One way(no so convenient) to reduce false positives in PET/CT scans
 

In an other thread about the the usefulness (or lack of it) of periodic scans there were various pros & cons depending on whether it applied to primary or metastatic cancer.
One of the arguments against PET/CT scan was their assumed high rate of false positives (as indicated in a small study of 160 cases presented at ASCO 2006).
I recently got some info from a technician in a hospital nuclear medecine department who is specialized in performing PET/CT scans.
He claims that on average about 7% of of detected lesions are false positives.
As a result, in the case of a patient with multiple lesions only one lesion has a probability of being false positive & in the case of metastatic cancer this would not change the treatment decision.
More than one false positive would be extremely rare to occur based on the 7% probability.
However if a patient has a scan showing a single doubtfull lesion, then there is a procedure to resolve the uncertainty.
It based on the fact that the radio isotopes injected in the patient blood system are still picked up by cancer cells well after the scan is completed while not so in normal cells. So if a second scan is performed say 2 hours after the first one the uptake (as measured in SUV units) is higher in cancer. cells & the comparison of the 2 scans would settle the doubt.

So why not always do a double scan?
-not convenient
-not always useful
-costly
-would overload the limited capacity of radiology machines