Infantile myofibromatosis of the iliac bone.

Introduction: Solitary infantile myofibromatosis (IM) of bone is a rare benign osseous tumor of childhood with low rate of recurrence. Well documented within the multicenter form, its solitary intraosseous location is less well described.

Case report: We present a rare case of intraosseous myofibromatosis arising the iliac bone of a 11-year-old girl, who was operated at 2 months of life for a retroauricular subcutaneous MF with unbalanced translocation t(9;16). She presented with a limping associated to a stiffness of the hip without pain. Imaging disclosed a 4×4×1cm intraosseous, lytic and heterogeneous mass with a soft tissue component on the medial cortical of the left iliac bone. Open biopsy was performed. Histology revealed proliferation of fusiform cells with eosinophil cytoplasm embedded in a myxoid and fibrous stroma without mitotic figures. On immunohistochemistry, cells were positive for actin, PS100, KL1, focally positive for EMA, CD34, P63, rarely CD31, which indicated diagnosis of new localization of IM. Cytogenetic analysis revealed absence of translocation t(9;16), which was found in the first tumor. Subsequent total resection was performed. The patient recovered normal function without recurrence of tumor at 3 years follow-up.

Conclusion: To our knowledge, this is the first case of solitary IM of the iliac bone, occurring 12 years after the first localization. Total resection resulted in excellent outcome. However recurrence can happen even long time after the first resection and new localization is possible, as in our case. This suggests close follow-up and clear information about the risk of recurrence.