A case of retroperitoneal tumor displaying epithelial differentiation, prominent myxoid stroma and loss of INI1/SMARCB1

Abstract

The clinicopathological spectrum of INI1 deficient tumors is expanding. Epithelioid sarcoma (ES) is a rare sarcoma of uncertain differentiation, more often occurring in the extremities and uncommonly in the deep soft tissues. Histopathologically, it manifests in the form of classical, proximal, or hybrid types, the latter two characterized by rhabdoid cytomorphology. Immunohistochemically, ESs display loss of INI1/SMARCB1 and genetically associated with high percentage of SMARCB1 deletions.

We report an extremely uncommon case of a retroperitoneal tumor in a 42-year-old male, who presented with abdominal discomfort. Radiologic imaging disclosed a 12 cm-sized retroperitoneal mass without involvement of any organ parenchyma. The patient underwent tumor excision with left-sided nephrectomy at another hospital. A review of the paraffin-embedded tissue sections revealed a multinodular tumor, composed of dyscohesive epithelioid tumor cells and focally arranged in cords, containing moderate to abundant, eosinophilic cytoplasm, vesicular nuclei, containing prominent nucleoli, including cells with rhabdoid cytomorphology, in a conspicuous myxoid stroma. A focal tumor area resembled proximal-type of ES. Immunohistochemically, tumor cells displayed positivity for pan cytokeratin (AE1/AE3), epithelial membrane antigen (EMA), vimentin and focally for CA125, while these were negative for CD34, S100 protein, CKIT, DOG1, and INI1/SMARCB1.

To the best of our knowledge, this constitutes the first case of a malignant tumor with epithelioid morphology, displaying myxoid matrix and loss of INI1/SMARCB1, resembling a myxoid variant of an epithelioid sarcoma and myoepithelioma-like tumor of the vulvar tumor, occurring in the retroperitoneum. A review of similar cases, differential diagnosis and treatment-associated implications are presented.

Keywords: INI1/SMARCB1; epithelioid sarcoma; myxoid tumors; rare tumor; retroperitoneal sarcoma.

Figure 1.

Computed tomogram (CT) scan showing a well-defined, large, heterogeneous soft tissue mass (dotted lines) in the left paravertebral region of the retroperitoneum.

Figure 2.

Microscopic appearance. Multiple ill-defined nodules of tumor cells embedded in a prominent myxoid stroma and surrounded by lymphoid aggregates. H and E, x 100.

Figure 3.

Loosely cohesive tumor cells arranged in cords and singly dispersed in an abundant myxoid stroma with interspersed lymphocytes and few thin-walled blood vessels. H and E, x 200.

Figure 4.

(A) Predominant polygonal-shaped cells with few interspersed multinucleate sarcomatous giant cells. H and E, x200. (B) Higher magnification showing cells with abundant eosinophilic cytoplasm and prominent nucleoli, arranged in cords in a myxoid stroma, focally surrounded by empty spaces, causing a pseudochondroid appearance. H and E, x 400. (C) Focal area of tumor necrosis. H and E, x 200. (D) Focal area displaying tumor cells with rhabdoid appearance, reminiscent of proximal-type epithelioid sarcoma. H and E, x 400.

Figure 5.

Immunohistochemical results. (A) Diffuse AE1/AE3 positivity. DAB, x 400. (B) Diffuse vimentin positivity. DAB, x 400. (C) CD34 negativity. DAB, x 400. (D) Focal CA125 positivity. DAB, x 400. (E) Tumor cells displaying diffuse loss of INI1/SMARCB1. DAB, x 400.

Figure 6.

EWSR1 gene rearrangement by fluorescence in-situ hybridization. Most tumor cell nuclei show double fused or single red green fused signals, indicative of lack of EWSR1 gene rearrangement. DAPI, x 1000.