ESTRO 2023 - Abstract Book

S1609

Digital Posters

ESTRO 2023

Figure 1: Boxplot of planned CTV Dmax of all simulated treatments for the two adaptation strategies.

Figure 2: Accumulated dose at the end of all simulated treatments for classical margins and the adaptive strategies. In figure 1 we show the maximum planned dose (as measure of CTV dose inhomogeneity) for the different strategies. The accumulated effect can be seen in figure 2. We found that the CTV Dmin was reached in all strategies, with an increase of CTV Dmax up to 110-115% in the adaptive strategies. We also found an almost 30% reduction of the mean dose in a 1 cm ring around the CTV. Conclusion Abolishing PTV margins for the first N-1 fractions, while correcting for missed dose was feasible. CTV coverage can be ensured at the desired level, at the cost of higher dose inhomogeneity in the CTV, increasing with random uncertainty. While the radiobiology of non-constant dose prescription requires further investigation, the absence of treatment margins in the first N-1 fractions, notwithstanding the higher CTV Dmax, substantially lowers the average normal tissue dose in the first centimetre compared to a classical margin and prescription.

PO-1867 Online adaption in short-course radiotherapy reduce bowel irradiation

J.A. Hundvin 1,2 , U.H. Lilleøren 1 , J.A. Brennsæter 1 , K. Lewinsen 2 , S. Pilskog 1,2

1 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway; 2 University of Bergen, Department of Physics and Technology, Bergen, Norway Purpose or Objective The RAPIDO trial in locally advanced rectal cancer (LARC), where short-course radiotherapy (RT) was followed by intensified chemotherapy, showed a reduction in distant metastases at the cost of an increased bowel toxicity. Cone beam CT (CBCT)- based online adaptive RT (ART) has potential to spare exposure of the adjacent healthy tissue, but requires knowledge on CTV-to-PTV margins to account for intra-fraction motion. As part of a clinical study, the goal of this study was therefore to explore margins robust to intra-fraction variation during online ART and quantify differences in dose to the bowel and bladder as compared to image-guided RT (IGRT).

Materials and Methods