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Head and neck cancer- PET scan results

Hi I was wondering, with a PET scan, should you be able to tell the difference between active scar tissue, and scar tissue from radiation treatment (which ended approx.7 weeks ago)? Also if a PET scan shows inflammation in the lymph nodes, but the lymph nodes appear normal how likely might it be that the inflammation is residual effects from the radiation,or rather that the cancer returned?
Asked On : Wed, 9 Feb 2011
Answers:  2 Views:  334
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Radiologist, Nuclear Medicine 's  Response

PET scan will certainly be able to differentiate between scar tissue and cancer.
But the timing of PET scan is very very important. After radiation therapy, it is ideal to wait for 12 weeks before radiation induced inflammation becomes normal. If FDG uptake occurs after 12 weeks from completion of radiation, it is not radiation induced inflammation.
At 7 weeks, it become difficult to conclusively say if its cancer or radiation induced inflammation. But the dictum if radiation induced FDG uptake is usually diffuse and low to moderate grade. A focal area of uptake usually indicates another problem like cancer or infection.

Recurrent cancer usually is focal and intense, though its not a dictum in all the cases.
Answered: Tue, 10 Apr 2012
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Radiologist Dr. Gaurav Sundar's  Response
welcome to
A PET scan can tell the difference between a metabolically active lesion and one that is not active (scar tissue). After radiotherapy, scar tissue will result which will not be metabolically active. Any residual disease or inflammation resulting from radiation exposure will show as activity.
A lymph node which is showing activity might suggest spread to that lymph node.
You will have to correlate with a previous report to check whether there is improvement or not.
Answered: Sun, 14 Aug 2011
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