SU-FF-J-81: A Feasibility Study for Real-Time Tomosynthesis-Guided Rapid Arc Therapy

Abstract

Purpose: To investigate the feasibility of real‐time mis‐alignment correction in Rapid Arc treatment and design a corresponding tomosynthesis acquisition protocol. Method and Materials: A CT image set of an anthropomorphic pelvic phantom was used in the study. Simulated projection images were produced to resemble a simultaneous kV fluoro in Rapid Arc treatment. A modified Feldkamp algorithm was used to reconstruct the tomosynthesis images. Various combinations of imaging reconstruction parameters including scan angle, angular interval, and slice thickness (mm) were tested: 1) 60°, 6°, 2.4; 2) 60°, 6°, 0.8; 3) 60°, 3°, 2.4; and 4) 30°, 3°, 2.4. A predefined 5 mm displacement in all three orthogonal directions modeled patient motion during treatment. After each successive tomosynthesis acquisition, registrations were performed between current reconstructions and reference images. The phantom position was corrected accordingly by shifting the treatment couch. Residual errors and their root mean square (RMS) values were recorded for evaluation. Results: The residual errors (L‐R, A‐P and S‐I directions in mm) for the 4 schemes after the first tomosynthesis acquisition were (1.2, 2.5, −0.2), (−1.2, 1.1, 0.0), (1.1, 1.9, 0.0) and (−1.2, 3.1, 0.0), and the corresponding RMSs were 1.6, 0.9, 1.3 and 1.9 respectively. The RMSs after a full arc delivery were 0.7, 0.5, 0.5 and 0.7. All schemes tested accurately corrected displacement in the SI direction after first acquisition. Scheme 2 performed better than scheme 1 at the expense of more computation time. By doubling projection numbers in scheme 1, scheme 3 improved correction ability in the L‐R direction. With a smaller 30° scan angle, scheme 4 was acceptable and will be improved after several acquisitions. Conclusions: Tomosynthesis scans can be used for real‐time mis‐alignment correction in Arc therapy after 30° gantry rotation.

CT reconstruction from portal images acquired during volumetric-modulated arc therapy

“…We note that how and why VMAT shows such effectiveness is not yet well understood (Webb and McQuaid 2009) and not all the potential equipment options are yet implemented and optimized (Webb 2010). A number of authors have discussed image-guidance in relation to VMAT, by means of twodimensional portal images (Burman et al 2009), pre-treatment kilovoltage CT (Bedford et al 2008), simultaneous kilovoltage CT (Nakagawa et al 2009) or tomosynthesis (Zhou et al 2009). Megavoltage images acquired during treatment with an electronic portal imaging device (EPID) have been considered as a planar image guidance tool (Liu et al 2009), a geometrical quality-assurance method (Yi et al 2009), or a dose-measurement method in two (Iori et al 2010) or three dimensions (Mans et al 2010).…”