Blue lacy matrix in giant cell tumour of bone with or without denosumab therapy.

Giant cell tumour of bone (GCTB) is genetically characterised by an H3F3A mutation. GCTB is treated with curettage or resection, and denosumab may be administered. Herein, we retrospectively analysed a large cohort of GCTB and identified a previously uncharacterised distinct blue matrix. Among 127 archival GCTB cases positive for the H3F3A G34 mutation, a blue lacy matrix was observed in 10 cases (7.9%). Five patients were previously treated with denosumab. Although a focal observation, the matrix was multifocal or relatively conspicuous in a subset. It appeared as blue mottles or bands, in which a delicate meshwork of basophilic, focally calcified, lacy material intricately surrounded the H3.3 G34W-positive mononuclear cells. The matrix was associated with depletion of osteoclast-like giant cells and fascicular proliferation of spindle cells, regardless of the history of denosumab therapy. Recognising this unique matrix will help avoid confusion with other bone tumours with different clinical management.