A 62-year-old man undergoes resection of a retroperitoneal mass. Histology shows spindle cells in a storiform pattern with thick hyalinized collagen bands and scattered plasma cells. IHC shows IgG4+ plasma cells > 50 per HPF with IgG4/IgG ratio > 40%. The WHO 2022 classification of soft tissue tumors would categorize this as:
- A Solitary fibrous tumor, malignant type
- B IgG4-related disease (IgG4-RD) — inflammatory pseudotumor variant ✓
- C Low-grade fibromyxoid sarcoma
- D Inflammatory myofibroblastic tumor (IMT) with ALK rearrangement
Explanation
IgG4-related disease (IgG4-RD) commonly presents as a pseudotumoral mass in the retroperitoneum and other sites. Diagnostic criteria include storiform fibrosis, lymphoplasmacytic infiltrate, >10 IgG4+ cells/HPF (often >50 in dense lesions), and IgG4/IgG ratio >40%. The WHO 2022 recognizes IgG4-RD as a distinct inflammatory condition rather than a neoplasm. Solitary fibrous tumor shows hemangiopericytoma-like pattern and STAT6 nuclear expression. Low-grade fibromyxoid sarcoma has bland spindle cells in alternating fibrous and myxoid zones with FUS-CREB3L2 fusion. IMT has ALK IHC positivity and lacks the IgG4 plasma cell predominance.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.