Residual Thymic Tissue and additional CT Scan Risk?

Back in 2/08 I went to the heart Dr. for a heart check up.  Mainly due to a family history of heart disease and I was having occasional chest pain.  I think it was brought on by stress.  

The Dr. suggested a calcium Score benchmark via a Cardio CT scan.  The results were good with a score of  0.  However, the radiology report shows the mediastinum to be abnormal, likely anterior mediastinal residual thymic tissue.  The radiologist requested a 6month follow up Chest CT with IV contrast.  The heart Dr. assured me that it was okay and not to worry,  They see a lot of "internal blemishes" with this type of high powered CT scan.  Don't stress over this finding.  

My concern is should I subject myself to more Radiation from a Chest CT to get a better understanding if this is a Thymic tissue or some type of tumor.   I'm concerned with my total level of  radiation exposure from two Chest CT scans in a period of 8 months. The equipment is a 64 slice multi row detector CT scanner.  I'm only 33 and I've read that multiple radiation exposure at young ages significantly increases your risk of developing cancer later in life.

Should I gamble and assume that I'm okay and not do the Chest CT scan? Or gamble on the radiation exposure and do the Chest CT scan?  Is there additional test that could rule out a tumor or thymic problem without exposing myself to unwanted radiation?  MRI, chest xray,   I'm not getting much help from my heart Dr.

Jay