Dr. Jared Gardner and Dr. Terence Lin review ten complex soft tissue pathology unknowns, focusing on diagnostic criteria, clinical pitfalls, and relevant molecular findings. The video covers a spectrum of mesenchymal tumors, including common entities presenting in unusual ways (e.g., massive lipidized dermatofibroma, Rosai-Dorfman in bone) and rare/translocation-associated sarcomas. Key diagnoses discussed include Cellular Dermatofibroma, Fibroma of Tendon Sheath, Angiomyolipoma (a PEComa), Extraskeletal Myxoid Chondrosarcoma, Solitary Fibrous Tumor, Kaposi Sarcoma, and Dermatofibrosarcoma Protuberans (including fibrosarcomatous transformation). The discussion emphasizes morphological clues (collagen trapping, fat entrapment, vessel patterns), the utility and pitfalls of immunostains (CD34, S100, STAT6, HHV-8), and modern risk stratification models (for SFT).
Features: Spindle cell lesion, bland cytology, fascicles of spindle cells, and classic collagen trapping at the periphery.
Key Takeaway: Cellular DF must be differentiated from Dermatofibrosarcoma Protuberans (DFSP). Scattered pleomorphism or mitoses favor DF, while DFSP is typically bland and defined by prominent fat entrapment and often CD34 positivity.
Clinical: Intra-articular knee mass in an 18-year-old male.
Features: Hypocellular, collagenized nodule composed of bland myofibroblastic cells and characterized by thin, cleft-like vessels. The background tissue is dense regular connective tissue (tendon/ligament).
Features: Triphasic lesion composed of mature adipose tissue, thick-walled vessels, and smooth muscle cells that appear granular, pale, or clear-celled. The smooth muscle components appear to grow directly off the vessel walls.
Immunophenotype: Positive for melanocytic markers (HMB45/Mart-1) and smooth muscle markers (SMA/MSA).
Key Association: Part of the PEComa family; often associated with Tuberous Sclerosis.
25:29 Case 4: Extra-skeletal Myxoid Chondrosarcoma (EMC)
Features: Multiple myxoid nodules containing round cells arranged in cords and chains (sometimes exhibiting a vague chondromyxoid or lacunar appearance).
Differential: Myoepithelioma (which can have similar morphology but usually stains S100/Keratin/EMA positive).
Molecular: Defined by a translocation involving the EWSR1-NR4A3 fusion gene.
Features: Sharply circumscribed tumor characterized by the staghorn (or hemangiopericytoma-like) vascular pattern, variable cellularity (hypocellular sclerotic zones next to hypercellular zones), and a pattern-less arrangement of plump, oval spindle cells.
Diagnosis Confirmation: The STAT6 immunostain shows strong nuclear positivity, reflecting the NAB2-STAT6 fusion.
Key Takeaway: Behavior is assessed using a risk stratification model (based on size, mitotic activity, necrosis, and patient age >55 years).
Key Detail: Lipidized DF often occurs on the lower leg/ankle, emphasizing the importance of location.
59:01 Case 8: Extranodal Rosai-Dorfman Disease (RDD) in Bone
Clinical Pitfall: RDD is rare, and its occurrence as a lytic bone lesion is unusual.
Features: Sheets of large histiocytes with abundant pale cytoplasm, large round nuclei, and prominent nucleoli. Associated with aggregates of lymphocytes and plasma cells.
Immunophenotype: Histiocytes are classically S100 positive and CD68 positive.
Classic Detail:Emperipolesis (lymphocytes/plasma cells "stopping by for tea" within the histiocyte cytoplasm) is often present but not required for diagnosis.
Clinical: 40-year-old male with HIV and multiple skin lesions.
Features: Variable presentation (patch, plaque, nodule stages). Infiltrating spindle cells forming slit-like vascular spaces often containing red blood cells. Hemosiderin deposition, and the tumor cells infiltrate and splay apart eccrine coils.
Etiology/Stain: Caused by HHV-8 (Human Herpes Virus 8); confirmed by the LANA1 (HHV-8) nuclear stain.
Key Takeaway: Must be distinguished from Angiosarcoma, as KS is typically managed non-aggressively (often resolves with HIV treatment), while Angiosarcoma requires aggressive therapy.
1:16:50 Case 10: Dermatofibrosarcoma Protuberans (DFSP)
Features: Highly infiltrating, poorly circumscribed bland spindle cell proliferation that extensively traps subcutaneous fat (leaving squeezed adipocytes/islands of fat). Cells are thin and bland.
Transformation: An area of fibrosarcomatous transformation (1:23:54) is suspected due to increased cellularity and the development of sweeping fascicles with a vague herringbone pattern.
Molecular: Defined by the COL1A1-PDGFB fusion (or rarely PDGFD fusion).
Diagnosis/Management: CD34 positive (but beware of normal background dermal staining). FISH testing is crucial for small/subtle biopsies to confirm the diagnosis, which dictates massive surgical excision (Mohs or wide local excision).