I have had a negative biopsy myself, but still have strong concerns, so I know where you are coming from there. If you still have concerns then I would suggest continuing to pursue the issue. In the meanwhile, though, try not to stress too much.

Perhaps you could request an imaging study of some sort (MRI, CT) if they haven't done one already. If there is an area of concern deeper than where they took tissue they may be able to see it that way, and it may put your mind more at ease. Another option I've heard of is having your biopsy sent to a second pathologist for review. If they took enough I believe that is possible. I don't know how to set that in motion, but I'd guess you can talk to whoever took your biopsy about it. I have mostly heard about this being done to confirm a malignant report, but I don't see why it couldn't be done to double check for cancerous cells. Maybe someone who knows more can weigh in there.