Early Experience with Mobile CT Used for External Beam Radiation Therapy Simulation and in-Room Image Guidance

Abstract

To demonstrate feasibility of using mobile CT for CT-simulation and in-room image guidance for external beam radiation therapy (EBRT). An electron density (ED) phantom was scanned using mobile CT to create a ED-HU table for TPS commissioning. A custom lock-bar was fabricated to allow for attachment of radiation therapy immobilization devices to the mobile CT table. To address larger table sag of the mobile CT compared to that of conventional CT simulators, a bubble level was attached to the inferior end of the table so that an operator could correct the table rotation (pitch and roll) after a patient is loaded. Due to the lack of external moving lasers, a traditional three points-setup was utilized for CT simulation. To test feasibility of in-room image guidance using mobile CT, the mobile CT was detached from its table and moved into a linac treatment room where it was positioned beside the linac couch. With the column rotated 90 degrees, the mobile CT was used to acquire a set of volumetric images (CTmobile). Thereafter, the couch was rotated back to the default position and a reference CBCT image was acquired (CBCTref). The first rigid body image registration was performed to register CTmobile to CBCTref, thus placing the CTmobile into the treatment room coordinates (CTmobile-reg). Benefiting from high soft tissue contrast of CTmobile-reg, the second image registration was performed between the planning CT and CTmobile-reg to compute the couch shift values. In-house sequential registration software was developed to perform three-image registration as described above and to calculate couch shifts. The ED-HU table generated by the mobile CT was very similar to those generated by the conventional CT simulators in our clinic. CT simulations using mobile CT were successfully performed for 83 patients with various treatment sites such as brain (13 patients), head and neck (7), thorax (11), spine (21), abdomen (3) and pelvis (28), where a moving laser system and 4D imaging were not required. A phantom test of in-room image guidance using the mobile CT demonstrated that the proposed three-image registration-based workflow is feasible and results in improved soft tissue contrast when compared to CBCT. Our in-house registration software can calculate the couch shift values accurately (< 1 mm and < 0.3° difference compared to commercial localization software). With minor customization, mobile CT can successfully be used for CT simulation in external beam radiation therapy. In-room image guidance using mobile CT allows for improvement of soft tissue contrast and is feasible if combined with a daily CBCT imaging used for the intermediate registration. In future, we plan to develop an optical tracking system to geometrically correlate the mobile CT with the treatment coordinates which will remove the need of the intermediate CBCT scan.