Page 14 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

Treatment planning
At the start of the radiotherapy process, a pre-treatment computed tomography (CT) scan of the patient in radiation treatment position is made for treatment planning purposes. The target (i.e., tumor) and surrounding OARs are delineated on this planning CT scan, which is usually a three- dimensional (3D) scan, consisting of two-dimensional images that are taken along the cranial-caudal direction. The scan yields a short acquisition time over repeatedly changing anatomy during respiration, thereby not providing information on breathing motion that sometimes causes motion artefacts in the image. This is especially of concern in the abdominal and thoracic region, where target volumes and OARs are more prone to motion than in the cranial area or in extremities. For abdominal and thoracic target locations, the pre-treatment 3DCT scan is therefore more often replaced by a four- dimensional CT scan (4DCT) [25–27], where the complete respiratory cycle, from end-inhalation to end-exhalation, is divided into typically 10 phase scans. For treatment planning purposes, either the averaged 4DCT scan, or other strategies including all phases or one phase scan, are used.
The radiation oncologist delineates the gross tumor volume (GTV), representing the macroscopic tumor visible on the planning CT [28]. However, in children, the tumor is often surgically removed before radiation treatment. For those cases, the target volume (i.e., tumor bed) includes the preoperative extent of disease or the residual macroscopic disease, and is delineated based on diagnostic information, preoperative imaging and surgical reports or clips (if present). The GTV is expanded with a margin to account for microscopic tumor growth, defining the clinical target volume (CTV). To account for geometrical uncertainties, such as a delineation error and anatomical variations, a margin is added to the CTV, resulting in the planning target volume (PTV) [28–30]. Similar margin definitions are also taken into consideration for OARs to define adequate planning risk volumes (PRVs) [31]. In the planning phase, a treatment plan is created including the delivery technique, describing the direction, intensity, and shape of the radiation beams.
Treatment delivery
Radiotherapy courses are typically delivered in several fractions. Before delivery of each treatment fraction, patients are positioned on the treatment table identical to the position during the acquisition of the planning CT. This is done by aligning the in-room laser system with the pre-treatment applied skin marks. However, this positioning provides no information on actual internal anatomical variations relative to the skin marks. This led to the introduction of image-guided radiotherapy (IGRT), including an in-room kilovoltage (kV) or megavoltage (MV) imaging system integrated to the linac (Figure 1.3) [32]. The most used form nowadays is (kV) cone beam CT (CBCT) imaging, enabling pre-fraction 2D imaging of the internal anatomy, which is subsequently reconstructed to a 3D image and registered to the planning CT for set-up verification [33]. Usually, bony anatomy serves as a surrogate of the target for the rigid registration, because tumor visibility is difficult due to poor soft tissue contrast of the CBCT image (Figure 1.4). According to the resulting translations and rotations, the patients’ position is corrected ideally to the position identical to the one during the acquisition of the planning CT.
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