My 10 Year Old Son Has Been Having Pain Behind

My 10 year old son has been having pain behind knee and u able to fully bend it for almost two years. He now has swelling or small bump on the outside of knee. I've gone through many doctor opionions ans tests and finally MRI confirms vascular anomalies. However I was just told the doctor wants to do a biopsy because "he saw the MRI looked atypical for a venous malformation, but still had some of the expected qualities. They want to be sure they match the correct treatment with the correct diagnosis. The doctor will take tissue out of the lesion (biopsy) around the knee area that is affected by the anomalous tissue."

The nurse at the children's hospital won't give me much more info. Are they looking for disease or cancer? Waiting for an appointment t is taking forever.
Thank you for any insight.