Complex sentence: Pancreatic adenocarcinoma and the other less common exocrine cancers have a very poor prognosis, as they are resistant to treatment and usually cause no early symptoms and so are diagnosed late, when the cancer is already locally advanced or has spread to other parts of the body.
Split into simpler sentences: Pancreatic adenocarcinoma and the other less common exocrine cancers have a very poor prognosis, as they are resistant to treatment and usually cause no early symptoms. Therefore they are normally diagnosed at a late stage, when the cancer is already locally advanced or has spread to other parts of the body.