The lymph node was removed (dissected) from level 2, according to the MO report from the PET scan. Jugulodigastic is in level II, and is very often the first lymph node that tonsil, and base of tongue cancer metasteses too. The majority of HPV postive cancers for H&N are from the tonsils. I assume they are treating the unknown primary as Oropharyngeal cancer, specifically tonsils, and base of tongue, which all suspicion it may be. I don't know about the body fighting off the cancer once it's present without treatment. I can see the body fighting off cancer before the cells has a chance to duplicate and become cancer, which it does, or in the case of cancer being present, the body fights off metasteses from spreading in the lymph or blood system, and other barriers, and really is a difficult process, so I don't know about the primary cancer just disappearing. PET scans usually do not pick up tumors less than 1cm or micrometasteses. Hopefully, and likely, the radiation kills off any remaining cancer cells, and the lymph node was removed, so hopefully that is it. Chemo would have added a boost to the radiation, but with HPV positive, maybe not these days, and remains to be seen what is the most effective lesser treatment. They do know chemoradiation works for advanced cancer.