I asked my radiation oncologist once how long does radiation keeps working being I had read 6 weeks, just as long as treatment lasted, he said in theory, but there is no way to prove that, but at least several weeks. Maybe they can do an FNAB to see if it is necrotic, cancerous or not. I had that done or a lymph node, and showed dead or dying cancer cells, but further testing showed it to be cancerous. Some have mixed reviews with FNAB, but all mine, around 10, were accurate, all done by a pathologist under ultrasound guidance. After that a PET scan was usually done if positive, which highlights uptake activity by the nuclear glucose injection to see of any other possible cancerous areas, then a neck dissection.

Good luck.