ESTRO 2020 Abstract book

S942 ESTRO 2020

PO-1629 A quantitative evaluation of deformable image registration based on Halcyon MVCBCT images Y. Huang 1 , H. Wu 1 , W. Wang 1 , Y. Zhang 1 1 Key Laboratory of Carcinogenesis and Translational Research Ministry of Education/Beijing- Department of Radiation Oncology- Peking University Cancer Hospital & Institute, Department of Radiotherapy, Beijing, China Purpose or Objective The incorporable imaging dose and fractional guidance of Halcyon MV cone beam CT images (MVCBCT) inspires its potential application on adaptive radiotherapy (ART) using deformable-image-registration-based (DIR). As a precondition, however, the accuracy dependence on the deformation magnitudes and image modalities remains unknown for Halcyon, which is the aim of this study. Material and Methods Planning CT images of an anthropomorphic Head phantom were aligned rigidly with MVCBCT and re-sampled to achieve the same resolution, denoted as pCT. MVCBCT was warped with twenty simulated pre-known virtual deformation fields (VFi , i= 1 to 20) with increasing deformation magnitudes, yielding warped CBCT (wCBCT). The wCBCT was registered to pCT and MVCBCT respectively (Multi- and Uni-modality DIR), generating deformation vector fields Vi and Vi' (i= 1 to 20). Vi and Vi' were compared with VFi respectively to assess the DIR accuracy geometrically. In addition, Vi, VFi and Vi' were applied to pCT, generating three sets of deformed CT: dCTi, wCTi and dCTi' respectively. The Hounsfield Unit (HU) on these virtual CT images were compared as dosimetric impacts. Results The impact of deformation magnitudes and imaging modalities on the geometric and HU accuracy of DIR has been investigated based on the new Halcyon MVCBCT system. As deformation magnitude increases, the mean errors of vector displacement continued to deteriorate. Uni-modality DIR consistently outperformed multi- modality DIR. For deformation magnitudes between 2.82 mm to 7.71 mm, the errors of uni-modality DIR were 1.16 mm~1.73 mm smaller than that of multi-modality (p=0.0001, Wilcoxon signed rank test). Considering direct dose comparison is rather plan-dependent and patient- specific and hence is ungeneralizable, HU was used as a surrogate dosimetric indicator of DIR accuracy in this work. By applying multi/uni-modality DIR, the maximum HU deviations could be reduced from 70.8 HU /208 HU to 12 HU/ 47 HU for signed/absolute HU errors respectively. Conclusion Although it is a preferable condition that imaging dose of Halcyon MVCBCT can be incorporated into the treatment dose, DIR-based ART utilizing the noisy MVCBCT images should be applied with caution. PO-1630 Accurate 3D-rotation correction for stereotactic intracranial treatments without a 6D couch S. Kwa 1 , A. Méndez Romero 1 , M. Hoogeman 1 1 Erasmus MC Cancer Institute - Rotterdam, Department of Radiation Oncology, Rotterdam, The Netherlands Purpose or Objective Rotation correction is considered a prerequisite for accurate stereotactic treatment (SRT) of elongated or simultaneously treated multiple lesions in the brain. However, a 6D couch is not always available. The purpose of this work is to provide an effective rotation correction in fractionated SRT in the absence of a 6D couch Material and Methods Fractionated SRT was performed on a standard linac for 18 patients with benign intracranial tumors (i.e. single lesions only) in N = 28–30 fractions. The patients were immobilized with a double shell mask (Double Shell Positioning System; DSPS - MacroMedics). Inter-fraction translational and

against the original labels associated to the range shift maps. Results The analytical method enables the detection of a perturbation of the calibration curve in the soft tissue region, characterized by a positive or negative mean range shift. Setup errors in the AP directions were identified with a gradient in the AP direction, a high standard deviation and a mean range shift around zero (see Figure 1(b)). Calibration curve errors in bone and adipose tissue regions within the selected error range, as well as setup errors in the IS direction are not detectable with this method since all metrics remained around zero in their range shift maps. The CNN method can correctly detect any type of isolated error, as 100% and 99% of the testing images were correctly classified (Table 1, cases I and II). Furthermore, combinations of calibration curve with setup errors can also be identified, since 73% of the maps were correctly classified and 27% were partly classified (Table 1, case III).

Conclusion The feasibility of the range probing method within head and neck patients was demonstrated for individual and combined sources of error affecting range accuracy. This outcome provides means of range accuracy quality control in the proton treatment delivery process, with the aim to trigger decisions on plan adaptations.