This video provides a microscopic pathology review of a nodule excised from dense connective tissue, identified as a Giant Cell Tumor of the Tendon Sheath (GCTTS), also known as Tenosynovial Giant Cell Tumor (localized type). The presenter highlights the key histological features: the presence of numerous large, plump, mononuclear neoplastic cells with abundant cytoplasm, mixed with scattered large osteoclastic giant cells. Although mitotic activity can be common, the focus should be on identifying the true neoplastic mononuclear cells rather than being intimidated by the mitotic count. Other characteristic findings discussed include dense collagen background, potential pools of discohesive mononuclear cells, and the presence of hemosiderin, sometimes forming a halo around the plump cells ("ladybird cells"). The presenter notes that while GCTTS is typically managed by orthopedic surgeons in deeper tissues, it is rare, but occasionally encountered in dermatopathology if the lesion pushes into the dermis, warning to differentiate it from the unrelated Giant Cell Tumor of Soft Tissue.
Microscopic Pathology of Giant Cell Tumor of Tendon Sheath
0:00:12 Nodule Attachment: The excised nodule is attached to a dense band of regular connective tissue, identified as a tendon sheath (hand or foot).
0:00:30 Key Cellular Components: The tissue contains large osteoclastic giant cells scattered throughout. The true neoplastic cells are the plump, massive mononuclear cells with abundant cytoplasm, sometimes exhibiting a plasma cytoid or rhabdoid appearance.
0:00:47 Mitotic Activity: Mitotic figures are relatively common or even abundant, which can be concerning but is typical for this entity.
0:01:04 Diagnosis: The tumor is a Giant Cell Tumor of the Tendon Sheath (GCTTS), or localized type Tenosynovial Giant Cell Tumor.
0:01:15 Neoplastic Cells vs. Giant Cells: The plump mononuclear cells are the neoplastic component; the giant cells are recruited histiocytes.
0:01:35 Background Features: The background often shows dense collagen and sometimes small pools of discohesive mononuclear cells.
0:01:44 Hemosiderin: Variable amounts of hemosiderin are often present, sometimes forming a characteristic halo around the plump mononuclear cells (known as "ladybird cells").
0:02:17 Rarity in Dermatology: GCTTS is usually excised by hand or orthopedic surgeons. It rarely pushes into the dermis to be seen by dermatologists.
0:02:48 Differential Diagnosis: If seen in the dermis, it is crucial to differentiate GCTTS from the unrelated Giant Cell Tumor of Soft Tissue.