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Larger lesions (as in Case 7) tend to undergo regressive changes with circumscribed myxoid transformation, liquefaction, and eventual fatty degeneration. This may produce a variegated imaging appearance with lucent zones in bone showing ground-glass transformation (soap-bubble pattern; see Case 7). Because areas of fibrous dysplasia usually undergo constant remodeling, they typically appear as “hot spots” on bone scintigraphy. Ground-glass attenuation is also the hallmark of fibrous dysplasia on CT scans (▶ Fig.

Ostertag of Hannover, the tumor is composed mainly of a dense soft-tissue mass (osteoid formed by the tumor). Matrix ossification is visible at the base of the lesion on CT (see ▶ Fig. 11) and is a fundamental property of Skull Fig. 11 a–f Differential diagnosis: osteoblastoma. Fig. 12 a–c Differential diagnosis of exophytic skull lesions. bone-forming tumors. Like the osteoma in ▶ Fig. 10, the lesion originates from the outer table, which it has destroyed and replaced with a thin new bony shell.

Histology at 7 months after the traumatic event identified the lesion as an epidermoid cyst with a focal granulating and fibrotic inflammatory reaction, interpreted partly as a foreign-body reaction. This last case naturally raises the question of whether the head trauma induced the epidermoid cyst, as the clinical presentation suggests. But epidermoid cysts or intraosseous epidermoids in the calvarium are generally asymptomatic. The young woman first noticed a depression in the right calvarium after the impact trauma and claimed that the palpable depression had enlarged over time.