[1562] Omental Fibromyxoid Tumor (OFT): A Distinctive Variant of Inflammatory Myofibroblastic Tumor? A Clinicopathologic and Immunophenotypic Analysis

R Alaggio, A Leszl, ESG d'Amore, PM Chou. University of Padua, Padova, Italy; Ospedale S. Bortolo, Vicenza, Italy; Children's Memorial Hospital, Northwestern University, Chicago, IL

Background: OFT, described by Gonzalez-Crussi as omental-mesenteric myxoid hamartoma, is considered part of the morphologic spectrum of IMT. It occurs in infants as abdominal nodules composed of mesenchymal and inflammatory cells in a myxoid stroma. We retrospectively investigated a series of OFT and compared their clinicopathologic and immunohistochemical features to classic IMT in order to explore their biological relationship.
Design: H&E stained section from 5 OFT and 11 abdominal IMT were evaluated as well as smooth muscle actin, desmin, ALK1, cytokeratins (AE1/AE3, Cam 5.2) and WT1 expression. Clinicopathologic information was retrieved from medical records.
Results: The mean age was 0.8 years for OFT and 8 for IMT. Mean follow up, available in all cases, but one OFT, was 4.6 years. All patients are disease free, after relapses in 1 OFT and 3 IMT. Histologically, OFT showed a myxoid stroma with hyalinized vessels, stellate or elongated mesenchymal cells and a discrete lymphocytic infiltrate, often perivascular . Pleomorphism and necrosis were absent, mitoses rare. IMT had a variable morphology, with myofibroblasts, ganglion-like cells and a prominent inflammatory infiltrate. A myxoid background was present in 3 cases and necrosis in 2. Mitoses were 2 to 7/HPF. Immunohistochemically, 90% of OFT and IMT expressed SMA. Desmin and cytokeratins were diffusely positive in OFT and only focally positive in 4 and 3 IMT respectively. Nuclear staining for WT1 was seen in OFT, but absent in IMT (although a cytoplasmic positivity was noted in 4). 4 OFT showed a peculiar dot-like ALK staining, whilst 4 IMT displayed a cytoplasmic positivity. Cytogenetic analysis in one OFT revealed a chromosomal mosaicism with inversion of chr 2(p14-16q37).
Conclusions: OFT may represent an infantile variant of IMT, with characteristic clinicopathologic, immunophenotypic and genetic features. OFT are histologically more homogeneous than IMT, with a prominent myxoid background and a bland cytology. The expression of cytokeratins and desmin and the nuclear staining for WT1, suggest a possible origin from sub-mesothelial fibroblasts. The dot-like pattern of ALK staining might not be related to a rearrangement of ALK gene, an hypothesis supported by the inversion of chr 2 in a different locus in one case. Further studies will clarify the cytogenetic features and the prognosis of OFT.
Category: Pediatrics

Monday, March 9, 2009 11:45 AM

Platform Session: Section H2, Monday Morning