This video provides a detailed pathological overview of Ossifying Fibroma-Myxoid Tumor of Soft Parts (OFMT), a rare soft tissue neoplasm usually found in the subcutis, most commonly in the lower extremity. The hallmark feature of conventional OFMT is the presence of a peripheral shell of ossification surrounding a central tumor composed of bland, uniform, round-to-oval cells. A critical diagnostic feature is the arrangement of these cells in distinct cords or trabeculae within a matrix that varies between fibrous (sclerotic) and myxoid. Immunophenotypically, OFMT is unique, often displaying a "scrambled" profile, notably the co-expression of S100 and Desmin. While typically indolent and associated with a PHF1 gene rearrangement, the video also demonstrates rare malignant variants characterized by nuclear atypia, high mitotic activity, necrosis, lymphovascular invasion, and an association with Chromosome 22 loss.
Ossifying Fibroma-Myxoid Tumor of Soft Parts (OFMT): Pathology and Variants
0:05 Introduction and Location: OFMT is an unusual and rare soft tissue tumor, often presenting in the subcutis of the lower extremity (thigh is the most common site). Most cases exhibit indolent behavior, but local recurrence is possible.
0:49 Classic Feature (Ossification): The majority of OFMTs are characterized by a peripheral shell (or incomplete shell) of bone/ossification surrounding the central tumor mass. However, a significant subset (approx. 20%) can lack bone entirely, requiring reliance on other histological features (5:37).
1:17 Cellular Morphology and Uniformity: The tumor cells are typically uniform, round-to-oval, with bland nuclei, fine chromatin, and very low mitotic activity (2:34). The uniformity of the cells is a clue suggesting a translocation-associated tumor (1:52).
3:13 Key Histological Pattern: The cells are arranged in highly distinct long rows, cords, or trabeculae within a variable background matrix that can be dense fibrous collagen or abundant blue myxoid material (3:26).
3:40 Differential Diagnosis: When cords or chains of cells are observed in soft tissue, the differential includes OFMT, Myoepithelial tumors, Epithelioid hemangioendothelioma, and Chordomas.
7:37 Scrambled Immunophenotype: IHC is tricky but helpful. OFMTs are often positive for S100. Critically, about half of cases co-express S100 (a neural marker) and Desmin (a muscle marker). This co-expression is a very unusual pattern and strongly suggests OFMT (8:00).
9:53 Malignant OFMT (Rare Forms): Rare atypical and malignant forms exist. Criteria for malignancy include marked nuclear atypia (pleomorphism, hyperchromatic nuclei) and high mitotic activity (e.g., >2 mitoses per 50 high-power fields, 10:14).
11:54 Transitional and Aggressive Cases: Examples of malignant OFMT show abrupt transitions from benign-appearing myxoid areas to highly cellular zones with dramatic atypia and high mitotic counts (e.g., 3 mitoses in one small field, 13:20), and tumor cell necrosis (13:45).
14:54 Metastatic Disease: Malignant OFMT can metastasize distantly and behave aggressively, evidenced by cases showing lymphovascular invasion (LVI, 14:57). However, even with metastasis, the clinical course can be variable (rapid vs. long, indolent disease progression).
15:21 Molecular Markers: Conventional OFMTs usually exhibit a rearrangement of the PHF1 gene. In contrast, malignant OFMTs seem to often lack the PHF1 rearrangement and instead show loss of Chromosome 22 (15:47).