Page 15 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

Next, the patient needs to remain alone in the treatment room during radiation dose delivery. For children, being separated from their parents, can cause additional anxiety, distress and inability of lying still [34–38]. To prepare children mentally and physically for this procedure, children are invited for a rehearsal session and are thereby guided by a specialized pediatric nurse who supports the child throughout the complete treatment course. During the rehearsal session, the child (and parents) experience a real treatment preparation. The steps of patient positioning and a dummy-session of dose delivery are undertaken, without the fear and pressure of actual treatment, to increase comfort and familiarity with the treatment room and procedure. Sometimes, in more complicated situations, children are immobilized in a vacuum matrass and some children, usually younger patients (< approximately 5 years [39, 40]) require general anesthesia (in the form of sedation).
After proper patient-positioning, irradiation with high-energy X-rays starts. According to the planned beam directions, intensity and shape, the gantry rotates around the patient and delivers the radiation dose. In the past, static beams from typically two to four directions were used, resulting in high doses to the target volume but also dose to a large volume of surrounding healthy tissues. To create a high dose area more conformal to the PTV, a multi-leaf collimator (MLC) was introduced to enable beam shaping to anatomical structures in order to match the shape around the tumor whilst avoiding the surrounding OARs [41]. This is known as 3D-conformal radiotherapy (3D-CRT). The MLC can also control and modulate the intensity of the beams, achieving a specific dose profile, which is so-called intensity-modulated radiotherapy (IMRT) [42]. Dose delivery by continuously modulating the shape and intensity of the beam, while the gantry rotates around the patient, is called volumetric modulated arc therapy (VMAT) [43]. Although these newer delivery techniques potentially improve conformity and spare OARs, the introduction of these delivery techniques are moving very slowly into the field of pediatric radiotherapy.
Image Guided Radiotherapy
IGRT, by using an in-room imaging device for patient-positioning, has played a crucial role over the last decades and has contributed to higher accuracy in radiotherapy [44]. However, image acquisition for IGRT purposes adds to the dose from therapeutic treatment and could contribute to the risk of late adverse events [45]. Especially in the pediatric population, who have higher susceptibility to radiation compared to adults, the limitation of additional dose has to be considered. Moreover, the increased treatment time for IGRT may also increase the risk of motion. Therefore, use of IGRT in children merits specific attention, but it was slowly integrated in pediatric radiotherapy [45]. The first publication evaluating clinical practice of pediatric IGRT dates from 2014 [46]. Among international multi- institutes, IGRT was commonly used but consensus in applied procedures for any given tumor type was lacking [46]. Efforts to reduce imaging dose have resulted in low dose protocols, thereby avoiding unnecessary toxicity without compromising image quality [47]. Although reports on pediatric IGRT are steadily increasing, little is known on exact numbers of current practice of pediatric IGRT. In our institute, pediatric IGRT was introduced in 2010. The acquisition of CBCT imaging did not only allow for position verification, but also enabled to retrospectively investigate the anatomical variations that could occur during the treatment course (Figure 1.4). Tumors and OARs located extra-cranially are more prone to anatomical changes than when they are located intra-cranially. These anatomical variations could limit the accuracy of the treatment plan and are accounted for in the CTV-to-PTV and
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