Page 147 - Organ motion in children for high-precision radiotherapy - Sophie Huijskens

(4D-) CBCT
To take full advantage of IGRT, acquisition of daily CBCT imaging is essential. More importantly, the process of ART relies on daily imaging. However, the additional imaging dose from CBCT imaging and increased risk for radiation induced adverse events is of great concern in children, with their sensitivity to radiation and long life expectancy. Efforts to reduce imaging dose have resulted in low dose CBCT protocols [72–74]. The design allows for considerable dose reduction, thereby avoiding unnecessary toxicity without compromising image quality for the registration accuracy of bony structures (Figure 8.1). However, poor soft tissue contrasts and lower image quality could hamper the registration of surgical clips or abdominal organs, which might be interesting for both research and clinical purposes. Lower doses also result in fewer 2D frames, which hamper the use of automatic methods to track intrafractional diaphragm motion as used in [75]. For potential online adaptive procedures including breathing motion, as suggested in chapter 7, sufficient image quality and number of projection images is crucial. Such a respiratory-correlated 4D-CBCT technique [76] requires additional imaging time and dose, but would allow for online verification of moving structures prior to treatment and adjustment of planning when needed. To overcome these limitations, different reconstruction methods are actively studied [77–79]. However, in chapter 7 we also showed that respiratory-induced diaphragm motion can vary within minutes and thus, motion as measured on a 4D-CBCT can again differ from actual motion during dose delivery.
Figure 8.1 | Example of optimizing imaging dose in pediatric CBCT imaging in the lumbar region. Exposure parameters used are 40ms, 32mA with a normal gantry rotation speed of 0.5 rpm (a) and 10ms, 10mA and a mimicked high gantry rotation speed (1.0 rpm) by reconstruction of the CBCT image with half of the projection frames (360 vs. 180 frames), resulting in imaging doses of approximately 2.0 cGy (left) and 0.2 cGy (right). Figure adapted from [72].
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