Very thickened inner and outer cortices. Patchy lytic and “fluffy” blastic medullary appearance - aka “cotton wool” appearance.

Answer: Classic appearance of Paget’s Disease of the skull .

Reference: http://www.nejm.org/doi/full/10.1056/NEJMicm0912945

Paget’s ref: http://radiopaedia.org/articles/pagets-disease-of-bone-1

If you see cortical thickening and bony expansion diffusely of an area of bone, Paget’s is upper there on the differential