My husband's biopsy of his lymph node came up suspicion of squamous cell carcinoma. All his doctors (ENT, Head & Neck surgeon,med onco and rad onco) said it was 99% tonsil cancer spread to the lymph node. They would not radiate until they knew 100%. His surgeon took another tonsil biopsy and it came back positive. After that we still went to one other head & neck surgeon who his cousin went to, just to get a second opinion. He agreed with the diagnosis and treatment.
This will be the most important diagnosis for your sister so I would definitely get another opinion. The doctors like to see the CD of the PET not just the report. So I would get a copy of it from the lab it was done at and bring it to the other doctor. A second opinion could never hurt.
God Bless you and your family,
Carol