This presentation is a detailed pathological review of a surgically excised, fragmented deep nodule from a 9-year-old female, initially suspected clinically to be a lipoma or cyst. The ultrasound findings indicated a complex mass with hemorrhage or hematoma suspected. The gross and microscopic examination revealed a highly heterogeneous lesion characterized by spindle to epithelioid cells with significant nuclear atypia, areas resembling herringbone patterns, sclerotic collagen, myxoid change, hemorrhage, and pseudocystic spaces, surrounded by a prominent lymphoplasmacytic inflammatory infiltrate. The presenter identifies this entity as Angiomatoid Fibrous Histiocytoma (AFH), emphasizing that despite its name, it is unrelated to dermatofibroma. AFH is characterized as a rare tumor of intermediate malignant potential, capable of local recurrence and rare metastasis. The discussion covers the classic histopathological triad (sheets of histiocytoid cells, central cystic space with blood/fibrin, and peripheral fibrous pseudocapsule with inflammation), common genetic fusions (most frequently $EWSR1-CRAB1$), and immunohistochemical findings (patchy Desmin/EMA, positive ALK). A key takeaway is the potential for misdiagnosis, especially when fragmentation or hemorrhage obscures the underlying neoplastic spindle cell component, highlighting the importance of vigilance for this entity in pediatric/young adult soft tissue nodules.
0:00 Case Presentation: Review of a fragmented, deep nodule excision from a 9-year-old female, initially clinically suspected as a lipoma/cyst.
0:34 Ultrasound Findings: Indicated a complex mass with internal echogenicity and a vascular component; differential diagnosis included hematoma or abscess, but neoplasm was not excluded.
0:51 Excision Result: The lesion fragmented during surgery, yielding serous fluid and fat, along with tissue submitted for pathology.
1:16 Heterogeneous Morphology: The fragmented tissue showed significant variation:
1:20 Cellular Areas: Spindle to epithelioid cells with significant hyperchromatic, pleomorphic atypia, sometimes showing a herringbone pattern.
1:43 Hemorrhage: Multiple areas of hemorrhage and extravasated erythrocytes.
2:00 Collagen/Cells: Sclerotic/keloidal collagen surrounded by spindle/boomerang-shaped cells.
2:34 Mixoid/Neural: Areas with myxoid background and loose cell arrangement, some foci appearing almost neural.
3:11 Pseudocystic Spaces: Large cystic spaces noted, potentially inherent to the lesion, containing blood and fibrin.
3:38 Inflammation: Brisk lymphoplasmacytic inflammatory infiltrate surrounding the lesion within a fibrous/sclerotic pseudocapsule.
4:51 Diagnosis: Angiomatoid Fibrous Histiocytoma (AFH): The presenter diagnoses the entity, noting its rarity and wide morphologic variability, which can lead to diagnostic confusion.
5:04 Diagnostic Warning: Stresses that AFH is not related to benign dermatofibroma, a point of historical confusion requiring explicit comments on pathology reports.
6:34 Classic Triad (Textbook Example): The classic presentation involves:
Sheets of histiocytoid cells with syncytial arrangement.
A central cystic area containing serum/blood/fibrin, often with hemosiderin.
A peripheral fibrous pseudocapsule with a thick band of lymphocytes and plasma cells, usually on the upper extremity of a child.
8:35 Molecular Genetics: AFH breaks the typical rule that fusion-associated tumors lack pleomorphism.
8:38 Common Fusions: Most common is $EWSR1-CRAB1$ (90% of cases). Others include $EWSR1-ATF1$ and $FUS-ATF1$.
10:08 Desmin/EMA: Patchy expression in about 50% of cases.
10:32 ALK: Expression noted in this case. ALK IHC is highlighted as a helpful diagnostic tool, even when ALK gene fusion is molecularly negative.
11:06 Behavior and Prognosis: AFH is considered a tumor of intermediate malignant potential.
11:29 Behavior: Usually indolent; local recurrence occurs, and rare regional lymph node metastasis is possible. Distant metastasis and death are very rare.
11:39 Management: Ideal management involves excision with negative margins.
12:10 Key Diagnostic Pitfall: Reminds viewers to look for the neoplastic spindle cell component when encountering fragmentation, blood, cystic change, and brisk inflammation in pediatric nodules, as reactive changes can mask the tumor.