ESTRO 2021 Abstract Book

S1275

ESTRO 2021

the pCT, sCT, iCBCT_clin and iCBCT_dev of twelve patients. Dose calculations on both CBCT reconstructions were compared to the corresponding results for the pCT and sCT, using the specific calibration curves. Resulting dose distributions were evaluated by gamma analysis, using a 2% / 2 mm criteria, as well as comparison of dose volume histogram (DVH) parameters. Results Calibration curves revealed good agreement between CT and CBCT, with a median root-mean square deviation of 24 HU. For the anthropomorphic phantom, the CT to CBCT dose comparisons resulted in gamma passing rates greater than 98.8% for all acquisition modes. For the patient data, dose profiles and DVH comparisons between CT and CBCT dose calculations were in general in agreement. Dose distributions calculated on the iCBCT_dev images had a median gamma passing rate of 93.5% and 97.1% for the pCT and sCT dose comparisons, respectively. Corresponding iCBCT_clin calculations had a median gamma passing rate of 88.7% and 93.1% for pCT and sCT, respectively (Table 1). The median gamma passing rates differences were not statistically significant. Overall, both DVH and gamma analysis indicated improvements in dose calculations using the artifact improved CBCT reconstruction.

Conclusion This study has demonstrated the feasibility of using CBCT images for direct dose calculations in an oART workflow; comparable to pCT and sCT in terms of calibration data and resulting dose calculations. Further improvements to CBCT reconstructions may support this even further, as demonstrated for the motion artifact reduced CBCT reconstruction. However, further studies with an extended pool of patient cases are required to confirm these initial results. PO-1551 Towards range-guidance in proton therapy to detect organ motion induced dose degradations K. Busch 1 , A.G. Andersen 1 , J.B. Petersen 2 , S.E. Petersen 1 , H.S. Rønde 1 , L. Bentzen 3 , S. Pilskog 4,4 , P.S. Skyt 1 , O. Nørrevang 1 , L. Muren 1 1 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 2 Aarhus University Hospital, Medical Physics, Dept of Oncology, Aarhus, Denmark; 3 Aarhus University Hospital, Dept of Oncology, Aarhus, Denmark; 4 Haukeland University Hospital, Dept of Medical Physics and Oncology, Bergen, Norway Purpose or Objective Internal organ motion and deformations may cause dose degradations in proton therapy (PT) that are challenging to resolve using conventional image-guidance strategies. The aim of this study was to investigate the potential of range -guidance using water-equivalent path length (WEPL) calculations to detect dose degradations occurring in PT. Materials and Methods Proton ranges were estimated using WEPL calculations. Field-specific isodose surfaces in the planning CT (pCT), from robustly optimized five-field proton plans (opposing lateral and three posterior/posterior oblique beams) for locally advanced prostate cancer patients, were used as starting points. WEPLs to each point on the field-specific isodoses in the pCT were calculated, whereafter the corresponding range for each point was found in the repeat CTs (rCTs). The spatial agreement between the resulting surfaces in the rCTs (hereafter referred to as iso-WEPLs) and the isodoses re-calculated in rCTs were evaluated for different dose levels and Hausdorff thresholds (2-5 mm). Finally, the sensitivity and specificity of detecting target dose degradation (V95%<95%) using spatial agreement measures between the iso-WEPLs and isodoses in the pCT was evaluated. Results The spatial agreement between the iso-WEPLs and isodoses in the rCTs depended on the Hausdorff threshold. For a 2 mm threshold the agreement was 65-88%, for 3 mm it was 83-96%, for 4 mm it was 90-99% and for 5 mm it was 94-99%, across all fields and isodose levels (Figure 1). Small differences were observed between the different isodose levels investigated. Target dose degradations were detected with a sensitivity of 82-100% and a specificity of 75-80% using a 2 mm Hausdorff threshold for the lateral fields (Figure