Acquisition of MV-scatter-free kilovoltage CBCT images during RapidArcTM or VMAT

Purpose: To perform kilovoltage (kV) cone beam computed tomography (CBCT) imaging concomitant with the delivery of megavoltage (MV) RapidArc treatment, and demonstrate the feasibility of obtaining MV-scatter-free kV CBCT images. Methods and materials: RapidArc/CBCT treatment and imaging plans are designed, and delivered on the Varian TrueBeam, using its Developer Mode. The plan contains 250 control points for MV-radiation delivery, each over an arc of 0.4-0.7o. Interlaced between successive MV delivery control points are imaging control points, each over an arc of 0.7-1.1o. During the 360o gantry rotation for the RapidArc delivery, CBCT projections of a phantom are acquired at 11 frames per second. The kV projections with minimal MV-scatter are selected, based on gantry angle, and the CBCTs image reconstructed. For comparison, a reference CBCTr image is acquired in the normal way. In addition, to examine the effect of MV-scatter we acquire CBCTc using the same treatment plan without the imaging control points, i.e. with continuous MV delivery during the 360o rotation. Quantitative evaluation of image qualities is performed based on the concepts of CNR (contrast-to-noise ratio) and NSTD (normalized standard deviation). Results: The different types of CBCT images were reconstructed, evaluated, and compared. Visual comparison indicates that the image quality of CBCTs is similar to that of the reference CBCTr, and that the quality of CBCTc is significantly degraded by the MV-scatter. Quantitative evaluation of the image quality indicates that MV-scatter significantly decreases the CNR of CBCT (from ∼7 to ∼3.5 in one comparison). Similarly, MV-scatter significantly increases the inhomogeneity of image intensity, e.g. from ∼0.03 to ∼0.06 in one comparison. Conclusion: We have developed a method to acquire MV-scatter-free kV CBCT images concomitant with the delivery of RapidArc treatment. Engineering development is necessary to improve the process, e.g. by synchronization of the MV and kV beams.