Radiation continues to work just as long as your treatment lasted. I had a similar enlarged lymph node, that was slowly shrinking, and didn't have my post treatment PET scan until 5 months, due to inflammation, and it came back clear, even though it did not shrink completely. The doctors can feel the nodule, and may get an idea if it's cancerous or maybe just necrotic, inflammation, fluids, although not certain proven that way, and may monitor to see if it's subsiding or not. They can also do an earlier scan, and have read at 8 weeks, PET scans are 90% accurate, and some even do them at 6 weeks, in case further treatment is necessary, an FNAB or even surgery for a neck dissection, if its suspicious for reminiscent cancer. Speak with your doctors with your concerns. I hope this helps.