If a pet scan shows no cancer, is the person truly cancer free?

After 6 months of chemo and radiation therapy with a stage 3b non-small cell lung cancer, the pet scan does not show any active cancer cells. Does this mean she is cancer free and truly better? Do the statistics still have meaning? Like the 15% survival rates, does that no longer count?

anuerodoc125 is correct.
A PET scan can usually show abnormalities approximately 1/3 of an inch or larger in size.

If her PET scan shows no active areas of concern then any cancer present is either very small in size or there is no cancer to be found - currently.

Cancer statistics show trends among large groups of cancer patients. Cancer patients are individuals. Statistics only show what usually happens among a large group of people but since each patient is only one percentile, what happens to the rest of the group is irrelevant to what happens with you. You either draw the long straw or the short one.

good luck to the lady with the good PET scan. Take this as great news.

Do you mean CT Scan? I haven't heard of a pet scan.
I have a coworker that went through having a cancerous tumor removed though and he has to get a CT Scan done every 6 months for a few times and as long as those are negative he is in the clear but then goes to the yearly one a few times and within 5 years he is then considered in remission and cancer free. It is standard to give that time period I am told to wait to be sure it doesn't come back. This is what I know from the guy that I work with who had cancer. I would take every CT Scan that is negative in the findings as good news and celebrate most when they reach five years and get the all clear to stop checking this. My coworker went through the chemo so they have to do this.

Positron Emission Tomography can miss tumors under 1 cm in size. Very good test, but not infallible.

PET scans are a very accurate means of identifying the presence and activity of some cancers, within limitations. A PET scan would normally be used in conjunction with a CT scan to obtain an effective overlay of the images. As the CT will show any abnormal growth the PET scan would reveal if there are any live cancerous cells in the given area.
However, PET scans are not full proof as they may not accurately identify cancerous growth that is less than about 8mm in size. This equates to approximately 1 million (1x10^6) cancerous cells that may go undetected. Therefore, to ensure the accuracy of a PET scan, the active cancerous growth would need to comprise of 1x10^9 cancerous cells. This would equate to a tumour size of between 1-2cm.