negative PET scan, but multiple Positive Nodes

I had negative PET scan prior to my lumpectomyand lymph node disscetion. The pathology report came back with multiple(12) positive nodes, 2 of which were completely replaced by tumor .
Why would a PET scan not pick this up?
Thank you.

[katarina777]

All I can say to this is that no test is perfect. my positive lymph nodes were also not picked up in my case by MRI. That's all I know. I think with PET scan it shows up everything that is inflamed and that hurts, like arthritis and stuff like that. it is really not specific for cancer, I think.