Answer: Calcifying fibrous pseudotumor

Histology: Images show a well-circumscribed mass with abundant hyalinized collagen and a minimal lymphoplasmacytic cell infiltrate. In some areas, psammomatous calcifications were seen (not shown). Immunostains for S100 and ckit were also negative.

Discussion: Calcifying fibrous pseudotumours were originally described by Fetsch et al. as a distinct lesion occurring mainly in the soft tissue. Since then, there have been reports of calcifying fibrous pseudotumours arising in the pleura, lung and mesenteric fat among others. Calcifying fibrous pseudotumour is a benign lesion characterized by the presence of abundant hyalinized collagen with psammomatous calcification and only a minimal lymphoplasmacytic cell infiltrate. It has been suggested that calcifying fibrous pseudotumours may represent an alternative evolution for some inflammatory pseudotumours given the rare case report showing overlapping histological features of calcifying fibrous pseudotumours and inflammatory pseudotumours in patients with multiple peritoneal nodules.

Reference(s):
– Pomplun S, Goldstraw P, Davies SE, Burke MM, Nicholson AG.
Calcifying fibrous pseudotumour arising within an inflammatory pseudotumour:
evidence of progression from one lesion to the other? Histopathology. 2000 Oct;37(4):380-2.