I Have Had A CT Scan 3 Months Ago And

I have had a CT scan 3 months ago and it showed that I have a pure ground glass nodule in the (middle) left lung. It is 13mm. I had another CT scan. This second scan showed that it has not increased in size. I have seen a respitory doctor who told me that he does not thinks it is cancer but he can not be 100% sure. He said that if it is it could be a slow growing cancer. I had a biopsy done. The results were negative BUT the doctor said that does not mean that it is not cancer as he is not sure if the sample of tissue was from the nodule. What else can I do to find out if is is cancer??? Thank you Eleni