You are not required to code from a path report if it is unavailable at the time of coding. As to pre and post operative diagnoses, if there is a variance between a preoperative diagnosis and a postoperative diagnosis, the postoperative diagnosis is considered more specific and should be reported instead of the preop diagnosis. For example, a patient might have a preoperative diagnosis of acute appendicitis, but intraoperatively, the physician confirms that the appendix has ruptured. The postoperative diagnosis would be acute appendicitis w/ rupture. In surgical coding, you should assign the most specific, confirmed diagnosis documented within the operative report (even if that diagnosis is in the body of the operative report - not all physicians fill in the formal fields of preoperative and postoperative diagnoses in the header of the note).
In the case of assigning a diagnosis of cancer, since that was one of the examples cited, you should not assign a diagnosis of cancer unless that diagnosis has been pathologically confirmed. Sometimes you will see a case where a biopsy was taken in the office, skin cancer was confirmed, and the patient now comes in for a total resection of that area of skin cancer (it would be permissible to use the preop diagnosis of skin cancer in that case because the pathology from the biopsy already confirmed the diagnosis of cancer). However, if the doctor "suspects" cancer, states it is "probable" cancer, that he is "ruling out" cancer, or any other terms indicating uncertainty, you should not assign a diagnosis of cancer until pathology confirms that for certain.