Aim. To report late development of sarcomas on sites of previously curetted and grafted benign tumours. Rare cases of development of sarcomas in sites of previous benign lesions are documented, and the development is generally considered secondary to progression of benign lesions, even without radiotherapy. Methods and Results. In our files, 12 cases curetted and grafted, without radiotherapy addition developed sarcomas from 6 to 28 years from curettage (median 18). Age at first diagnosis (9 GCT, 1 benign fibrous histiocytoma, ABC and solitary bone cyst) ranged from 13 to 55 (median 30). For all cases radiographic and clinic documentation was available. Histology was available for 7 of the benign lesions and for all malignant lesions. The type of bone used to fill cavities was autoplastic in 4 cases, homoplastic in 2 cases, homoplastic and tricalciumphosphate/hydrossiapatite in 1 case, autoplastic and homoplastic in 1 cases, heteroplastic in 1 case. In 3 cases the origin was not reported. Secondary sarcomas, all high grade, were 8 OS, 3 MFH, and 1 fibrosarcoma. Conclusions. It is impossible to calculate the exact incidence of this transformation, but from a comparison with 137 secondary sarcomas treated in the same years (1975-2005) at the Rizzoli Institute, it is similar to the risk of a sarcoma on fibrous dysplasia or lower than a sarcoma on bone infarcts or on Paget's disease. Recurrence with progression from benign tumours can occur, but the very long intervals reported in the present series suggest a possible different pathogenesis. Recent preclinical papers report development of sarcoma in mice after transplantation of mesenchymal stem cells, independently from the type of scaffold used. The fact that reparative proliferative changes occuring in the area of dead
