Non‐ossifying fibroma: A RAS‐MAPK driven benign bone neoplasm

Non‐ossifying fibroma (NOF) has been an intriguing entity since its first description. It is the most common bone tumour, is usually asymptomatic affecting children and adolescents, is composed of a heterogeneous cell population, and undergoes spontaneous regression after puberty. In a recent article in The Journal of Pathology, Baumhoer and colleagues demonstrate mutations activating the RAS‐MAPK pathway (KRAS, FGFR1 and NF1) in ∼80% of the tumours. Activation of the RAS‐MAPK pathway by somatic mutations is found in a plethora of tumour types, both benign and malignant, while germline mutations cause a wide range of syndromes collectively termed the RASopathies. Their findings indicate that NOF, for long thought to be reactive, should be considered a true neoplasm. Moreover, their data suggest that only a subset of cells in the lesion contain the mutation. A second cell population consisting of histiocytes and osteoclast‐like giant cells appears to be reactive. This intimate relation between WT and mutant cells is also frequently encountered in other benign and locally aggressive bone tumours and seems essential for tumourigenesis. The spontaneous regression remains enigmatic and it is tempting to speculate that pubertal hormonal signalling, especially increased oestrogen levels, affect the balance between mutant and WT cells. © 2019 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.