I am so sorry to hear that they have found a shadow on your lung. I can imagine you are feeling rather anxious and that you would like to have some information about this appointment after waiting for three weeks. I am not sure whether your referral was urgent or not but if you are worried and would like to be given a timeframe, I would suggest you ring the GP who made the referral to ask how long it might take for you to get your appointment. Do mention that you have been waiting for three weeks and that you would just like to know roughly what the waiting times may be in your area at the moment.
I am keeping everything crossed for you that you find out more very soon and that you get seen quickly. I also hope that it all turns out to be nothing of concern.
