Can a diagnosis of lung cancer which already has spread to the liver, be mistaken for something else?

Not usually: If the lung pathology was confirmed by histology then most likely the metastasis are lung origin. Some other malignancies can be confusing spreading to liver and lung.

Dr Friedlander and another doctor agreed

A biopsy should tell: The only way to be absolutely sure that lung cancer has spread to the liver or anywhere else is to biopsy the area of concern. Other tests like pet/ct and bone scan my suggest areas of spread( aka metastasis) but a biopsy is usually needed to confirm. Once biopsied it should be clear if there is metastic disease or not.

Dr Lebowitz and another doctor agreed

Infection: Clinical presentation of the patient and a confirmatory biopsy would be necessary if there was a question as to the diagnosis.

Biopsy is best: If a patient has a history of lung cancer and develops a tumor in the liver, it is most likely lung cancer. Under these circumstances, a biopsy is necessary for confirmation since a stage IV (metastatic lung) is treated far differently than a primary liver cancer.

Dr Stringer and another doctor agreed