We present a 43-year-old man who came to our department for assessment of a left frontal tumour of 6 months' evolution without previous trauma at that level. The patient reported no pain in the tumour, only a feeling of heaviness and left fronto-occipital headache. Physical examination revealed a tumour measuring 1 cm in diameter with minimal perilesional erythema. On palpation it appeared as a hard mass, not adherent to the bone plane without alteration of the frontal branch of the facial nerve. He also reported a similar nodule in the right costal region. A CT scan and an ultrasound-guided fine needle puncture-aspiration (FNAUS) of the costal tumour were requested. The CT scan showed a mass of approximately 0.7 cm at the level of the left frontal region, well defined, which did not invade adjacent tissues and showed no bone destruction. FNAUS of the right costal region was reported as a lipoma.

It was decided to perform a biopsy-excision of the tumour under local anaesthesia. Intraoperatively a lesion firmly attached to the fascia of the frontalis muscle and the pericranium was observed. A subperiosteal dissection including frontalis muscle and pericranium was performed, no erosion of the underlying frontal bone was observed. Closure of the surgical defect was straightforward and simple. The postoperative period was uneventful and there was no recurrence after 18 months of postoperative follow-up.
The anatomopathological study describes macroscopically an irregular white-pardusc nodular fragment measuring 0.9 × 0.7 × 0.4 cm. Microscopy revealed a connective tissue with a circumscribed, poorly demarcated nodular lesion composed of a proliferation of oval-fusiform elements arranged in a lax matrix with a swirling-storiform pattern together with osteoclastic giant cells, some extravasated red blood cells and a scattered mononuclear inflammatory component. Immunohistochemistry showed positivity for CD68 antibody and vimentin protein in the giant cells and with less intensity in the oval cells. Negative for S-100 protein. Positivity for Ki 67 antibody, representing a proliferative index of less than 5%.

The study concludes with the diagnosis of benign neoplasia of mesenchymal origin type FN.