  The concept of using kilovoltage (kV) and megavoltage (MV) beams concurrently
has potential applications in cone beam computed tomography (CBCT) guided
radiation therapy, such as single breath hold scans, metal artifact reduction,
and simultaneous imaging during MV treatment delivery. However, MV
cross-scatter generated during MV beam delivery degrades CBCT image quality. To
address this, a 2D antiscatter grid and cross scatter correction method were
investigated. a 3D printed, tungsten 2D antiscatter grid prototype was utilized
to reduce MV cross-scatter fluence in kV projections during concurrent MV beam
delivery. Remaining cross-scatter was corrected by using the 2D grid itself as
a cross-scatter intensity sampling device, referred as Grid-based Scatter
Sampling. To test this approach, kV CBCT acquisitions were performed while
delivering 6 and 10 MV beams, mimicking high dose rate treatment delivery
scenarios. MV cross-scatter suppression performance of the proposed approach
was evaluated in projections and CBCT images of phantoms. 2D grid reduced the
intensity of MV cross-scatter in kV projections by a factor of 3 on the
average. Remaining MV cross-scatter estimated by Grid-based Scatter Sampling
was within 7% of measured reference intensity values. CBCT image quality was
improved substantially during concurrent kV-MV beam delivery. Median Hounsfield
Unit (HU) inaccuracy was up to 191 HU without our methods, and it was reduced
to 3 HU with our 2D grid and scatter correction approach. Our methods provided
a factor of 2-6 improvement in contrast-to-ratio. Results indicate that our
approach can successfully minimize the effects of high energy cross-scatter in
concurrent kV CBCT imaging and megavoltage treatment delivery.
