Room: Exhibit Hall | Forum 6
Purpose: To demonstrate feasibility of using mobile CT for in-room image guidance to improve soft-tissue target localization in external beam radiation therapy.
Methods: A mobile CT (AIRO, Brainlab AG, Germany) was driven into a linac treatment room and positioned beside the treatment table. With the table column rotated 90 degrees, the mobile CT was used to acquire a set of volumetric images (CTmobile) of a pelvis phantom. Thereafter, the couch was rotated back to the default position and an intermediate CBCT image was acquired (CBCTinter). The acquired images as well as a planning CT image set (CTplan) were transferred to in-house sequential registration software based-on an open source image registration engine (plastimatch). CTmobile was rigidly registered to CBCTinter using mutual information to geometrically correlate the mobile CT with the treatment coordinates. Thereafter, to calculate couch shifts, CTmobile was registered to CTplan in two steps: (1) automatic rigid registration with mean square error metric; (2) manual fine tuning based on soft-tissue matching. To investigate possibility of imaging dose reduction, the CBCTinter images were acquired with a default setting (~700 projections) and a fast scan setting (~350 projections).
Results: The pelvis phantom was successfully localized with the mobile CT in conjunction with the intermediate CBCT scan. The couch shift discrepancies between the in-house and the commercial software CBCT software (XVI, Elekta, UK) were 0.67Â±0.41 mm / 0.16Â±0.07Â° and 0.50Â±0.28 mm / 0.18Â±0.10Â° for the default and the fast CBCT scan settings, respectively.
Conclusion: In-room image guidance performed with mobile CT is feasible if combined with a daily CBCT imaging used for the intermediate registration. It allows for better visualization and localization of low-contrast soft tissue targets. By lowering the number of CBCT projections, the radiation dose from the intermediate CBCT scan can be significantly reduced while maintaining overall registration accuracy.