Last night I posted this lovely gross picture; I’m now adding a couple of histologic images. Kudos to @kriyer68 and @DrAldehyde for being the first to submit the correct answer- this is indeed a synovial sarcoma! Not a bad time for a #Tweetorial #BSTPath 
Synovial sarcoma is actually uncommon in joint cavities & does not recapitulate synovium in any way. Its rare amongst soft tissue sarcomas, representing only 5-10% of the cases. The disease is most prevalent in teenagers & young adults; 50% of cases are seen in pediatric patients
These most commonly present as pain in the affected site with an associated mass. In many cases the duration of symptoms is 2-5 years. They are seen most commonly in the extremities and are often associated with tendons. #BSTPath
Calcifications may be seen on imaging studies, particularly on plain radiographs and CT scans. Grossly they tend to be well circumscribed. They can have multiple lobules and are often yellow to grey. #BSTPath
Histologically, these tumors are moslty biphasic (the earliest described type) or monophasic. The monophasic variants can be purely spindle cell or purely epithelioid. There are poorly differentiated tumors which are round cell morphologically and carry a poor prognosis #BSTPath
The tumors can be intensely vascular and may contain course calcifications. Some cases demonstrate “hemangiopericytoma like” vasculature. #BSTPath
Most synovial sarcomas are immunoreactive to CK and EMA. Many demonstrate immunoreactivity to CD99 and BCL2. TLE1 is reactive in over 90% of synovial sarcomas. #BSTPath
Synovial sarcomas demonstrate a classic translocation, t(X:18)(p11.2:q11.2) which results in a fusion product between SS18(18) and either SSX1, SSX2, or SSX4 (X). This can be done by traditional karyotyping or by FISH studies. #BSTPath
With proper surgical resection and neoadjuvant or adjuvant therapy, local recurrence rates are around 40%. Unfortunately ~50% of cases metastasize with lung being one of the most common sites. Notably this is a sarcoma that metastasizes to lymph nodes. #BSTPath
And to add everyone who was tagged or responded (this may take a few tweets... @Alipathy @pembeoltulu @PPatilMD @EAljufairi @ALBoothMD @marcoslepe @marenwhymd @bluehatcomics85 @Dr_DR_Cells @ElviraGlezO @DrAhmedKalebi @ElshaikhMD @VijayPatho @histolover @DraEosina @lara_pijuan
One thing I forgot to add yesterday; differential diagnosis! Considerations include solitary fibrous tumor, malignant peripheral nerve sheath tumor, carcinosarcoma, fibrosarcoma, & leiomyosarcoma. Consider primitive neuroectodermal tumor in poorly differentiated round cell tumors
