Flavokawain B, a kava chalcone, induces apoptosis in synovial sarcoma cell lines:
http://www.ncbi.nlm.nih.gov/pubmed/22213202
This is an in vitro study using 2 synovial sarcoma cell lines that may or may not be representative of all synovial sarcoma. And unfortunately, flavokawain B is apparently hepatotoxic: