mammography result

I had a screening mammogram last week and got a letter today stating only this
"The radiologist has recommended additional imaging studies are needed to complete the evaluation. Further studies may include additional mammographic views or breast ultrasound."  I had a benign adenoma removed several years ago, but no other problems. or lumps detected by my doctor.
What are the reasons for additional views needed, could it be the scar tissue.

[Cleveland Clinic]

Dear loveslight 57:  It is not uncommon to be called back for additional views after a screening mammogram.  Often some tissue may fold over on itself and appear as a density.  Spot compression will often smooth out the problem.  If the density remains, additional investigation may be necessary, including ultrasound etc.  

[loveslight57]

Hi thank you for your reply.  It was very helpful.  Could a scar from a previous breast surgery cause the density?.  It was about 7 years ago that I had an adenoma removed in that breast.  The technician didn't mark the scar because she said it was so long ago.