Dad's lung biopsy came back benign. he's having a 2nd biopsy done going in through his back. what are the chances this one will be benign too?

No way to tell: Lesions mimicking lung cancer include everything from old histoplasmosis (ubiquitous where i live) to stray dog heartworms. Or a first biopsy can be "benign" because they missed the cancer which is very common. Cherish each day with dad, and if the more definitive biopsy is benign, be happy.

Hard to tell: It would be impossible to tell from the above information. The question they should ask when a biopsy comes back negative is whether the lesion in question was actually sampled or did they sample normal tissue beside the lesion in question.

No way of knowing: We would need to know the case and be able to review imaging to even make a guess. Lung biopsies can be done and still not come up with a diagnosis.