Retroperitoneal Sarcomas: An Update on the Diagnostic Pathology Approach

Abstract

Retroperitoneal sarcomas are a heterogenous group of rare tumors arising in the retroperitoneum. Retroperitoneal sarcomas comprise approximately 10% of all soft tissue sarcomas. Though any soft tissue sarcoma histologic types may arise in the retroperitoneal space, liposarcoma (especially well-differentiated and dedifferentiated types) and leiomyosarcoma do so most commonly. Retroperitoneal sarcomas are diagnostically challenging, owing to their diversity and morphological overlap with other tumors arising in the retroperitoneum. An accurate diagnosis is necessary for correct management and prognostication. Herein, we provide an update on the diagnostic approach to retroperitoneal sarcomas and review their key histologic findings and differential diagnoses.

Keywords: leiomyosarcoma; liposarcoma; pathology; retroperitoneal space; sarcoma.

Figure 1

Well-differentiated liposarcoma, inflammatory subtype. (a) The tumor shows an abundant chronic inflammatory cell infiltrate and scattered atypical pleomorphic stromal cells. (b) The atypical stromal cells are positive for MDM2 (H&E stain, original magnification 100× a; MDM2 immunostain, original magnification 200× b).

Figure 2

Dedifferentiated liposarcoma. (a) The tumor shows an abrupt transition from well-differentiated liposarcoma component to a non-lipogenic, high-grade dedifferentiated area (right). (b) The dedifferentiated component shows MDM2 amplification by FISH. The green signal corresponds to the MDM2 probe, while the red signal corresponds to the chromosome 12 centromeric probe (H&E stain, original magnification 100× a; MDM2 FISH, original magnification 1000× b).

Figure 3

Dedifferentiated liposarcoma with homologous lipoblastic differentiation. (a) The tumor shows an abrupt transition from well-differentiated liposarcoma component to dedifferentiated component. (b) The dedifferentiated component shows homologous lipoblastic differentiation with scattered lipoblasts. The morphology resembles pleomorphic liposarcoma (H&E stain, original magnifications 100× a and 200× b).

Figure 4

Leiomyosarcoma. (a) The tumor cells have cigar-shaped, blunt-ended nuclei with brightly eosinophilic cytoplasm and a fascicular pattern. Mitoses are present. (b) The tumor cells are diffusely positive for SMA (left) and desmin (right) (H&E stain, original magnification 20 × a; SMA and desmin immunostain, original magnification 200× b).

Figure 5

Dedifferentiated solitary fibrous tumor. (a) The tumor shows an abrupt transition from typical solitary fibrous tumor to high-grade dedifferentiated area. (b) The typical solitary fibrous area shows strong and diffuse positive nuclear expression of STAT6 and the dedifferentiated area shows loss of STAT6 expression (H&E stain, original magnification 100× a; STAT6 immunostain, original magnification 100× b).

Figure 6

Epithelioid inflammatory myofibroblastic sarcoma. (a) The tumor cells are round to epithelioid-shaped and have prominent nucleoli, with mild infiltration of inflammatory cells, including neutrophils and lymphocytes. (b) The tumor cells show nuclear membranous staining for ALK (H&E stain, original magnification 200× a; ALK immunostain, original magnification 200× b).

Figure 7

Embryonal rhabdomyosarcoma. (a) The tumor is composed of primitive round to spindle cells and eosinophilic rhabdomyoblasts. (b) The tumor cells are positive for desmin (left) and myogenin (right) (H&E stain, original magnification 200× a; desmin and myogenin immunostain, original magnification 200× b).

Figure 8

Malignant peripheral nerve sheath tumor. (a) The tumor is composed of spindle cells with alternating hypercellular and hypocellular areas. (b) The tumor cells show loss of H3K27me3 expression (H&E stain, original magnification 100× a; H3K27me3 immunostain, original magnification 200× b).

Figure 9

Extraskeletal osteosarcoma. (a) The tumor consists of hyperchromatic tumor cells producing lace-like neoplastic bone. (b) The tumor shows a cartilaginous area (H&E stain, original magnification 200× a and 200× b).

Figure 10

Poorly differentiated synovial sarcoma. (a) The tumor shows hypercellular rounded tumor cells. Slightly spindled tumor cells are also present. Mitotic figures are frequently seen. (b) The tumor cells are strongly and diffusely positive for TLE1 (left) and focally positive for cytokeratin (AE1/AE3) (right) (H&E stain, original magnification 200× a; TLE1 and cytokeratin (AE1/AE3) immunostain, original magnification 200× b).

Figure 11

Desmoplastic small round cell tumor. (a) The tumor shows nests of small round tumor cells within desmoplastic fibrous stroma. (b) The tumor cells are positive for cytokeratin (AE1/AE3) (left) and show perinuclear dot-like expression for desmin (right) (H&E stain, original magnification 200× a; cytokeratin (AE1/AE3) and desmin immunostain, original magnification 200× b).

Figure 12

Sclerosing PEComa. (a) The tumor shows nests of epithelioid tumor cells with abundant clear cytoplasm. The densely hyalinized collagenous stroma is present. (b) The tumor cells are positive for SMA (H&E stain, original magnification 100× a; SMA immunostain, original magnification 200× b).

Figure 13

Extraskeletal Ewing sarcoma. (a) The tumor is composed of uniform small round tumor cells arranged in vaguely lobular pattern. The tumor cells have finely dispersed chromatin and scanty cytoplasm. (b) The tumor cells show diffusely membranous expression for CD99 (left) and diffusely nuclear expression for NKX2.2 (right) (H&E stain, original magnification 100× a; CD99 and NKX2.2 immunostain, original magnification 100× b).