Hey there...
and welcome.

Let me first start by saying the type of scans you've had and the the person reading the results could very well make up for the differences in the size. What you see as a reduction could very well be just a margin of error. There are differences between whether you've had contrast dye or not, whether you've had a PET/CT, CT, or MRI. All give a different kind of picture, some more accurate than the others. The Tesla of the machine also makes a difference - higher tesla, more accurate the scan. Also the fact that some scans (PETs) work on a sugar based dye that shows a higher up take in the tumor - may also have something to do with the fact you area diabetic, the meds you are taking and your blood sugar at the time. Since diabetes medication stimulates the cells to uptake sugar, if you had your meds and your blood sugar was higher you may get more infused area... (normally diet effects uptake too)

My first CT was done with contrast at my local hospital, my second at a cancer hospital. The differences were minimal - mere millimeters but this is why the cancer center wanted their own in house team to look at a scan done by them. It gave them a clear accurate picture by a radiologist they trust and know. Since they design radiation plans around these scans.

There is a minor possibility that the some of what showed up on the initial scan was inflammation (in the surrounding area), and maybe the second scan, was less inflamed and that would make up for the difference in size since it is often hard to determine the differences between inflammation, infection and a tumor. This is why there are always differences and why we leave the reading up to those who are trained to.

Sometimes a small primary will resolve itself (this is how we get some occult findings) but this is rare and as far as I know usually it shows up in the nodes regardless. So basically what the means is they may not find the original primary tumor but they find the cancer elsewhere - usually the neck nodes. Your tumor being a stage three is quite large. I am assuming you've had a biopsy so you know it's cancer. At this point if there is no nodal involvement there's a good chance that despite what the scan says there is likely cancer on a cellular level in at least one of your nodes. So in answer to your question - chances of it resolving itself... likely nil. Chances of it spreading if you wait any longer - VERY HIGH.

Deal with it now before it becomes life threatening. Healthwise you are somewhat compromised - despite the fact that everything is stable right now, you want to do battle when you are at your healthiest, not when you get sicker. And waiting could be deadly.

best of luck.