Any type of cancer consultation no matter what you do will be a scary thing. I certainly sympathize with your concern. The best thing for you to do is to bring a loved one with you to the evaluation so that you have someone that can help you listen and understand the information that is given to you. If you have an episode of hyperventilation there, the staff will be able to help you through it.
On a side note if you are particularly susceptible to hyperventilating, then you may have an anxiety disorder or panic disorder. Panic disorder is a condition characterized by anxiety and the propensity for panic attacks. This type of thing can be treated. You should see your primary care physician
soon for an evaluation and possible treatment of this issue.
I suggest that you schedule an appointment with your ophthalmologist
(or which ever doctor
you are seeing for your ocular tumor). Let the doctor know up front that you are scared. He or she will have a lot of experience discussing your disease with your in terms that will make sense. Your doctor will likely only discuss issues that are pertinent to the very next step. This way there is not too much to think about on your first visit. I hope that helped.