ESTRO 2023 - Abstract Book

S183

Saturday 13 May

ESTRO 2023

dose calculations were then compared to dose calculations on the pCT with the relative dose difference between the calculations in specific regions of interest (ROIs) as the metric. The investigated ROIs were the clinical target volumes (CTV) and eight organs-at-risk (OARs) used to calculate the NTCP models scores, like the submandibular and parotid glands (see figure 1). A second comparison was done by calculating the Normal Tissue Complication Probabilities (NTCP) for dysphagia and xerostomia, on the rCTs and the same-day sCTs to investigate the clinical relevancy of possible dose differences. Results The relative mean dose difference found in the CTV 70 Gy was 0.1±0.3% in the pCT/rCT and -0.2±0.9% in the pCT/sCT group. Figure 1 shows the relative difference per OAR, where the largest disagreement in mean relative difference is found in the left parotid gland with 1.6±9.6% (pCT/rCT) and 6.9±11.6% (pCT/sCT). The best agreement is found in the superior pharyngeal constrictor muscle with mean relative differences of 1.8±10.9% (pCT/rCT) and 4.0±12.2% (pCT/sCT). Figure 2 shows the NTCP score difference between the rCT and same-day sCT calculations. The mean difference of the overall NTCP score was found to be -1.6±2.8%. The shifts towards positive dose difference in the OARs, especially the submandibular and parotid glands, complies with the minor NTCP score increase.

Conclusion This study showed the potential of sCTs for dose calculations in daily treatment robustness evaluation and online adaptive proton therapy of HN cancer patients. In comparison to rCTs, only minor differences were observed. Further investigation is required to identify where the differences originate from and if they are relevant for clinical decision making. PD-0248 Can surface imaging predict the impact of anatomical deformations on proton breast treatment? V. Hamming 1 , D. Cannavò 1 , B. Strbac 1 , G. Guterres Marmitt 1 , J.H. Maduro 1 , J.A. Langendijk 1 , N.M. Sijtsema 1 , S. Both 1 1 University Medical Center Groningen, Radiotherapy, Groningen, The Netherlands Purpose or Objective Currently proton breast cancer patients receive weekly repeat CTs for treatment robustness evaluation purposes, often showing that the original treatment remains robust. Therefore, the goal of this feasibility study is to assess if breast surface anatomy deformation can predict the need for repeat CT acquisition and plan adaptation in proton breast cancer patients. Materials and Methods A phantom test and seven proton breast cancer patients with 109 daily Surface Guided RadioTherapy (SGRT) images in treatment position were included in this proof of principle study. The phantom test consisted of imaging (CT and SGRT) a mannequin with known deformations of 5;10;15;20;25 and 35mm. In the Treatment Planning System (TPS), a proton plan was created for this phantom based on a cylindrical target. The plan was subsequently re-calculated on the CT sets with the deformations. Firstly the SGRT images were rigidly registered to the BODY contour from the planning CT by an iterative closest point algorithm employing a novel in-house developed tool. Secondly, a landmark based deformation analysis was performed to determine the local deformation. The magnitude of the local deformation was defined as the ratio of the maximum and the minimum vector length between landmarks, captured in the value T. Hence, T is a unique estimator whose value is close to 1 for similar shapes and increases with deformation present. Within the phantom test the correlation between T and the dose difference with respect to the original CT was determined for the CTV dose-volume parameters D99, D98, D95 and mean dose. For the clinical patients, deformations of 0 to +20mm were applied to the BODY contour at the location of the target area to determine the sensitivity of the tool and similar analyses were performed as for the phantom. Results Five landmarks were used, one was placed around the sternum as a reference point, whereas the other four landmarks were placed inside the field of view of the SGRT images and within the target area. Figure 1 shows two examples of the BODY including landmarks, the corresponding registration between the BODY and the SGRT image and the resulting difference in landmark position. For the phantom test, the correlation coefficient of T with the known deformation was 0.91, with the D99, D98 and D95 it was 0.99 and with the mean dose it was 0.82. For the patient group, Figure 2 shows the