How do you create an ITV for liver SBRT (metastatic lesions) when you don't have a 4D-CT or fiducials?

I will add a comment about ITV generation. Even with abdominal compression, there can still be movement of 8-10 mm or more depending on the abdominal compression system, body habitus of the patient, and breathing pattern.

Fusing the MR and PET-CT only helps you delineate a GTV but it does not addres...

One of the most important things to remember about metastases is that sometimes they are infiltrative sometimes they're not. When they appear infiltrative on imaging I add a 5 to 10 mm low-dose PTV in addition to the 5 mm PTV margin on the gross tumor.

I would use abdominal compression for the liver...

Liver motion in particular is more challenging than other organs as it is typically not in the standard XYZ axis’ that we think of and very difficult to always fully appreciate in 2D views like fluoroscopy, such that one really needs to draw the full extent of motion rather than use an estimate of X...