This is a very tricky case! Sometimes monophasic synovial sarcoma can be quite bland and fibroblastic in appearance (I once saw a case on needle biopsy that was so bland and hypocellular it made me consider low-grade fibromyxoid sarcoma! On resection it was obvious synovial sarcoma).
Synovial sarcoma has very uniform monotonous slightly plump spindle cells. As with most translocation sarcomas, all of the cells look just like each other (Uniform/monotonous tumor nuclei with no pleomorphism). Sclerotic collagen and calcifications (both seen on left side of image) are useful clues. Synovial sarcoma often has dilated Branching staghorn vessels very similar to those seen in solitary fibrous tumor, making SFT a potential mimic of synovial sarcoma and vice versa.
More info about synovial sarcoma:
Video (101): https://kikoxp.com/posts/4105
Synovial sarcoma has very uniform monotonous slightly plump spindle cells. As with most translocation sarcomas, all of the cells look just like each other (Uniform/monotonous tumor nuclei with no pleomorphism). Sclerotic collagen and calcifications (both seen on left side of image) are useful clues. Synovial sarcoma often has dilated Branching staghorn vessels very similar to those seen in solitary fibrous tumor, making SFT a potential mimic of synovial sarcoma and vice versa.
More info about synovial sarcoma:
Video (101): https://kikoxp.com/posts/4105
Video (short - biphasic synovial sarcoma): https://kikoxp.com/posts/4685
WSI digital slide (biphasic synovial sarcoma): https://kikoxp.com/posts/8229
Image (biphasic with striped pattern & immunostains I like for synovial): https://kikoxp.com/posts/11073
Image (tricky monophasic synovial sarcoma bland & benign-appearing!): https://kikoxp.com/posts/11075
On totally classic biphasic synovial sarcoma cases, I personally feel comfortable making the diagnosis on H&E only. But in monophasic synovial sarcoma or in my face that cases with unusual appearance or clinical scenario, I perform immunostains. I like pancytokeratin and EMA as general screening stains for synovial sarcoma and I love the new SSX and SSX-SS18 immunostains (thanks Jason Hornick et al!!!) as specific markers to confirm the diagnosis. TLE1 works but it’s not very specific. I’ve seen it stain a lot of other tumors so I’m not a huge fan of it personally. In very difficult cases or where the immunostaining is unusual, I sent the case for SS18 break apart FISH to confirm the diagnosis. Keep in mind that S100 expression can be seen in a subset of synovial sarcoma. CD34 can also be useful here by its absence…it is usually totally negative in synovial sarcoma. I don’t find CD99 or BCL2 useful. They are too non specific. I pretty much only use CD99 for small round blue cell tumors (as a screening marker for Ewing sarcoma); I don’t use it for spindle cell tumors. I never use BCL2 for soft tissue tumors. And before you even ask, you KNOW how I feel about vimentin… (Detailed explanation of why I don’t like vimentin: https://kikoxp.com/posts/4771).
A complete organized library of all my videos, digital slides, pics, & sample pathology reports is available here: https://kikoxp.com/posts/5084 (dermpath) & https://kikoxp.com/posts/5083 (bone/soft tissue sarcoma pathology).
A complete organized library of all my videos, digital slides, pics, & sample pathology reports is available here: https://kikoxp.com/posts/5084 (dermpath) & https://kikoxp.com/posts/5083 (bone/soft tissue sarcoma pathology).