Hi Linh,
There are rules for this in the Tirads system,
Tirads has a couple of supporting documents which are fabulous to read as they explain all of these small issues and provide guidance,
I must get around to recording my thyroid lecture soon as I think it will be helpful.
The Tirads rules say that if you cannot see the internal architecture of a lesion due to complete or near complete egg shell calcification you do the following :
Assume it is solid (2 points)
Assume it is isoechoic ( 1 point)
Allocate peripheral calcification points (2 points)
So they get 5 points.
I agree with you that these are often benign and often there is evidence they have been there for ages unchanged,
But they are the RULES !!!!
Steve