaphragm motion in children on a single pre-treatment 4DCT can accurately predict respiratory-induced diaphragm motion as observed on daily CBCTs. We also analyzed possible time trends in respiratory-induced diaphragm motion over the complete treatment course. In 9 out of 12 patients, we observed a significant difference between the amplitude measured on 4DCT and the amplitude obtained from the daily/weekly CBCTs. The overall pre-treatment amplitude was smaller than the amplitude on CBCTs (10.4 mm vs. 11.6 mm). No obvious time trends in respiratory-induced diaphragm motion over the course of treatment were found, but significant baseline shifts of the diaphragm position were present in 7 out of 12 patients. These findings suggest that respiratory-induced diaphragm motion as measured on 4DCT was not representative for respiratory motion during the treatment course in children. Our results show the limitations of using a single pre-treatment 4DCT to take the patient-specific respiratory motion for treatment planning purposes into account. Therefore, regular monitoring of respiratory motion with CBCTs could yield a higher accuracy when a treatment plan is adapted to the actual breathing amplitude when necessary.
Small pediatric patient cohorts and large variation in childhood cancer types, subtypes, and treatment sites make it difficult to derive population-based margins for children. Next, the variety in age, height, and weight and the lack of correlation of organ position variation and respiratory motion with these patient-specific factors, make margins based on age or height not (yet) feasible. Nevertheless, we have summarized our results with the available literature (chapter 8), which form the basis of a first
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