A rare case of deep fibrous histiocytoma with low-grade myxoid and dedifferentiated liposarcoma features: clinical, radiological, and histopathological insights.

Deep fibrous histiocytoma (DFH), also termed deep benign fibrous histiocytoma, is an uncommon fibroblastic neoplasm that typically arises in the dermis or subcutis. Occurrence in deep soft tissues is rare and retroperitoneal presentation is exceptional. Distinguishing DFH from soft tissue sarcomas can be challenging when lesions show atypical morphology or focal lipogenic change. We report a giant retroperitoneal DFH displaying low-grade myxoid and dedifferentiated liposarcoma-like area and summarize diagnostic pearls and management considerations. A 53-year-old man presented with postprandial dyspepsia and increased bowel movements. Imaging revealed a multilobulated retroperitoneal mass (35 × 32 × 18 cm) displacing the inferior vena cava, aorta, and bowel loops, with the left kidney ectopically located in the right paramedian pelvis. Through a xipho-pubic laparotomy, a well-encapsulated 12-kg tumor was excised en bloc without rupture. Histology showed a spindle-cell proliferation with storiform and meningothelial-like architecture consistent with DFH, with foci of low-grade myxoid change and areas mimicking dedifferentiated liposarcoma. Immunohistochemistry demonstrated CD34 positivity and negativity for S100 and smooth muscle actin in the fibrohistiocytic component. The early postoperative course was uneventful. Retroperitoneal DFH is a diagnostic mimic of liposarcoma and other sarcomas. Correlation of morphology with an appropriate immunophenotype and clinical-radiologic context is essential to avoid overtreatment. Complete surgical excision is the mainstay of therapy; long-term surveillance is advisable given the deep location and size.