Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. The brainstem and the C1 levels are pretty close, the C1 is the last spinal cord level before it joins the brainstem. However you seem to speak of the mass being around the C1 vertebrae not the spinal cord, in that case it is separated by the spinal cord (and the brainstem naturally) by the vertebral body and ligaments, doesn t seem to originate from the brainstem tissue or represent any imminent threat to compress the brainstem if that s what you mean. Of course it is still a delicate matter since the doctors haven t identified the nature, I suppose they might want to take biopsy material from it to create a better idea. Hope I have answered your query. Let me know if I can assist you further.