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Accuracy of PET scan
My dad had a pet scan to check for a return of cancer.  It came back with no hot spots.  But what is strange is that a CAT scan showed a mass on his pleura of his lung.  They thought it may be a recurrent thymoma which he had removed 4 years ago.  He did have a bx of the area.  No results available yet.  If it were a recurrence of the thymoma wouldn't the pet scan have showed a hot spot?  What could this mass be.  They said it was about the size of a nickle.
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Hi.  Positron Emission Tomography or PET scan involves the use of a radioactive form of glucose (fluorodeoxyglucose or FDG), which is introduced into the body and taken up by the body's organs in differing quantities, depending on that organ's metabolic rate.  The assumption is that cancer cells will have a higher metabolic rate than normal cells, would take up FDG more avidly and show up on the scan as a "hot spot".  While this assumption is true most of the time, it's possible for the growth rate (and consequently, the metabolic rate) of a cancerous mass to slow down after it has attained a certain size.  Hence the mass doesn't show up too well on PET since the FDG uptake has also slowed down.  Another possibility is that the mass, although malignant, is largely necrotic.  Lots of dead cells means less FDG uptake.  The third possibility is that the mass seen on CAT scan is non-cancerous and does not have an increased metabolic rate.  

Anyway, the biopsy was the right thing to do, since it's going to provide you with a more definitive answer than PET or CAT scan.
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