pancreatic cancer

My father has been diagnosed with PC since early July.  He started Chemo in NYC and taking Gemzar, Leucovorin, Irinotecan, 5Fu and oxiplatin.  His tumor markers are very low and doing pretty well.  It has been around 5 rounds every two weeks.  Now its time to do a follow up to see how things are working.. However, we can not see the tumor in any of the scans. Even before he was diagnosed.  He had a Cat, Pet done but they cant see it! He had an endoscopy? done in July but it said it was very small, abut 2 cm so then went for the whipple surgery to remove it and the doctor could not because it was too big. arund 5 cm  Much bigger than the endoscopy said.  So my question is....... Are there any other test done to try to detect the size of the tumor?  if not does anyone have any suggestions as what to do?  Thanks so much....This message fourm is always helpful and so supportive...... I appreciate it! Bridget