Tumor does NOT show on PET Scan

I have a 5.5. cm (!) tumor that has both ductal and lobular cancer characteristics.  It did not show on the mammogram, US or the PET scan.  It did show on the MRI.  Do you know why this would not show up on the something as sensitive as a PET?  Thank you.

[CCF-RN,MSN-rf]

Dear tigris6:  A PET scan is not necessarily a "more sensitive" test.  Rather, it is a different test.  A PET scan works to identify areas of hypermetabolic activity.  It does this by attaching a radioactive substance to glucose, which is attracted to cells that are hypermetabolic.  When the scan is done, these areas "light up."  Often, but not always, cancer cells fall into this category of being hypermetabolic.  So, the point is, every test we have in medicine has it's application but no test is perfect.  Often, we must correlate various tests with physical examinations in order to come up with a diagnosis.