Is this a simple cyst?
No it is not. Not now, not ever. This is a complex lesion with a large solid mural component and a residual solid mass post aspiration.
Why not observe this lesion and follow up?
If it bugs you now, it will big you when the patient comes back. This rationale applies to mammography too.
A complex cystic structure with a thick mural component. The lesion is poorly defined with no capsule.