ELI5: The main cancer has a different look, size and pattern from the spread cancer. Liver cancer is also one of the only cancers that can be diagnosed with nearly 100% accuracy using only a CT or MRI and does not require a biopsy.
Non Eli5: Without a biopsy, there are a few ways to make an educated conclusion, but you wouldn’t 100% know for sure. Risk factors plays an important role. For example, if you have hepatitis B or cirrhosis, you are far more likely to have liver cancer than lung cancer. On the other hand, a smoker is far more likely to have lung cancer than liver cancer. The pattern of masses in your liver versus your lung will also be an indicator. Usually the primary cancer is larger and tends to be singular or only a few masses while the metastatic lesions are smaller, and usually multiple (scattered). In liver cancer, a diagnosis is typically not made with a biopsy but rather with a multi-phase CT which looks at the contrast pattern of the lesion.
If the suspicion is for liver cancer, spread to the lung, you would get a multi-phase CT of the liver which will likely diagnose liver cancer. Then, you *could* do a biopsy of the lung lesion to confirm that this is indeed liver cancer spread to lung.
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