This video presents a detailed pathology case of a large, deep-seated tumor from the thigh of an elderly patient, identified as Monophasic Synovial Sarcoma. The focus is on the tumor's characteristic highly cellular morphology and its classic architectural arrangement of streaming fascicles forming a "herringbone" pattern. The presenter discusses the historical misclassification of herringbone pattern tumors as fibrosarcoma and emphasizes the broad differential diagnosis required in modern practice (including MPNST and DFSP). The diagnosis relies on immunohistochemistry (Keratin, EMA, TLE1 positivity) and molecular studies targeting the SS-18 fusion gene, for which newer fusion product immunostains are now available. A key takeaway highlights that translocation-associated sarcomas, such as synovial sarcoma, typically lack nuclear pleomorphism.
0:00:08 Case Overview: The tumor is sourced from the thigh of an elderly adult, presenting as a very large, deep-seated cellular mass that has grown up to involve the skin surface.
0:00:50 Microscopic Features: The tumor is highly cellular with closely packed nuclei and streaming fascicles. The cells have a plump, oval morphology and demonstrate readily identifiable mitotic activity (0:06:28).
0:01:19 Herringbone Pattern: The tumor displays the classic "herringbone" (also called fishbone or chevron) pattern, a visually striking feature where cellular fascicles converge at acute angles.
0:01:41 Differential Diagnosis: Historically, the herringbone pattern was considered diagnostic of fibrosarcoma, but modern pathology links it to Synovial Sarcoma, Malignant Peripheral Nerve Sheath Tumor (MPNST), DFSP (fibrosarcomatous DFSP), and dedifferentiated liposarcoma.
0:02:24 Superficial Herringbone: If the herringbone pattern occurs primarily in the skin, the primary consideration should be fibrosarcomatous DFSP, with Synovial Sarcoma being an exception to this rule.
0:04:00 Additional Features: The tumor exhibits dilated vessels in some areas, commonly presenting as a staghorn dilated vascular pattern, which can also be seen in solitary fibrous tumors and DFSP.
0:04:19 Final Diagnosis and IHC: The diagnosis is Monophasic Synovial Sarcoma, supported by positive immunohistochemical stains for Keratin, EMA, and TLE1.
0:04:57 Molecular Basis and Newer Stains: Synovial sarcomas are characterized by a fusion between the SS-18 gene and one of three SSX genes. The presenter notes the availability of newer, highly specific immunostains that detect this fusion product, which are useful alongside FISH for SS-18 break-apart.
0:05:48 Key Takeaway: Pleomorphism: Translocation-associated sarcomas (like synovial sarcoma) typically lack nuclear pleomorphism because the cells all share the same defining molecular abnormality.
0:06:56 Monophasic vs. Biphasic: The monophasic pattern (spindle cell only) is seen much more frequently than the classic biphasic pattern (which includes epithelial gland structures), making its recognition crucial.
0:07:16 Tricky Areas: Pathologists must be aware of less cellular, paler, or myxoid areas within the tumor that can mimic benign entities like plantar fibromatosis or low-grade fibromyxoid sarcoma, potentially leading to misdiagnosis on needle biopsy.