Try to remember that a PET scan is not a definitive determiner of a cancer, though given the history, it at first glance seems a likely bad sign. Since your armpit is full of node chains, and the area arond the clavicle on the route form the neck to the arm pit full of them, connecting the two areas, I am really surprised to read that there wasn't some node involvement between the two sites. For a metastasis to jump like this seems unusual. Remember that often nodes are encapsulated when cancerous and this does not mean it is in surrounding tissues if it is cancer.
I need to ask someone that knows me that i do, but it seems like if this is an isolated node, that surgical removal of it since they are close to the surface would be the first plan, and would be a determiner of cancer for sure, and removal of it.
Nodes can show up on PETS for lots of reasons besides cancer. I had a bunch of them in my chest hot after my last scan and of course my mind went right to cancer. I had some aspiration pneumonia going on that was undiagnosed, and those nodes were hot because of the infection in the adjacent area.