ESTRO 2024 - Abstract Book


Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

deformably propagated from pCT to each pre-fraction scan and, when necessary, edited manually by the same physician who contoured the pCT. The planned doses were re-calculated on the deformed anatomy using the fiducial markers alignment center to rigidly register the pCT to each pre-fraction scan (Figure 1). Constraints of the deformed OARs were considered met at V35Gy<1 cc. When 2 pre-fraction CT scans were acquired, average V35Gy values were considered. Data were analyzed to define a threshold for EIV values: EIV above or below threshold should predict when constraints are likely to be or not to be exceeded.


All planned dose distributions met the initially required duodenum, stomach and bowel constraints (V35Gy<0.5 cc). EIV values calculated on the planning CT anatomy ranged from 0 to 21.4 cc (median 1.2 cc). Comparison between observations on pre-fraction deformed anatomy and EIV-based predictions is shown in figure 2. The recalculated doses on repeated CT scans met constraints (V35Gy<1cc) for the three OARs in 7/20 patients. An optimal threshold EIV value of 1.5 cc was identified. When EIV values for duodenum, stomach and bowel calculated on the pCT were all below 1.5 cc, dose constraints (V35Gy<1 cc) were never violated for the 3 deformed OARs contoured on pre-fraction CT scans (Figure 2). On a patient basis, 3 false positive values were observed (that is EIV> 1.5 cc but no constraint violation observed) and EIV prediction rate was 85% . Considering each OAR separately, prediction rate using a 1.5 cc threshold was the highest for duodenum (80%) and lower for stomach and bowel (75%).

Made with FlippingBook - Online Brochure Maker