Pathology: plexiform fibrohistiocytic tumour

Multinodular

Clusters of histiocytes with abundant eosinophilic cytoplasm; monomorphic round-ovoid nuclei with prominent nucleoli and inconspicuous mitotic activity 

Occasional interspersed osteoclast-type giant cells

Desmoplastic background

Multinodular

Clusters of histiocytes with abundant eosinophilic cytoplasm; monomorphic round-ovoid nuclei with prominent nucleoli and inconspicuous mitotic activity 

Rare interspersed osteoclast-type giant cells frequently surrounded by small pools of erythrocytes

Siderophages and haemosiderin deposition

Desmoplastic background

Multinodular

Clusters of histiocytes with abundant eosinophilic cytoplasm; monomorphic round-ovoid nuclei with prominent nucleoli and inconspicuous mitotic activity 

Interspersed osteoclast-type giant cells frequently surrounded by small pools of erythrocytes

Siderophages and haemosiderin deposition

Desmoplastic background; vascular invasion

Multinodular

Clusters of spindle cells with fascicular pattern; eosinophilic cytoplasm; ovoid nuclei with mild atypia and inconspicuous mitotic activity 

Interspersed osteoclast-type giant cells

Myxoid-collagenous stroma; focal metaplastic bone