This is the first part of a two-part lecture by Dr. Jared Gardner on Fibroblastic/Histiocytic Lesions and more, presented to the Australasian Dermatopathology Meeting. This segment focuses heavily on the diagnosis and differential diagnosis of Dermatofibroma (DF), contrasting its features with Dermatofibrosarcoma Protuberans (DFSP).
Dr. Gardner details the classic histological features of DF, including spindle cell proliferation, collagen trapping, and epidermal changes (like acanthosis and rete ridging). He emphasizes recognizing benign mimics, such as epidermal induction alongside DF, and notes that features like plump cells, hemosiderin, and fat necrosis argue against DFSP. Specific variants discussed include cellular DF, aneurysmal DF, and lipidized DF. The section on DFSP highlights its characteristic bland, monotonous spindle cells, CD34 positivity (often with a peripheral halo), and characteristic honeycomb fat entrapment, distinguishing it from the reactive changes seen in DF.
The lecture then pivots to address other entities that can mimic DFs or be confused with each other: Fibrous Hamartoma of Infancy (noting its characteristic triad), Myofibroma, Inclusion Body Fibromatosis, Giant Cell Tumor of Tendon Sheath (emphasizing non-giant cell features), and Nodule/Proliferative Fasciitis (highlighting myxoid cystic degeneration). Finally, the discussion expands to other histiocytic-appearing lesions, including Juvenile Xanthogranuloma (JXG), Reticular Histiocytoma, and Langerhans Cell Histiocytosis (LCH), concluding with a detailed case example of an unexpected Mycobacterial Pseudotumor mimicking Rosai-Dorfman disease.
Summary of Transcript: Fibroblastic Lesions (Part 1)
Dermatofibroma (DF) Basics and Histology (0:00:05 – 0:17:00)
0:00:05: Dr. Jared Gardner presents the first part of a lecture on fibroblastic/histiocytic lesions, noting that these lesions might originate from fibroblasts or myofibroblasts, not true histiocytes.
0:01:28: Classic DF presents as firm, often pigmented papules. Histologically, it shows poorly defined spindle cell proliferation in the dermis with collagen trapping (0:04:01).
0:02:34: Look for epidermal hyperplasia with elongated, flattened rete ridges ("tabling").
0:03:02: Epidermal changes can include induction of hair follicle epithelium, mimicking basal cell carcinoma (BCC).
0:04:08: Collagen trapping at the edge can be extensive but is not perfectly specific; it can occasionally be seen in DFSP.
0:04:46: DF spindle cells are often haphazardly interspersed or form short fascicles, sometimes showing a storiform pattern, but this pattern is not exclusive to DFSP (0:05:11).
0:05:26: Key feature distinguishing DF from DFSP: DF cells are often plump/chubby with abundant cytoplasm; DFSP cells are thin and stretched (0:05:37).
0:06:04: Mitoses are common in DF and should not typically lead to a diagnosis of sarcoma.
0:06:44:Hemosiderin and blood-filled spaces (aneurysmal DF) are very reassuring findings against DFSP.
0:08:06: DF commonly extends into the subcutis; look for fat necrosis, foamy histiocytes, and inflammation upon subcutaneous extension, contrasting with the clean wrapping seen in DFSP (0:09:12).
0:11:15: DF can exhibit a beautiful storiform pattern (0:11:17) and contain plump/chubby cells (0:11:33), supporting a DF diagnosis despite alarming features.
0:12:11: DF rarely shows true sarcomatous transformation; pigment in DF is hemosiderin, unlike the melanin in the pigmented DFSP variant (Bednar tumor).
0:13:00:Cellular DF can look scary (cellular, intersecting fascicles, mitoses) but is technically benign, though it recurs more often (approx. 10% recurrence rate, 0:14:47).
0:15:05: Extensive surgery (e.g., Mohs) is usually overkill for cellular DF; removing the bulk is often sufficient (0:15:21).
0:16:04: "Recurrence" of cellular DF is often persistence due to incomplete (shaved) biopsy of a large lesion (0:16:50).
0:17:04:Necrosis can occur in up to 10% of cellular DFs, which is a concerning but reported feature (0:17:14).
0:20:33: DF can contain monster cells (atypical giant cells), which are benign findings unless they form a substantial, pleomorphic/sarcoma-like component (Atypical Fibrohistiocytic Cytoma, 0:22:09).
0:24:04:Lipidized DF (often on the lower leg/ankle) is characterized by foamy cells resembling xanthoma cells, with diagnostic rings and arcs of sclerotic collagen (0:25:19).
0:26:06:CD34 is typically negative in the center of DF but shows a strong peripheral halo as the tumor pushes out normal CD34+ dermal dendritic cells (0:27:28). Shave biopsies picking up only this halo can be misleading.
0:34:36: Classic DFSP presents as a multinodular, fungating mass, common on the trunk, head, or neck (20% of cases are H&N, 0:35:45).
0:37:12: DFSP spindle cells are bland, uniform, and monotonous (a characteristic of translocation sarcomas), rarely showing atypia or necrosis (0:37:51).
0:38:10:CD34 is highly sensitive but not 100% specific for DFSP.
0:38:30: The most common fusion is COL1A1::PDGFB; if negative, reflexively test for PDGFD fusions (0:39:15).
0:40:19: Classic DFSP shows honeycomb fat entrapment; the fat is cleanly wrapped, unlike the dying, inflamed fat in DF (0:40:31).
0:41:51: Strong, diffuse, blazing CD34 positivity is characteristic of conventional DFSP.
0:42:00:Fibrosarcomatous change in DFSP (FS-DFSP) shows highly cellular, intersecting fascicles in a herringbone/chevron pattern (0:42:17).
0:43:30: The term "Fibrosarcoma" (adult type) is vanishingly rare; most historical cases are now classified as other sarcomas or DFSP.
0:45:08: Acceptable diagnoses of fibrosarcoma include: DFSP (or FS-DFSP), Infantile Fibrosarcoma (ETV6::NTRK3 translocation, good prognosis), or tumors with "fibrosarcoma" in their proper name (e.g., Myxoid Fibrosarcoma).
0:46:34:Giant Cell Fibroblastoma (GCFB) is a rare, usually pediatric variant of DFSP; it is myxoid and hypocellular with large cystic spaces and large multinucleated cells (0:47:07).
0:50:16:Fibrous Hamartoma of Infancy (FHI) is recognized by a triad: mature fat, fascicles of fibroblasts, and nodules of immature mesenchyme (0:51:18).
0:51:49: FHI must be distinguished from DFSP as FHI contains immature mesenchyme, whereas DFSP has characteristic storiform patterns and CD34 positivity.
0:52:48: Well-formed, hypocellular, parallel fascicles are common in FHI but rare in DFSP.
0:53:43:Fat trapping is not exclusive to DFSP; it is seen in DF, FHI, and Neurofibroma. CD34 staining helps differentiate Neurofibroma (positive) from DF (peripheral halo only).
Other Spindle Cell/Histiocytic Tumors (0:55:27 – End)
0:55:27:Myofibroma is common in kids but also seen in adults; classically biphasic with pseudo-chondroid/myoid nodules separated by hypocellular spindle cell zones, often with Staghorn vessels.
1:01:19:Inclusion Body Fibromatosis (Juvenile Digital Fibroma) shows bland fibroblasts/myofibroblasts in fascicles, key feature being small, clumped actin filament inclusions (0:01:55).
1:02:48:Giant Cell Tumor of Tendon Sheath (GCTTS) is usually deep, attached to tendon sheath; look for mononuclear histiocyttoid cells with abundant cytoplasm (plasma-cytoid/rhabdoid appearance) and sclerotic collagen (1:06:37).
1:08:35: GCTTS often has hemosiderin, sometimes forming a halo around the mononuclear cells ("ladybird cells").
1:10:29:Nodular/Proliferative Fasciitis is a benign tumor-like process, usually small (<3cm) and arising from fascia; key clue is myxoid cystic degeneration and loose/feathery ("tissue culture") pattern (1:12:04).
1:16:05: Proliferative Fasciitis/Myositis can show ganglion-like cells (large nucleus, central nucleoli, abundant cytoplasm) that look scary but are benign (1:16:16).
1:18:32:Juvenile Xanthogranuloma (JXG) classically shows Touton giant cells (foam around a central creamy core, 1:19:36). Eosinophils may be absent.
1:21:42: Early JXG may look like a sheet of monotonous histiocytes, potentially mimicking Mastocytoma (differentiated by negative CD117/tryptase).
1:22:01:Reticular Histiocytoma (Solitary Epithelioid Histiocytoma) shows large epithelioid histiocytes, often with two-toned/biphasic cytoplasm (purple center, pink periphery) (1:23:32).
1:24:25:Langerhans Cell Histiocytosis (LCH) cells live in both the papillary dermis AND the epidermis; they have characteristic grooved/bean-shaped nuclei and stain for CD1a, S100, and Langerin (1:25:03).
1:27:00:Indeterminate Dendritic Cell Tumor mimics LCH in adults but is negative for Langerin.
1:28:00:Rosai-Dorfman Disease shows large histiocytes with peripolieses (intranuclear lymphocytes/plasma cells) and often has lymphoid aggregates in the background; look for pink (histiocytes) and blue (lymphoid) pattern on low power (1:28:08).
1:33:19:Mycobacterial Pseudotumor can mimic Rosai-Dorfman or other spindle cell lesions; identification requires demonstrating Acid-Fast Bacilli (AFB) within the histiocytes (1:33:33). (A case showing numerous AFB-laden cells is presented, 1:37:19).