How are carcinoma in situ and benign neoplasm different?

Good question: And difficult to tell sometimes. The most straightforward way to put it is that a benign neoplasm will never turn into an invasive malignancy within your lifetime. Cis will eventually turn into one. But, because we don't live forever, cis might not turn into a malignancy during one's lifetime anyway. Pathologists can usually, but not always, tell the difference by microscopic cell exam.