What clinical and pathologic features do you use to discern whether >= 2 synchronous lung nodules, biopsy proven lung adenocarcinoma, are different primaries versus metastatic disease?  

These cases are always discussed at a multi-disciplinary tumor board with a review of pathology slides from the operative specimen. In some cases, there are clearly different morphologies or levels of differentiation. These can be initial clues, but not definitive to determine synchronous primaries ...