On-board kV imaging
Modern linear accelerators have on-board kV imaging equipment. This enables image guided radiation therapy (IGRT), using either kV or CBCT. Implanted fiducial markers makes it easy and safe for the medical staff to determine the need for patient repositioning prior to each fraction (interfraction motion management).
Matching kV to DRR
The fastest and easiest method is to capture two orthogonal kV images and compare them to Digitally Reconstructed Radiographs (DRRs). The two sets of images are blended over each other and moved so that the markers in the kV image overlap with the corresponding structure in the DRR. Images courtesy of Dr Marcio Fagundes.
Matching CBCT to CT
It is sometimes beneficial to also visualize soft tissue using cone beam computed tomography (CBCT). The CBCT is then compared to CT. The two image sets then have to be compared in all three dimensions. Images courtesy of Dr Marcio Fagundes.
Fiducial markers makes positioning more precise
A recent study, with corresponding author from the Icahn School of Medicine at Mount Sinai, compared 2-dimensional kV orthogonal imaging with fiducial markers (kV-FM) and soft-tissue CBCT (ST-CBCT) for IGRT of prostate.
They concluded that “The kV-FM-based daily image guided alignment for IMRT of prostate cancer is more precise than ST-CBCT, as assessed by a physician’s ability to reproducibly align images. Given the magnitude of the error introduced by inconsistency in making ST-CBCT alignments, these data support a role for daily kV imaging of FM to enhance the precision of external beam dose delivery to the prostate.”