This pathology session, led by Dr. Jared Gardner and Dr. Fatima Opsar, examines a range of challenging soft tissue and bone lesions, focusing on morphology and key diagnostic criteria. Cases include Epithelioid Fibrous Histiocytoma (EFH)—now recognized by ALK fusions and distinct from conventional Dermatofibroma (DF)—and Atypical Fibrous Histiocytoma (AFH), which features high-grade pleomorphism but retains benign DF architectural characteristics, essential for differentiation from Pleomorphic Dermal Sarcoma. The discussion also covers the classic presentation of Traumatic Neuroma (damaged, regenerating nerve bundles), the bland findings of Solitary Bone Cyst, the defining features of Schwannoma (Antoni A/B and degenerative atypia), and two synovial lesions: Lipoma Arborescens and Synovial Chondromatosis. The final case is a Paraganglioma, illustrating the Zellballen pattern and noting that bizarre nuclear atypia does not indicate malignant behavior in this tumor type.
Pathology Case Review: Soft Tissue, Bone, and Synovial Lesions
0:15 Epithelioid Fibrous Histiocytoma (EFH): Characterized by the collarette sign (epidermis reaching down around the lesion), a sharply circumscribed border, and plump epithelioid cells.
Key Detail: Formerly considered a DF variant, EFH is now confirmed to be a distinct entity, often positive for ALK fusions (0:04:11).
Takeaway: EFH is a benign, indolent lesion; rule out melanocytic lesions using S100/SOX10. Morphology can vary, including spindled or vacuolated (foamy) areas.
0:13:59 Dermatofibroma (DF): Conventional DF shows epidermal hyperplasia (tables and chairs pattern) and collagen trapping (0:15:16).
Variant:Lipidized Dermatofibroma (0:16:21) features abundant foamy/xanthomatous cytoplasm and hyalinized sclerotic collagen arcs; commonly found near the ankle.
Takeaway: DFs can include "monster cells" (scattered bizarre pleomorphic nuclei) which are a benign, incidental finding and should not be used in the diagnosis line.
0:19:18 Atypical Fibrous Histiocytoma (AFH): This diagnosis is given to a lesion that morphologically resembles a DF at low power (collagen trapping, epidermal changes) but displays diffuse high-grade atypia and atypical mitoses (0:21:24).
Key Distinction: AFH/Atypical DF is usually small, confined to the dermis, and found on the extremity or trunk of younger patients, differentiating it from AFX/PDS, which typically occurs in the sun-damaged head/neck of older patients (0:24:14).
Takeaway: AFH has a very good prognosis but usually requires excision with negative margins.
0:28:54 Traumatic Neuroma: Presents as a nodule of tangled, regenerating nerve bundles (0:33:34), resulting from the severance of a large nerve (e.g., amputation stump).
Key Detail: Normal nerve fascicles appear gently undulating (0:29:26) and, if large, show myelinated axons. This is contrasted with fascia/tendon, which exhibits dense wavy collagen ("ramen noodle sign").
0:38:15 Solitary Bone Cyst (SBC): A fluid-filled lesion (importance of clinical/radiology) often found in the long bones of skeletally immature patients.
Microscopic Features: Thin fibrous wall, but lacks a true epithelial lining. The most helpful finding is the presence of homogenized pink material (fibrinoid/osteoid-like - 0:41:51).
Takeaway: SBC is much less cellular and "busy" than an Aneurysmal Bone Cyst (ABC).
0:46:33 Schwannoma (Neurilemoma): A well-circumscribed neural tumor featuring highly cellular Antoni A areas (often with palisading nuclei forming Verocay bodies) and hypocellular, loose Antoni B areas (0:49:10).
Key Detail: Schwannomas commonly exhibit degenerative (ancient) atypia (scattered bizarre, ugly nuclei - 0:50:30) and hyalinized vessels (0:53:37), which are entirely benign features.
0:58:12 Lipoma Arborescens (Synovial Lipomatosis): Benign condition presenting as polypoid projections of mature adipose tissue pushing into the joint space (0:59:57), lined by loose synovium.
1:02:25 Synovial Chondromatosis: Characterized by multiple, discrete islands of benign hyaline cartilage (1:04:50) in the synovium or soft tissue near a joint, often showing chondrocyte clustering and ossification.
1:07:55 Paraganglioma (e.g., Carotid Body Tumor): Displays a classic Zellballen pattern (multiple small nests of epithelioid cells surrounded by stroma/vessels - 1:09:23).
Key Takeaway: Like Schwannoma, Paragangliomas can show significant nuclear pleomorphism and large nuclei (1:11:56), which is not predictive of malignant behavior.
Differential: Must be distinguished from Alveolar Soft Part Sarcoma (ASPS), which can have morphologic overlap, particularly in non-classic areas (1:15:24).