Pathology: synovial sarcoma

Lobulated; loose fascicles of spindle-stellate cells

Scant pale eosinophilic cytoplasm

Ovoid nuclei with mild pleomorphism; mitotic activity

Myxoid-collagenous stroma with wiry collagen between the cells; metaplastic bone and cartilage; dystrophic calcification

IHC: S100, keratin (AE1/AE3), epithelial membrane antigen

RNA-seq: SS18::SSX1 gene fusion

Variably cellular; loose fascicles of spindle cells

Pale eosinophilic cytoplasm

Ovoid nuclei with mild pleomorphism; inconspicuous mitotic activity

Myxoid-collagenous stroma; Ectatic-hyalinized blood vessels

IHC: S100, SOX10, epithelial membrane antigen

RNA-seq: SS18::SSX2 gene fusion

Fascicles of spindle cells

Eosinophilic cytoplasm

Ovoid nuclei with fine chromatin and inconspicuous nucleoli; occasional mitotic activity

Wiry collagen interspersed between the cells; dystrophic calcification

IHC: AE1/AE3, SATB2

RNA-seq: SS18::SSX1 gene fusion

Cellular; sheets and nests of polygonal cells and fascicles of spindle cells

Scant amphophilic cytoplasm

Round-ovoid nuclei with crowding, mild pleomorphism; mitotic activity

Wiry collagen interspersed between the cells;  thin-walled, branching vessels; necrosis 

RNA-seq: SS18::SSX2 gene fusion

Cellular; spindle cell fascicles

Eosinophilic cytoplasm with indistinct cell borders

Ovoid nuclei with striking palisading, mild pleomorphism; mitotic activity

Abundant necrosis 

RNA-seq: SS18::SSX1 gene fusion

Cellular; spindle cell fascicles

Pale eosinophilic cytoplasm with indistinct cell borders

Ovoid nuclei with striking palisading, mild pleomorphism; mitotic activity

RNA-seq: SS18::SSX2 gene fusion

Cellular; biphasic; spindle-epithelioid cells in nests and fascicles with areas of gland formation 

Abundant pale amphophilic cytoplasm

Plump ovoid nuclei with crowding, prominent nucleoli and mild pleomorphism; mitotic activity

Wiry collagen interspersed between the cells; thin-walled, branching vessels; necrosis 

IHC: keratin (AE1/AE3)

RNA-seq: SS18::SSX2 gene fusion

Cellular; sheets-nests of polygonal cells 

Scant pale amphophilic cytoplasm

Round-ovoid nuclei with mild pleomorphism; mitotic activity

Wiry collagen interspersed between the cells; prominent branching thin-walled vasculature 

IHC: epithelial membrane antigen

RNA-seq: SS18::SSX2 gene fusion

Cellular; biphasic; spindle cell fascicles with epithelioid gland formation 

Abundant pale amphophilic cytoplasm

Plump ovoid nuclei with crowding, prominent nucleoli and mild pleomorphism; inconspicuous mitotic activity

Dystrophic calcification

RNA-seq: SS18::SSX1 gene fusion

Cellular; biphasic; spindle cell fascicles with epithelioid gland formation 

Abundant pale amphophilic cytoplasm

Ovoid-round nuclei with minimal pleomorphism; inconspicuous mitotic activity

RNA-seq: SS18::SSX1 gene fusion

Spindle cells with herringbone pattern

Amphophilic cytoplasm

Plump ovoid nuclei with crowding, mild pleomorphism; mitotic activity

Wiry collagen interspersed between the cells; occasional broad bands of collagen; focal myxoid stroma; prominent dystrophic calcification 

RNA-seq: SS18:SSX1 gene fusion 

Cellular; spindle cells with herringbone-storiform pattern

Scant amphophilic cytoplasm

Plump ovoid nuclei, minimal pleomorphism; mitotic activity

Focal myxoid stroma

RT-PCR: SS18::SSX gene fusion 

Spindle-stellate cells with herringbone-fascicular pattern

Scant amphophilic cytoplasm

Hyperchromatic ovoid nuclei, minimal pleomorphism; inconspicuous mitotic activity

Myxoid stroma

RNA-seq: SS18:SSX2 gene fusion 

Cellular; spindle cells with herringbone pattern

Minimal eosinophilic cytoplasm with long tapering processes

Elongated fusiform nuclei with mild pleomorphism; inconspicuous mitotic activity

Prominent wiry collagen interspersed between the cells

RT-PCR: SS18::SSX gene fusion 

Spindle-ovoid cells with vague fascicular pattern

Pale eosinophilic cytoplasm with indistinct cell borders

Monomorphic ovoid nuclei; rare mitotic activity 

Nodules of hyaline stroma (reminiscent of cartilage / osteoid)

RNA-seq: SS18:SSX2 gene fusion 

Cellular; sheets-nests of round cells 

Scant amphophilic cytoplasm

Round-ovoid nuclei with mild pleomorphism; mitotic activity

Prominent branching thin-walled vasculature 

IHC: keratin (AE1/AE3)

RT-PCR: SS18::SSX gene fusion