This lecture, presented by Dr. Jared Gardner, focuses on the diagnosis and differentiation of various fibrohistiocytic and histiocytic lesions encountered in dermatopathology, intended for the Australasian Dermatopathology Meeting. The presentation is split into two parts, with this transcript covering the first part. Dr. Gardner begins by reviewing the classic features of the Dermatofibroma (DF), emphasizing the spindle cell proliferation, collagen trapping, and epidermal changes, while cautioning against over-interpreting findings like storyform patterns or deep extension into fat, which can mimic other entities like Dermatofibrosarcoma Protuberans (DFSP). He then details key differentiating features for DFSP, focusing on its uniform, bland spindle cells, CD34 positivity (especially at the periphery), and honeycomb fat entrapment. The lecture continues by addressing rarer differential diagnoses, including Fibrous Hamartoma of Infancy (FHI), Myofibroma, and Inclusion Body Fibromatosis. A significant portion is dedicated to distinguishing aggressive-appearing but benign variants like Cellular DF and Aneurysmal DF from true sarcomas, emphasizing that "chubby" spindle cells are reassuring for DF. Finally, the presentation transitions to other histiocytic-looking lesions, covering Juvenile Xanthogranuloma (JXG) (highlighting two-tone giant cells), Reticular Histiocytoma, Langerhans Cell Histiocytosis (LCH) (noting its epidermal involvement), and Rosai-Dorfman Disease (RDD) (characterized by abundant plasma cells and emperipolesis). The lecture concludes with a striking case example of a Mycobacterial Spindle Cell Pseudotumor, which mimicked RDD due to the presence of granulomas and histiocytes, underscoring the importance of thorough investigation.
Fibrohistiocytic Lesions and More: Distinguishing Features in Dermatopathology
0:00:05 Introduction & Disclosures: Dr. Jared Gardner addresses the Australasian Dermatopathology Meeting, noting financial disclosures related to YouTube and Kiko content hosting his educational material.
0:01:26 Dermatofibroma (DF) Basics: Classic features include spindle cell proliferation in the dermis, collagen trapping, and epidermal hyperplasia/acanthosis ("tables and chairs").
0:03:39 DF Mimics/Variations: Epidermal induction (mimicking BCC) is common; DF can have subtle scroll-like cells wrapping collagen.
0:04:16 Collagen Trapping Caveat: While common in DF, collagen entrapment can also be seen in DFSP, making it non-specific.
0:04:46 DF Spindle Cell Pattern: Cells are often haphazard, in short fascicles, or even storyform, but typically appear "plump" or "chubby" ($\text{chiflu}$), which is atypical for DFSP.
0:06:09 Mitosis and Atypia in DF: Mitoses are common and atypia (plump cells) are reassuring signs of benign DF, contrasting with DFSP.
0:06:44 Hemosiderin: Presence of hemosiderin (and aneurysmal change/blood) is reassuringly against DFSP.
0:08:06 Subcutaneous Extension in DF: DF extension into fat (subcutus) often shows fat necrosis, foamy histiocytes, and inflammation, unlike the clean fat wrapping seen in DFSP.
0:12:32 Cellular DF (cDF): These are large, deep, cellular DFs that can look scary but are technically benign; recurrence risk is low (around 10%), and removing the bulk is often adequate (0:15:09).
0:34:36 Dermatofibrosarcoma Protuberans (DFSP): Classically presents as a multinodular mass; spindle cells are bland, monotonous, and usually lack pleomorphism (characteristic of translocation sarcomas).
0:38:10 DFSP Immunohistochemistry & Genetics: Highly sensitive for CD34 (often a peripheral halo), but not entirely specific. Look for $\text{COL1A1-PDGFB}$ fusion, or $\text{PDGFD}$ fusion in $\text{CD34}$-negative cases.
0:40:19 DFSP Fat Entrapment: Features clean, honeycomb-like entrapment of fat lobules, often replacing the entire subcutis.
0:43:24 Fibrosarcoma (Historical Term): Adult-type fibrosarcoma is vanishingly rare; the term should generally be reserved for $\text{DFSP}$ (fibrosarcomatous change), Infantile Fibrosarcoma, or specific entities like Myxofibrosarcoma (0:46:21).
0:47:07 Giant Cell Fibroblastoma: A rare pediatric variant of DFSP, very myxoid and hypocellular, with multinucleated cells aggregating around cystic spaces.
0:50:18 Fibrous Hamartoma of Infancy (FHI): Defined by a triad: mature fat, fascicles of fibroblasts, and nodules of immature mesenchyme (0:51:18).
0:55:27 Myofibroma: Biphasic tumor with pseudo-chondroid/myoid nodules separated by hypocellular zones of oval/spindle cells; usually benign even if atypical-looking (0:59:49).
1:01:19 Inclusion Body Fibromatosis: Bland fascicles of fibroblasts/myofibroblasts, often on digits, characterized by actin filament aggregates (inclusion bodies).
0:12:47 Giant Cell Tumor of Tendon Sheath (GCTTS): Characterized by mononuclear histiocyttoid cells with abundant cytoplasm, often with multinucleated osteoclastic-type giant cells and sclerotic collagen.
0:12:47 GCTTS Pigmentation: Often shows hemosiderin, sometimes forming a "Halo" pattern around the tumor cells ("Ladybird cells").
1:07:29 GCTTS Mononuclear Cells: The true neoplastic cells resemble plasma/rhabdoid cells with abundant cytoplasm and pale nuclei, distinct from giant cells.
1:10:29 Nodular Fasciitis (NF): Defined by a myofibroblastic proliferation with myxoid/cystic degeneration and "tissue culture" (loose/feathery) appearance; mitosis is common but true hyperchromatic atypia is absent.
1:16:05 Proliferative Fasciitis/Myositis: A variant of NF with abundant ganglion-like cells (big nuclei, large central nucleoli) scattered in a loose background.
1:18:32 Xanthogranuloma (JXG): Characterized by histiocytes and classic two-tone giant cells (creamy center, foamy shell); eosinophils are often present but not required for diagnosis.
1:22:01 Reticular Histiocytoma: Resembles JXG but epithelioid histiocytes have dense, often two-toned (purple/pink) cytoplasm; $\text{S100}$ positive but $\text{Langerin}$ negative.
1:24:25 Langerhans Cell Histiocytosis (LCH): Fills papillary dermis and epidermis; cells have grooved/bean-shaped nuclei and are $\text{S100}$/$\text{CD1a}$/$\text{Langerin}$ positive.
1:27:59 Indeterminate Cell Tumor: Resembles $\text{LCH}$ in adults ($\text{S100}$/$\text{CD1a}$ positive, $\text{Langerin}$ negative, no Birbeck granules).
1:28:02 Rosai-Dorfman Disease (RDD): Low power shows pink (histiocytes) and blue (lymphoid nodules/plasma cells); hallmark is emperipolesis (lymphocytes/plasma cells within histiocyte cytoplasm).
1:35:32 Mycobacterial Pseudotumor: Presented as a case mimicking RDD with granulomas and plasma cells; diagnosis confirmed by $\text{AFB}$ stain showing organisms within histiocytes (1:37:21).