solitary fibrous tumor; fibrous mesothelioma; pleural fibroma

Introduction

originally described as a mesenchymal pleural tumor; can occur at many different sites.

Microscopic pathology

• patternless architecture
• alternating hypo- & hypercellular areas separated by thick bands of hyalinized collagen and branching vessels
• round to spindle shaped cells
• little cytoplasm
• indistinct borders
• vesicular nuclei
• myxoid areas, fibrosis common
• mitoses scarce, rarely > 3 per 10 HPF malignant solitary fibrous tumor:
• hypercellular
• at least focal moderate to marked cytologic atypia
• tumor necrosis
• numerous mitoses (> 4 per HPF)
• infiltrative margins

Immunophenotype

• CD34 + (~90-95%)
• CD99 + (70%)
• bcl-2 variable + (~20-35%)
• EMA variable + (~20-35%)
• vimentin +
• cytokeratin usually -
• CD31 -
• actin -

Clinical manifestations

• well delineated, slow growing, painless mass
• compression symptoms if large (nasal cavity, orbit or meninges)
• most cases benign
  • ~10-15% behave aggressively (not strict correlation between morphology and behavior although most can be predicted by histology)
• tumors in mediastinum, abdomen, pelvis and retroperitoneum tend to be more aggressive than in extremities
• metastases to bone, liver, lung

More general terms

• intermediate grade fibroblastic/myofibroblastic neoplasm