Abstract Book

ESTRO 37

S532

RT can be used for accurate dose calculation in proton therapy for prostate cancer. Material and Methods Ten prostate cancer patients with rigidly aligned CT and 3T MRI data acquired in RT position within less than 2.5 hours were included in this study. A bulk-assignment method with a constrained bone shape model was used for sCT generation from a T1w gradient echo MRI scan with Dixon reconstruction [1]. As addition to the photon solution, an algorithm for detecting internal air on only the MRI data was implemented. Still, in order to minimize the impact of inter-scan differences in sCT and CT, internal air cavities were copied from CT to sCT during data evaluation (Figure 1). Body contours of both data- sets were matched by cropping and filling operations. Opposing field IMPT treatment plans optimized on CT were recalculated on sCT for comparison in terms of gamma-index analysis and clinically relevant target and OAR DVH parameters using the same structure set on CT and sCT. The proton range in beam’s eye view (BEV) was compared utilizing single field uniform dose (SFUD) plans.

Conclusion Results suggest that MR-only proton dose calculation of comparable accuracy to CT is feasible using a fully automatic bulk-assignment sCT generation algorithm originally designed for photon RT. Adaptation of the assigned bone sHU values and accurate localization of internal air was required. Acknowledgments DFG-MAP, Deutsche Krebshilfe, ZonMw IMDI. Ethical protocol 15-444/C. [1] Helle et al., 2014, Proc. Int. Soc. Magn. Reson. Med. PO-0967 Quality assurance of 4DCT in the EORTC Lungtech trial on SBRT for patients with NSCLC M. Lambrecht 1 , J.J. Sonke 2 , U. Nestle 3 , H. Peulen 1 , D. Weber 4 , M. Verheij 2 , C.W. Hurkmans 1 1 Catharina Hospital, Department of Radiation Oncology, Eindhoven, The Netherlands 2 The Netherland Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands 3 Kliniken Maria Hilf, Department of Radiation Oncology, Mönchengladbach, Germany 4 Paul Scherrer Institute- ETH Domain, Center for Proton Therapy, Villigen, Switzerland Purpose or Objective To provide an overview of the 4DCT acquisition methods and achievable accuracy to image tumour volumes in institutions participation in the EORTC SBRT Lungtech trial. Material and Methods 3DCT and 4DCT images were acquired of a 008A CIRS phantom with 2 spherical inserts of 7.5 and 12.5mm diameter placed inside a cylinder of lung equivalent material using the institutional scan protocols. Regular asymmetric (cos 6 (t)) tumour motion was simulated with amplitudes A=15mm and periods t=3s and 6s and with A=25mm and t=4s. All CT scans were imported centrally in a treatment planning system to enable auto-contouring of the reconstructed sphere. Knowing the exact volume of the sphere, an institution dependent HU threshold was determined on the static CT images. This threshold was thereafter used for auto contouring the 4D-CT datasets. Acquisition parameters were saved and volume and amplitude deviation were assessed in comparison to the static volume. Results Acquisition parameters were rather comparable over the 11 institutions analysed (table 1). However, substantial inter-institution variations were found in the observed volume deviations. Average volume deviations for the 12.5mm sphere were 14.6%(-3.5% to 68.5%) at end of inspiration, 12.2%(0.05% to 36.0%) at mid-ventilation and 1.6%(-1.6% to 9.0%) at end of expiration. For the smaller 7.5mm sphere deviations were 13.1%(-98.7% to 64.8),

Results Optimization of the assigned synthetic Hounsfield Unit (sHU) values of soft and cortical bone, using data of two exemplary patients, was crucial to yield satisfying proton range agreement on CT and sCT. Accurate localization of internal air was also critical. After adapting the bone sHU and copying internal air from CT to sCT, a mean difference in SPR of 0.1±0.3% (1σ) over all patients was found comparing CT and sCT adp in the region receiving at least 10% of the prescribed dose. The mean proton range difference for the SFUD plans was 0.1mm. More than 96% of all analyzed dose profiles in BEV had a range agreement better than 3mm (Figure 2). The average pass- rate for a (2%,2mm) gamma criterion with 10% dose threshold was 98±1%. Mean differences in all considered DVH parameters were below 1Gy (1.5%).