You can use PET-CT fusion. And still, liver is an organ, that can very well heal itself. If the location of the lesion in the liver is not too problematic (close to bowel or gall bladder), then just make your CTV bigger and hit it. I haven't seen any patient with a normal liver developing chronic toxicity because of SBRT originating from the liver itself.
On a side note: Is it just me or do more of you have the impression that SBRT-liver patients tend to:
A) develop lots of fatigue after treatment
A) sometimes experience cold shivers or fever after the first (couple of) fraction(s).
I have witnessed A several times and the magnitude of fatigue is in my opinion lots more than in SBRT-lung patients with lesions of the same size. B is something I've seen in large lung-SBRT volumes as well but more often in liver. I guess B is some kind of cytokine rush and A the liver repairing itself after treatment?