Page 16 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

PRV margin. In order to appropriately account for these uncertainties, quantification of abdominal and thoracic motion is essential to reach high accuracy in pediatric radiotherapy. This thesis particularly focusses on organ motion, which can be described by interfractional position variation (i.e., day-to-day variations of the anatomy) and intrafractional motion (mainly caused by respiration).
1.3 | Organ motion
With the introduction of in-room CBCT imaging the interfractional setup error is minimized. However, since the image quality of CBCT is often too poor to clearly visualize the tumor or tumor bed, a surrogate is required for the rigid registration. Therefore, the position verification is then either based on bony structure or implanted markers, which are clearly visible on CBCT images. This means that a residual error between the tumor and the surrogate remains (Figure 1.4). OARs also move relative to the surrogate. Interfractional position variation is often related to variations in bowel- and bladder filling, tumor size, patient weight and treatment-induced tissue changes. This interfractional position variation (after setup verification) needs to be determined in order to be considered when defining the CTV-to-PTV margin.
Additionally, the CTV-to-PTV margin accounts for intrafractional motion, which is mainly induced by respiration. Efforts to manage respiration, such as breath holding, gating techniques or real-time monitoring, have been introduced in radiotherapy, but reports in pediatric radiotherapy are scarce [48, 49]. Although also children could potentially benefit from these techniques, they experience radiotherapy already as a stressful procedure [36, 50, 51] and these techniques may cause further distress and anxiety. Additionally, it is questionable if the youngest children (e.g., <8 years) would be able to follow a breath-hold procedure. Furthermore, due to the ALARA principle (keeping doses As Low As Reasonably Achievable), and previously reported radiation risks in children from CT [52, 53], 4DCT is not frequently used in children. Therefore, respiratory motion in children needs to be accounted for in the CTV-to-PTV margin. In case a pre-treatment 4DCT is available, respiratory-induced motion can be measured pre-treatment. Solely accounting for the internal motion then leads to the internal target volume (ITV), which includes the CTV plus an internal margin, covering the entire respiratory-induced motion range. However, this leads to large margins and increased dose to healthy surrounding tissues. An alternative approach, the mid-ventilation based PTV planning, leads to smaller margins, simultaneously accounting for respiratory motion and other geometrical uncertainties [54, 55].
Margin recipe
Geometrical uncertainties are a superposition of a systematic and a random error, and form the basis for the CTV-to-PTV margin recipes [56–60]. Systematic errors originate in the treatment preparation phase and therefore affect all treatment fractions. Random errors occur arbitrarily and could have a different effect each single fraction [58], effectively blurring the dose distribution. Thus, the effect of the systematic error on the dose distribution is more significant than that of random errors. Interfractional position variation and intrafractional motion have both systematic and random components. However, when a 4DCT is acquired to assess respiratory motion pre-treatment, the intrafractional motion will only add to the random deviation.
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