The pancreas is an organ located behind the stomach. It produces enzymes that digest food and hormones that regulate blood sugar levels. An abnormal growth (mass) in the pancreas can be cancer or a benign (noncancerous) tumor. Finding out whether a pancreatic mass is cancer usually involves biopsy—putting a thin needle into the pancreatic mass to obtain a tissue sample that can be examined under a microscope. One way to do needle biopsies is to use an ultrasound instrument, which makes images of the pancreas that help identify the best place for the biopsy. The ultrasound images can be made either by passing the ultrasound instrument across the outside of the abdomen or by using an instrument built into a tube (endoscope) that gets passed through the patient's mouth into the stomach (endoscopic ultrasound). Unfortunately, no biopsy method is perfect, and some methods miss as many as 20% of pancreatic cancers.