Welcome. A brachial cyst, should be highly suspicious of HPV, as is indicated, and oropharyngeal cancer. This is sometimes missed by doctors, and just diagnosed as just a brachial left cyst, but can be both, and sometimes cancer is even missed, but good they did a biopsy, but surprised they did not inform you of this possibility. They should do another type scan in the H&N, and an endoscopy, with biopsy under anesthesia of the oropharynx identify the primary. 90% of HPV are in the oropharynx anyways, as well as 80% of the unknown primaries. This also can be an unknown primary, since the body may have fought off the HPV infection at the primary, and or died off once it metastasized to the cervical neck or too small to be picked up by the PET scan. You will probably be treated as if the primary is in the oropharynx, if not found. Having no tonsils doesn't matter, actually there are 6, and mainly those involved with HPV cancer are the palenine, and lesser the lingual tonsils, and then the BOT can also be involved. Are you at a CCC, Comprehensive Cancer Center?

Good luck.