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 address the issue of tumor motion and there can be challenges in image registration of MR and PET-CT.

I think that a 4DCT is a must for more accurate evaluation of tumor motion for SBRT. There is not simply a superior inferior motion of the liver. The liver can have translational motion that would not be appreciated well on fluoroscopy. Particularly if the tumor is situated next to hollow viscus, a 4DCT is critical for evaluation of motion relative to ...

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 I think it's a great option because organs at risk are not being compressed into the target. I think fusion of the PETCT is a great idea because the halo around the GTV will be a measure of the free breathing motion.

Lastly, there is almost never a concern about the liver dose in these cases because the tumors are usually tiny. Therefore when in doubt use larger margins. The biggest challenge comes when the tumor is close to the stomach or duode...

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 XYZ directional expansions (which you might be able to get away with in lung where you can directly measure tumor motion).

PET fusion may be a reasonable tool for a center lacking 4DCT as the region of avidity does represent to some extent a region of uncompressed free breathing motion, however there is some averaging and regions where tumor spends little time may be missed, and this is likely only reliable for high SUV tumors where contrast to ba...