This video focuses on the histopathology of Giant Cell Tumor of Tendon Sheath (GCTTS), also known as Tenosynovial Giant Cell Tumor, localized type, emphasizing its relevance to dermatopathology despite its typically deep location in the distal extremities. The presentation highlights that relying solely on the presence of multinucleated giant cells is insufficient, as "giant cell poor" variants exist which can be misdiagnosed as malignancy due to high cellularity and mitotic activity. Key diagnostic features include the multi-nodular, well-circumscribed nature, sclerotic collagen background, and, most importantly, heavy hemosiderin deposition. The neoplastic cells are mononuclear histiocytoid cells with abundant plasmacytoid cytoplasm; when these cells contain hemosiderin rings, they form a characteristic "ladybird cell" pattern considered pathognomonic for this family of tumors.
Summarizing Giant Cell Tumor of Tendon Sheath (GCTTS) Pathology
0:06 Tumor Identification and Location: Giant Cell Tumor of Tendon Sheath (GCTTS) is the localized type of Tenosynovial Giant Cell Tumor. It usually arises deep from the tendon sheath, primarily in the digits or distal extremities (hands, feet, wrist, ankles).
0:34 Derm Path Context: Most cases go to surgeons, but dermatopathologists may occasionally see GCTTS if the tumor pushes up into the dermis or ulcerates the overlying skin.
1:17 Low Power Appearance: The tumor is characteristically multi-nodular, well-circumscribed, and typically found deep, attached to the tendon sheath.
1:55 Key Background Features: The stroma contains variable cellularity with sclerotic, hyalinized collagen. Hemosiderin deposition is a very helpful and usually abundant clue.
2:24 Diagnostic Pitfall: Cases can be "giant cell poor" or even "giant cell free," meaning diagnosis cannot rely solely on the presence of giant cells. Because these tumors can be highly cellular and mitotically active, there is a risk of misdiagnosis as something malignant.
2:51 Lesional Cells: The true neoplastic cells are the mononuclear histiocytoid cells (the giant cells are secondary). These cells often form "discohesive pools" or aggregates within the sclerotic background.
3:53 Mononuclear Cell Cytology: These cells are often plasmacytoid or rhabdoid-like, featuring large nuclei, pale chromatin, punctate nucleoli, and abundant, dense, pink cytoplasm that pushes the nucleus eccentrically.
5:31 Pathognomonic Finding (Hemosiderin Halo): Hemosiderin deposition is very common. When the iron pigment arranges itself as a "halo" around the cytoplasm of the histiocytoid cells, these are termed "ladybird cells" or "ladybug cells" and are considered highly diagnostic.
6:38 Nomenclature Distinction: The localized GCTTS (found distally) and the diffuse type (known as Pigmented Villonodular Synovitis, found in proximal, larger joints) are made up of the same cell types and look identical at high power.
6:41 Other Cellular Features: Foamy histiocytes (xanthoma cells) are often found, particularly around the periphery of the lesion.