Pathology: aneurysmal bone cyst

Aneurysmal bone cyst

• Solid and cystic spaces

• Fibrous septa containing fibroblasts with a fascicular pattern; monomorphic ovoid nuclei; inconspicuous mitotic activity

• Intervening stroma contains osteoclast-type giant cells, osteoid deposition with osteoblasts, and occasional siderophages

• RNA-seq: PAFAH1B1::USP6 gene fusion

Aneurysmal bone cyst

• Solid and cystic spaces

• Fibrous septa containing fibroblasts, with fascicular pattern; monomorphic ovoid nuclei with prominent nucleoli; inconspicuous mitotic activity 

• Intervening stroma contains osteoclast-type giant cells, osteoid deposition with osteoblasts, and occasional siderophages

Aneurysmal bone cyst

• Cystic spaces filled with blood separated by fibrous septa

• Septa contain fibroblasts with fascicular pattern; monomorphic ovoid nuclei; inconspicuous mitotic activity 

• Intervening stroma contains osteoclast-type giant cells, woven bone rimmed by osteoblasts, and so-called "blue bone"

• RNA-seq: KLF3::USP6 gene fusion

Aneurysmal bone cyst

• Cystic spaces filled with blood separated by broad fibrous septa

• Septa contain fibroblasts with fascicular pattern; monomorphic ovoid nuclei with prominent nucleoli; inconspicuous mitotic activity 

• Intervening stroma contains osteoclast-type giant cells, woven bone rimmed by osteoblasts, and extravasated erythrocytes with occasional siderophages

• RNA-seq: TAF7::USP6 gene fusion*

* To date unreported in the literature

Aneurysmal bone cyst

• Focal cyst lining

• Fibrous septa containing fibroblasts with fascicular pattern; monomorphic ovoid nuclei; inconspicuous mitotic activity 

• Stroma contains osteoclast-type giant cells, woven bone rimmed by osteoblasts, and so-called "blue bone"