I thought I remembered reading on this board about a PET scan that missed a cancer because the cancer was too small. This person's cancer was less than 0.5 cm (=5mm) in size and was missed.
http://www.cancercompass.com/message-board/message/all,6887, "...because the tumor was less than a half of a centimeter in size that that is why none of the scans picked it up. He said that usually PET Scans do not pick up a tumor until they are about 1 centimeter in size."
this person was likely scanned on an older PET scanner, probably not a combination PET/CT machine. had this person been scanned on a 3rd generation PET scanner, the tumor was actually just barely large enough that it might have been found.
this goes back to my belief that going to top cancer places for treatment gives better results. top places have the latest technology and the cutting edge treatments. had this person gone to MD Anderson or Sloan-Kettering, he would have gotten a better PET scan.