This video presents a pathology case study of a lower thigh mass in a 25-year-old male, confirming the diagnosis of monophasic synovial sarcoma. The discussion focuses on the key histological features, including the monotonous proliferation of plump spindle cells, hypercellularity, lack of nuclear pleomorphism (a characteristic of translocation sarcomas), and secondary features like prominent cystic breakdown, hemorrhage, and dilated staghorn vessels (a feature shared with solitary fibrous tumor, or SFT). Molecular confirmation requires testing for the SS18-SSX fusion. The video outlines the expected immunohistochemistry profile, emphasizing positivity for Keratin, EMA, and TLE1, while noting the diagnostic pitfall of patchy S100 positivity and the utility of CD34 negativity in differentiating it from SFT.
Synovial Sarcoma: Histology, Molecular Markers, and Immunoprofile
0:10 Case Introduction: A 25-year-old man presents with a lower thigh mass, diagnosed as monophasic synovial sarcoma.
0:27 Key Histological Features: The cells are described as monotonous, uniform, plump, slightly fat spindle cells, closely packed together resulting in a hypercellular, blue/purple appearance.
0:56 Translocation Sarcoma Traits: A crucial feature is the lack of pleomorphism, which is typical for translocation sarcomas. Mitoses are often present.
1:14 Growth Pattern: The cells tend to run in long, streaming fascicles that sometimes intersect at sharp angles (herringbone pattern).
1:25 Secondary Changes: The tumor frequently exhibits cystic breakdown, hemorrhage, and sometimes calcifications.
1:40 Diagnostic Pitfall (Hypocellularity): The tumor can contain hypocellular areas, sometimes with myxoid change, which can make diagnosis difficult on small biopsies.
2:03 Vascularity and Mimics: The presence of dilated, branching, staghorn vessels is noted (similar to Solitary Fibrous Tumor/SFT), making SFT a close mimic.
2:25 Molecular Abnormality: The characteristic genetic finding is the SS18-SSX fusion (SSX 1, 2, or 4). This can be tested using FISH or RT-PCR, and new SS18-SSX immunostains are becoming available.
3:45 Key IHC Profile: The tumor is expected to be positive for Keratin and EMA (often patchy and focal in spindle cells) and show strong, diffuse nuclear staining for TLE1.
4:40 Distinguishing Markers:CD34 is usually negative in synovial sarcoma, which helps differentiate it from SFT (which is strongly positive for CD34). CD99 often stains positive but is non-specific (mostly used to rule out Ewing sarcoma if negative).
5:27 S100 Pitfall: It is important to know that S100 can sometimes stain synovial sarcomas (usually patchily), which is a potential pitfall in distinguishing them from melanocytic or neural tumors.