Do you use a metal biliary stent as a fiducial marker for cholangiocarcinoma?   

Yes, I use expandable metal stents as fiducials in pancreas as well as biliary tract cancers. There is no concern for either migration or forward scatter causing complications. Please see discussion in the below letter to the editor. My only concern in the pancreatic patients is that the distal...

I agree with @Christopher H. Crane (who did a much more thorough explanation of the physics - thanks!).  From a practical view, as we are increasingly dose escalating in the treatment of unresectable GI cancers, the need for image guidance is essential.  In the case of a hepatobiliary...