The synovium or synovial membrane can be a primary site of development of various tumor types. These tumors can be benign or malignant. While benign tumors behave in an indolent manner, malignant tumors show high metastatic potential and can therefore be lethal for the patients. The latest editions of the World Health Organization (WHO) classification for soft tissue and bone tumors initiated also the intermediate category, where tumors are neither indolent nor frankly malignant. In this category, tumors can be locally aggressive or can rarely metastasize, but the metastatic potential is in this case rather low. For the soft tissue tumors, grade and stage are better predictors of disease outcome than the histologic type. On the other hand, grading of bone tumors is very difficult to assess, giving their degree of heterogeneity in combination with the rarity of these neoplasms. Therefore, histologic (sub)type is in these cases a better prognostic marker. The only true synovial tumors are the fibroma of the tendon sheath and the tenosynovial giant cell tumor. In very rare instances, synovium can also be the primer site of involvement for a variety of soft tissue and bone tumors, either benign, intermediate, or malignant.
Kerckhofs, A., & Siozopoulou, V. (2023). Classification of Synovial Tumors According to WHO 2020. In Filip M. Vanhoenacker, Mohamed Fethi Ladeb (ed.), Imaging of Synovial Tumors and Tumor-like Conditions (p. p. 15-19). Springer Cham. https://doi.org/10.1007/174_2023_408