Room: Room 202
Purpose: We investigated the feasibility of using intrafractional 4D CBCT reconstructed from kV images acquired with MV treatment beams to verify that the moving tumor stays inside the planning target volume during the entire delivery process, especially for lung SBRT that requires accurate patient positioning.
Methods: The XSight lung tracking phantom kit (CIRS, USA) was used for the dynamic simulation of a target by moving lung-equivalent solid epoxy rod containing a soft tissue target toward the superior-inferior direction. Four-dimensional CT and CBCT (without and with MV treatment beams delivered) were acquired using a CT simulator (Aquilion, Toshiba) and a gantry-mounted kV imager (XVI R5.0, Elekta Limited, Stockholm, Sweden). Ten-phase 4D CT was reconstructed using respiratory signals obtained by Abches, a respiration monitoring device. For intrafractional 4D CBCT, a VMAT plan for a lung SBRT patient (10 MV, flattening filter free) was delivered as quality assurance mode. Acquired 4D CBCT projections were divided into ten groups according to the target position and each phase group was then independently reconstructed using the Feldkamp-Davis-Kress algorithm. Using MIM (MIM Software, Inc.), the target was manually localized for each of ten phases. The motion range was calculated as the maximum difference in the phase-dependent target positions.
Results: The motion range of the moving target measured using 4D CT, 4D CBCT, and intrafractional 4D CBCT was 28.6, 28.8, and 29.1 mm, demonstrating the feasibility of using intrafractional 4D CBCT to detect the motion range of a moving target. The target in the phantom was clearly identified at extreme phases. However, for five out of ten phases, projections were not correctly sorted, especially at intermediate phases.
Conclusion: The intrafractional 4D CBCT can make it feasible to localize a lung tumor target, thus providing self-assurance that the actual treatment is delivered to the right area.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by Radiation Technology R&D program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2017M2A2A6A01070330).