How do you use a 4D CT to account for hepatocellular carcinoma tumor motion?

If your IGRT solution is KV imaging, then implanted fiducals are very important for multiple reasons. However, to take full advantage of having them in place, I recommend using an inspiration breath hold technique as well using the Varian RPM system or gating belt that comes with the True Beam. This will cut down the ITV margin to 1-2mm. This approach also lends itself to a solution to your other question. For simulation, we use inspriation BH and get a phase IV contrast CT scan (image every 30 ...

This is easiest if you use permanent implanted fiducial markers. I typically ask the radiologists to implant the markers "near" the tumor, and as with all fiducials, they serve as a surrogate for being able to see the tumor. The more fiducials you have in place, the easier time you'll have in creating your ITV based on the variability in position of your markers. Imperfect, but it works.