ESTRO 2021 Abstract Book

S1289

ESTRO 2021

cohort of head-and-neck patients with daily CBCT images to assess the influence of the treatment execution uncertainty on the efficacy of daily adaptation. Virtual daily CT images were generated by deformable image registration of the planning CT to daily CBCT images. The �� i uncertainty is dominated by the robot’s accuracy that we conservatively assumed to be 1 mm. For the second uncertainty �� m , we considered three scenarios with �� m =1, 2, and 3 mm. Therefore, the total treatment execution uncertainties were �� =1.41, 2.24, and 3.16 mm. Those were used to generate random Gaussian-distributed offsets corresponding to displacements occurring after daily plan adaptation but before the dose delivery. The offsets were applied to each axis posterior-anterior, left-right, and inferior-superior. For online plan adaptation, we used our fast GPU Monte-Carlo algorithm. Results Results showed no significant differences in accumulated DVHs and dose distributions for �� =1.41 mm and 2.24 mm (figure 1). Offsets with �� =3.16 mm resulted in underdosage to high- and low-risk CTV, more hot spots of considerable volume, and noticeably higher doses to OARs. For high-risk CTV, the mean D 98 was found to be 97.9%, 98.0%, 96.7% for �� =1.41 mm, 2.24 mm, 3.16 mm, respectively, and 97.9% for online adaptation without offset. Figure 2 shows the evolution of the high-risk CTV coverage over the course of a 32- fraction treatment. Only the offset corresponding to �� =3.16 mm and the un-adapted base plan are far from meeting the clinical goal of D 98 =95% for several fractions.

Figure 1: Comparison of accumulated DVHs for un-adapted base plan, online adaptation (OA) without offset and OA with �� =2.24 mm.

Figure 2: Evolution of D 98

for high-risk CTV.

Conclusion If the total treatment execution uncertainty due to imaging with CT on rails does not exceed 2 mm, the efficacy of daily adaptation is not significantly affected. Given other advantages of fan-beam CT, such as lower whole-body dose and better image quality, CT on rails can be considered a good alternative to CBCT for adaptive proton therapy.

PO-1565 Evaluation of online AI-driven adaption for palliative radiotherapy A. Kejda 1 , S. Wong 1 , T. Eade 1,2 , J. Booth 1,3 , T. Schuler 1,4 , S. Roderick 1

1 Northern Sydney Cancer Centre, Royal North Shore Hospital, Sydney, Australia; 2 Sydney Medical School, University of Sydney, Sydney, Australia; 3 School of Physics, University of Sydney, Sydney, Australia;