Aims Osteoid osteoma and osteoblastoma have, in the past, been variously regarded as both similar and distinct entities. Currently, WHO classifies these tumours separately. We compared archetypal cases to identify novel histomorphological and immunohistochemical features attempting to clarify their mutual relationship.

Methods and results 10 osteoid osteomas and 20 osteoblastomas (10 spinal and 10 non-spinal) were retrieved and reviewed clinically, radiologically and histologically. Immunohistochemistry was performed for: desmin, SMA, neurofilament, S100, vimentin, PGP9.5, GFAP, EMA, caldesmon, CD34, broad-spectrum cytokeratins, claudin-1. We identified features, common to both osteoid osteoma and osteoblastoma, namely, areas of lesional non-osteoblastic stroma and the presence of scattered, large cells with smudged/degenerate nuclei. Immunohistochemically, we confirmed the innervated status of osteoid osteomas, and found that osteoblastomas were similarly innervated. The non-osteoblastic lesional stroma was distinctive owing to expression of EMA and NSE by the mesenchymal spindle cells and expression of desmin, PGP9.5 and S100 by the scattered, large cells with ‘smudged’ nuclei.

Conclusions Both osteoid osteoma and osteoblastoma are innervated bone-forming lesions which share novel histomorphological and immunohistochemical features supporting the view that separate classification is unjustified, and we offer a pathogenetic explanation for their apparent clinical and radiological variance.