Dermatofibromas and the Fibrohistiocytic Spectrum: From Classic Bumps to the Bet-Your-Career DFSP Call

Evidence Snapshot

• The classification has flipped: USP6 gene fusions are present in 74–92% of nodular fasciitis cases (Erber & Agaimy, 2018; Patel et al., 2017); PRKCG and PRKCD fusions define cellular and metastasizing variants of dermatofibroma (Wang et al., 2025; Della Mura et al., 2026); ALK rearrangements define epithelioid fibrous histiocytoma (Hornick, 2020)
• Dermoscopy is highly informative: A typical dermoscopic pattern — most commonly a central white scar-like patch with a peripheral delicate pigment network — is identified in 70–92% of dermatofibromas across published series (Zaballos et al., 2008; Ferrari et al., 2013)
• The DFSP separator on H&E: The alternating ovoid-spindled nuclear morphology of DFSP carries 98% sensitivity and 100% specificity — visible on routine staining (Khamdan et al., 2023)
• D2-40 is one of the highest-yield single markers: Strongly and diffusely positive in dermatofibroma — sensitivity has approached 100% in the original Bandarchi series and is high but variable across subsequent studies (~87–100% in most large reports). It is negative in DFSP, which is the clinically useful direction (Bandarchi et al., 2010; Kaddu & Leinweber, 2009)
• Cellular dermatofibroma is not always indolent: Recurrence rates run 10–33%, with deep extension, lesion size >1 cm, and positive margins driving the high end (Gaufin et al., 2019; Le et al., 2025); facial lesions recur up to 22% (Alsawas et al., 2022)
• The evidence base for this article: This article synthesizes 70 peer-reviewed sources from PubMed, JAMA, and the WHO classification framework including the 2018 WHO framework, the Doyle and Fletcher 16-case metastasizing series, the Khamdan nuclear morphology study, and the 2022 German S1 guidelines on AFX/PDS

What is a dermatofibroma — and why has the definition changed?

Dermatofibromas are among the most common benign skin tumors in adults. They are the small, firm, often pigmented bumps that show up on the lower legs (especially in women), the arms, or the trunk — frequently after what seems like a minor trauma such as a shaving cut, an insect bite, or an ingrown hair. For more than a century, dermatology textbooks classified them as reactive lesions: a chronic, exuberant overgrowth of fibroblasts and histiocytes in response to injury. Patients were told they were "scars from the inside" and to leave them alone.

That framing was wrong. Or, to be more precise, it was a clinically useful approximation that has now been overturned by sequencing data. Between 2017 and 2026, a series of molecular studies identified recurrent, clonal gene fusions across nearly every entity in the fibrohistiocytic family. The MYADM::PRKCG fusion in a cellular pseudoangiomatous dermatofibroma. The KIRREL1::PRKCD and LAMTOR1::PRKCD fusions in dermatofibromas that recurred and metastasized. The ALK rearrangements that now define epithelioid fibrous histiocytoma. The PDGFRB activating mutations in dermatomyofibroma. The MYH9-USP6 fusion in nodular fasciitis — present in 74–92% of cases and completely absent in morphologically similar reactive proliferations (Patel et al., 2017; Erber & Agaimy, 2018; Wang et al., 2025; Della Mura et al., 2026; Flucke et al., 2025; Hornick, 2020).

A clonal driver mutation is the definition of a neoplasm. The 2018 WHO Classification of Skin Tumors reflects this shift: dermatofibroma and its variants are now classified as benign mesenchymal tumors — not reactions. The keloidal variant, long held up as proof of a reactive origin, was put to rest by a 52-case series showing that the keloidal collagen most likely results from secondary injury to a preexisting neoplasm, not from a simultaneous reactive process (Nishimoto et al., 2023).

The classic dermatofibroma: how to recognize it and what to do

A classic dermatofibroma is a single, firm, 5–10 mm papule or nodule, usually on a limb. It is brown to red-brown, sometimes with a pale halo. It feels harder than it looks — the cardinal physical exam finding — because the bulk of the lesion is a dense storiform proliferation of fibroblastic and histiocytic cells within the dermis. The overlying epidermis is often hyperplastic and hyperpigmented, which is why patients call it "a mole that won't fade."

The dimple sign

The single best bedside test is the dimple sign (also called the Fitzpatrick sign): pinch the skin from either side of the lesion and watch what happens. A dermatofibroma tethers to the overlying epidermis and dimples inward as you compress the tissue around it. A melanocytic nevus, a melanoma, a basal cell carcinoma, and a cyst typically do not. Dimpling is not absolutely pathognomonic — Meffert and colleagues described non-dermatofibroma lesions that can produce the same finding (Meffert et al., 1997) — but for the common differential a patient brings into the clinic, the dimple sign is the highest-yield 10-second test in general dermatology.

Dermoscopy when the dimple sign is equivocal

The most common dermoscopic finding is a central white scar-like patch with a peripheral delicate pigment network, originally described by Ferrari and colleagues in 2000 and confirmed by Zaballos and colleagues in a prospective 412-case series. Across published series, 70–92% of dermatofibromas display a typical dermoscopic pattern of one or both of these features (Zaballos et al., 2008; Ferrari et al., 2013; Ferrari et al., 2000).

The catch: up to 29% of dermatofibromas show atypical dermoscopic patterns that mimic melanoma, vascular tumors, or BCC — especially the hemosiderotic and aneurysmal variants (Ferrari et al., 2013; Genc et al., 2020). For those, Zaballos and colleagues described a more specific dermoscopic clue in a 110-case series: a prominent homogeneous bluish or reddish area combined with a peripheral delicate pigment network — 100% specificity, 69% sensitivity (Zaballos et al., 2023). When the peripheral network is absent, the overlap with melanoma is real, and the lesion should be biopsied.

When to leave it alone, when to remove it

A classic dermatofibroma in a typical location, with a positive dimple sign and a classic dermoscopic pattern, needs no treatment. The recurrence risk after complete excision is exceedingly low, metastasis is extraordinarily rare, and removing it produces a scar that often looks worse than the lesion itself (Shen-Wagner et al., 2024).

Excision is indicated when:

• The lesion is symptomatic (frequently catches on clothing or razors)
• It is cosmetically disturbing
• The diagnosis is uncertain — especially when dermoscopy is atypical, the location is unusual (face), the growth has accelerated, or the lesion is larger than 1 cm
• The pre-biopsy differential includes melanoma

Excisional biopsy is preferred over shave biopsy whenever there is any doubt. Shave biopsy frequently transects the deep component, which is exactly where the cellular and atypical features that drive prognosis live (Shen-Wagner et al., 2024; Hornick, 2020).

The bet-your-career call: dermatofibroma versus dermatofibrosarcoma protuberans

Of every diagnostic decision in dermatopathology, the distinction between a cellular dermatofibroma and an early dermatofibrosarcoma protuberans (DFSP) is the one most often missed — and the one with the largest stakes. DFSP is a locally aggressive sarcoma with a strong propensity for local recurrence and, in its fibrosarcomatous form (FS-DFSP), metastatic potential. Calling it a dermatofibroma and watching it is a career-defining error in the wrong direction.

The good news: with a thorough panel, the separation is now almost perfect on a partial biopsy.

The H&E features that distinguish them

Sources: Hornick, 2020; Khamdan et al., 2023; Hanly et al., 2006

The Khamdan and colleagues 2023 study deserves special mention. They examined nuclear morphology alone — no immunostains, no FISH — and found that the alternating ovoid-spindled pattern of DFSP nuclei distinguished it from dermatofibroma with 98% sensitivity and 100% specificity. That is a cost-free diagnostic tool on the very first H&E slide (Khamdan et al., 2023).

The immunohistochemistry panel that closes the case

Sources: Bandarchi et al., 2010; Kaddu & Leinweber, 2009; Yamada et al., 2023; Hengy et al., 2022; Hardy et al., 2025; Agarwal et al., 2017; Hanly et al., 2006; Hsi & Nickoloff, 1996; West et al., 2014

The single highest-yield stain is D2-40 (podoplanin). In the original Bandarchi series of 56 dermatofibromas (including six cellular variants), D2-40 was positive in every case, while it was negative in DFSP (Bandarchi et al., 2010). Subsequent series have reported somewhat lower sensitivity (Kaddu and Leinweber found 87% in 30 cases, and lower numbers have been reported with different antibody clones), so D2-40 is best understood as highly sensitive but not perfectly so (Kaddu & Leinweber, 2009). The clinically useful direction is unchanged: a CD34-positive, D2-40–negative lesion in the deep dermis is far more likely DFSP than dermatofibroma. ERG is a newer addition — positive in approximately 90% of conventional dermatofibromas and negative in DFSP, with the bonus of distinguishing dermatofibroma from hypertrophic scars (Yamada et al., 2023; Hengy et al., 2022).

A note on Ki-67. Three large studies (Hsi 1996, Hanly 2006, Agarwal 2017) have reported the Ki-67 relationship between dermatofibroma and DFSP differently, with each finding directionally inconsistent results because of differences in counting methodology and tumor sampling. The defensible teaching point is Hanly's rule: a very high proliferative index essentially excludes classic dermatofibroma — but Ki-67 is not a reliable stand-alone discriminator between cellular DF and DFSP and should always be interpreted alongside CD34, factor XIIIa, D2-40, and ERG (Hanly et al., 2006; Agarwal et al., 2017; Hsi & Nickoloff, 1996).

The molecular safety net

When histology and immunohistochemistry remain equivocal — and especially on a small or partial biopsy — molecular testing for the COL1A1::PDGFB fusion confirms DFSP. The fusion is canonical in conventional DFSP and drives autocrine PDGFR-β signaling, which is why imatinib is FDA-approved for unresectable cases.

There is a critical pitfall here: approximately 8.6% of DFSPs are negative for PDGFB break-apart FISH because they harbor alternative fusions — COL6A3::PDGFD, EMILIN2::PDGFD, FGL2::PDGFD, or TGFBI::PDGFB. A negative PDGFB FISH does not rule out DFSP. Comprehensive RNA sequencing for COL1A1, PDGFB, and PDGFD partners is the safer approach (Lee et al., 2022; Cloutier et al., 2026). Sclerosing variants of DFSP overlap morphologically with sclerotic fibroma and both show CD34 positivity — in that scenario, the COL1A1::PDGFB fusion is the only reliable separator (Abdaljaleel & North, 2017).

When "benign" misbehaves: cellular dermatofibroma and metastasizing fibrous histiocytoma

The word "benign" in pathology means predictably indolent. For most dermatofibromas, that is exactly right. For the cellular variant, it is approximately right. For a small subset of cellular dermatofibromas, it is not right at all.

Cellular dermatofibroma

Cellular dermatofibroma shows increased cellularity, fascicular growth, frequent extension into the subcutis, and higher mitotic activity than classic dermatofibroma — but no atypical mitoses. Two large recent series have settled the recurrence question:

• Gaufin et al., 2019 — 218 cases. Recurrence concentrated in lesions >1 cm and those with positive margins. Gaufin et al., 2019
• Siegel et al., 2020 — 93 cases. Confirmed the role of complete excision with histologically clear margins. Siegel et al., 2020
• Le, Fung & Eisen, 2025 — pooled review. Recurrence rates of 10–33% across modern series; the upper bound is driven by deep extension, positive margins, and cellular features. Le et al., 2025
• Alsawas et al., 2022 — facial dermatofibromas recur in up to 22% and frequently turn out to be cellular or subcutaneous on histology, even when the clinical impression was conventional. Alsawas et al., 2022

The clinical implication: a cellular dermatofibroma is not a lesion to "remove and forget." It warrants complete excision with histologically negative margins and a follow-up plan.

Metastasizing fibrous histiocytoma

Approximately 0.1% of dermatofibromas behave malignantly — recurring widely, invading deeply, and occasionally metastasizing to lung, regional lymph nodes, or distant soft tissue. Doyle and Fletcher described 16 such cases in 2013, the largest series to date, and emphasized that the original biopsies in many cases were histologically indistinguishable from ordinary cellular dermatofibromas (Doyle & Fletcher, 2013; Lodewick et al., 2014; Liu et al., 2025).

For years, the molecular driver of these aggressive cases was a mystery. In 2025, Wang and colleagues identified recurrent KIRREL1::PRKCD and LAMTOR1::PRKCD fusions in "benign" metastasizing fibrous histiocytomas — providing the first molecular marker that flags aggressive biology before it declares itself clinically (Wang et al., 2025). Charli-Joseph and colleagues had previously shown that complex chromosomal abnormalities by array-comparative genomic hybridization identify a similar subset on a karyotype level (Charli-Joseph et al., 2014).

Perineural invasion in a dermatofibroma is a clue

Santos-Briz and colleagues described perineural invasion as a histologic clue to adverse prognosis in otherwise conventional-appearing dermatofibromas — a single feature that should trigger wide local excision plus surveillance, regardless of what the rest of the lesion looks like (Santos-Briz et al., 2020).

Surveillance for the rare aggressive case

For high-risk cellular dermatofibroma, atypical dermatofibroma, or documented metastasizing fibrous histiocytoma, current literature supports clinical examination every 3–6 months for the first 2–3 years and annually thereafter for at least 5 years, with chest imaging and regional lymph node ultrasound every 6–12 months for the first 2–3 years in the highest-risk patients (Doyle & Fletcher, 2013; Liu et al., 2025).

Atypical fibroxanthoma and pleomorphic dermal sarcoma: one feature set separates them

AFX and PDS sit at the malignant end of the fibrohistiocytic spectrum. They arise on the sun-damaged skin of elderly patients — bald scalp, ear, cheek — and both look terrifying under the microscope: highly pleomorphic spindle and epithelioid cells with atypical mitoses. They are diagnoses of exclusion: cytokeratins, S100, SOX10, desmin, and CD34 are all negative, while CD10 is diffusely positive in a "block-type" pattern (Ørholt et al., 2022; Agaimy, 2023; Helbig et al., 2022).

Despite the shared histology and the shared UV-driven mutational landscape (TP53, CDKN2A, NOTCH1/2, FAT1, TERT promoter), the prognosis is wildly different — and the difference is decided by a single histologic feature set (Griewank et al., 2018; Klein et al., 2025; Caprini et al., 2026).

Sources: Miller et al., 2012; Helbig et al., 2022; Agaimy, 2023; Bowe et al., 2021

The 2022 German S1 guideline put it cleanly: AFX is essentially cured by complete excision, and Mohs is the procedure of choice because these tumors live on cosmetically unforgiving skin where every millimeter matters. PDS, by contrast, demands wide excision with at least 2 cm margins and a low threshold for adjuvant imaging because its recurrence and metastatic risks are real (Helbig et al., 2022; Miller et al., 2012).

Nodular fasciitis: the tumor that destroys itself

Nodular fasciitis is the entity that broke pathology's old reactive-versus-neoplastic dichotomy. It looks alarming on a biopsy — rapidly growing, cellular, mitotically active, sometimes with atypia — and it is the lesion most commonly misdiagnosed as a sarcoma. For decades it was considered the prototypical reactive myofibroblastic proliferation.

In 2011, Erickson-Johnson and colleagues identified the MYH9-USP6 gene fusion in nodular fasciitis and proposed a new conceptual model: transient neoplasia. The fusion drives clonal proliferation, but the tumor self-limits and regresses, often spontaneously (Erickson-Johnson et al., 2011).

Subsequent work confirmed the model and refined the frequency:

• USP6 fusions are present in 74–92% of nodular fasciitis cases (Patel et al., 2017; Erber & Agaimy, 2018; Fusco et al., 2026)
• Over 20 different fusion partners have now been described (Patel et al., 2017; Fusco et al., 2026)
• USP6 rearrangements are absent from morphologically similar reactive myofibroblastic proliferations (Erber & Agaimy, 2018)

The clinical translation is straightforward: nodular fasciitis is benign and frequently does not require excision. Spontaneous regression is well documented (Fusco et al., 2026; Cloutier et al., 2021). The pathologist's job is to make the molecular diagnosis confidently enough that the surgeon does not over-treat.

When nodular fasciitis is not benign

A small but real subset of cases — usually with unusual USP6 fusion partners like PPP6R3-USP6 or COL3A1-USP6, often with concurrent TP53 mutations — have shown aggressive behavior including distant metastasis and death (Saoud et al., 2025; Teramura et al., 2019; Suchi et al., 2026; Aboulafia et al., 2026). These are the malignant transformation cases. They are rare enough to be reportable, common enough to keep on the radar.

The other entities worth recognizing

Three additional members of the fibrohistiocytic family deserve specific mention, because misidentification carries clinical consequences.

Epithelioid fibrous histiocytoma (EFH)

Composed of epithelioid cells with abundant eosinophilic cytoplasm arranged in nests or sheets, EFH is now defined by recurrent ALK gene rearrangements (most commonly TPM3, DCTN1 partners). The molecular signature distinguishes it from Spitz nevus and epithelioid sarcoma — both of which can look similar on H&E. In the rare unresectable or metastatic case, ALK inhibitors are a theoretical therapeutic option (Hornick, 2020; Fischer & Papke, 2023; Cazzato et al., 2026).

Dermatomyofibroma

A plaque-like proliferation of bland myofibroblasts arranged parallel to the skin surface, typically on the shoulders or upper back of young women. For decades it was lumped with reactive myofibroblastic processes. In 2025, Flucke and colleagues identified activating PDGFRB mutations in 6 of 7 cases (exons 12 and 14), placing dermatomyofibroma firmly within the family of tyrosine kinase–driven neoplasms (Flucke et al., 2025; Mentzel et al., 1993).

Angiomatoid fibrous histiocytoma (AFH)

An intermediate-malignancy tumor with multinodular sheets of ovoid-to-spindle cells, pseudoangiomatoid blood-filled spaces, a thick fibrous pseudocapsule, and a peripheral lymphoplasmacytic cuff. It is most easily confused with aneurysmal dermatofibroma. The molecular hallmark — EWSR1-CREB1, EWSR1-ATF1, EWSR1-CREM, or FUS-ATF1 fusions in 75–95% of cases — is absent from aneurysmal dermatofibroma, making molecular testing the gold standard for the separation (Thway & Fisher, 2015; Tanas et al., 2010; Rossi et al., 2007; Calonje & Fletcher, 1995).

Plexiform fibrohistiocytic tumor (PFHT)

An intermediate-grade neoplasm of children and young adults (median age 14–21, female predominance), with a biphasic plexiform proliferation of histiocyte-like cells, osteoclast-like giant cells, and spindle fibroblastic cells. No recurrent gene fusions have been identified; recent work points to a CSF1-producing "null cell" population as the likely neoplastic compartment. Local recurrence runs 12–20%, with rare metastasis to lymph nodes or lungs (Remstein et al., 1999; Thangaiah et al., 2022).

Giant cell fibroblastoma (GCF)

The juvenile variant of DFSP. Same COL1A1::PDGFB fusion, same CD34 diffuse positivity, same biology — just with pleomorphic multinucleated giant cells and angiectoid spaces. Affects boys in the first decade. Local recurrence up to 50%; metastasis has not been documented (Terrier-Lacombe et al., 2003; Shmookler et al., 1989).

Special situations

Children

Dermatofibromas are rare in children — only 4.8% of cases occur in children under 5 years. In younger children, the trunk is the predominant location, not the limbs. The clinical diagnosis is often missed; "cyst" is the most common pre-biopsy clinical impression in 43% of pediatric cases. Pediatric dermatofibromas may show a distinctive retiform architecture with reduced factor XIIIa–positive dendritic cells (Berklite et al., 2020; Kelly et al., 2024).

Face

Facial dermatofibromas are more common in geriatric patients, present unique surgical challenges, and recur in up to 22% of cases. The histology is more frequently cellular or subcutaneous than the clinical exam suggests. Wider excision and a follow-up plan are warranted (Alsawas et al., 2022).

Collisions with melanocytic lesions

A junctional nevus or even a melanoma in situ can grow directly over a dermatofibroma — a "collision lesion." The dermatofibroma drives epidermal hyperplasia and dendritic-cell recruitment, which in turn can stimulate adjacent melanocytic proliferation. The diagnostic risk is overcalling the melanocytic component as melanoma when it is responding to the underlying tumor. S-100/MART-1 for the melanocytic component, factor XIIIa for the dermatofibroma component, and careful architectural assessment together prevent the misdiagnosis (King et al., 2005).

Rare variants

The morphologic spectrum of dermatofibroma includes the palisading variant (mimics schwannoma or granuloma annulare), the keloidal variant (must be distinguished from keloid scars), the sclerotic epithelioid variant with aberrant cytokeratin and p40/p63 expression (mimics sarcomatoid squamous cell carcinoma or epithelioid sarcoma), and the collapsing angiokeloidal variant. Each is uncommon, each has a published name, and each demands a careful immunohistochemistry workup (Pun & Cassarino, 2022; Kuo et al., 1998; Álvarez Bobillo et al., 2026; Schnebelen et al., 2012).

What to ask your dermatologist

For most readers, the answer is going to be reassuring: a classic dermatofibroma with a positive dimple sign in a typical location does not need treatment. If you want it removed for cosmetic or functional reasons, excisional biopsy gives the pathologist enough tissue to characterize the lesion confidently.

The questions worth asking — when the lesion is on the face, is larger than 1 cm, has changed recently, is recurrent, or comes back with an "atypical" or "cellular" descriptor on pathology — are these:

1. Is the diagnosis classic dermatofibroma, cellular dermatofibroma, or something more unusual? The distinction changes the follow-up plan.
2. Were the margins clear on pathology? Positive margins in a cellular variant are the single biggest driver of recurrence.
3. Was the histology reviewed by a dermatopathologist? Some of these cases are subtle, and the IHC panel (CD34, factor XIIIa, D2-40, ERG, Ki-67) is what closes the case.
4. If the lesion is on a sun-damaged elderly patient and the pathology is pleomorphic, is this AFX or PDS? That single distinction changes the entire treatment plan.
5. For an enlarging, deep, fibrous plaque — has DFSP been ruled out? If not, request the H&E review with attention to the alternating nuclear morphology and, if equivocal, COL1A1::PDGFB testing.

Frequently Asked Questions

1. Is a dermatofibroma a kind of skin cancer?

No. A classic dermatofibroma is a benign mesenchymal neoplasm — a small, well-behaved tumor that does not invade nearby tissues, does not spread, and does not become cancerous over time. The 2018 WHO Classification of Skin Tumors places it firmly in the benign category. The "cellular" variant is also classified as benign but recurs more often, and an exceedingly rare subset can metastasize — which is why the variant matters on the pathology report.

2. My dermatologist said it appeared after an insect bite. Did the bite cause it?

Almost certainly not in a direct sense. For decades, dermatology taught that dermatofibromas were reactive responses to trauma. Modern molecular sequencing has overturned that view — these are clonal neoplasms driven by gene fusions and other tumor-defining alterations. What likely happened: the bite irritated a small preexisting lesion enough that you noticed it, or it triggered the secondary inflammation that drew your eye to it. The trauma was the messenger, not the cause.

3. What is the dimple sign and why do dermatologists use it?

Pinch the skin from either side of the lesion. A dermatofibroma tethers to the overlying epidermis and dimples inward — the surface of the lesion drops below the surrounding skin as you compress the tissue around it. Moles, melanomas, basal cell carcinomas, and cysts typically do not. The maneuver takes ten seconds, costs nothing, and changes the diagnosis in the overwhelming majority of common-clinic cases. It is not absolutely pathognomonic — other lesions can occasionally dimple — but for the practical differential a patient most often brings in, it is one of the highest-yield bedside tests in dermatology.

4. Should a dermatofibroma be removed?

Usually not. A classic dermatofibroma in a typical location, with a clear dimple sign and a classic dermoscopic pattern, can be safely observed. Removal is appropriate when the lesion is symptomatic (catches on clothing or razors), is cosmetically disturbing, is changing or growing, is on the face, or when the diagnosis is uncertain. When excision is performed, excisional biopsy is preferred over shave biopsy because the deep component carries diagnostic information that shave techniques routinely transect.

5. What is the difference between a dermatofibroma and a dermatofibrosarcoma protuberans (DFSP)?

This is the most consequential distinction in this entire family of lesions. A dermatofibroma is a small, indolent, dermal-based tumor that almost never recurs or spreads. DFSP is a locally aggressive sarcoma that grows progressively, infiltrates the subcutaneous fat in a "honeycomb" pattern, and — in its fibrosarcomatous variant — can metastasize. They can look similar on a partial biopsy, but a panel of CD34 (diffusely positive in DFSP, negative or focal in dermatofibroma), factor XIIIa (positive in dermatofibroma, negative in DFSP), D2-40 (highly sensitive for dermatofibroma, negative in DFSP), and a careful look for the alternating ovoid-spindled nuclear morphology of DFSP separates them with high reliability. The COL1A1::PDGFB fusion confirms DFSP molecularly when histology is equivocal.

6. What is "cellular dermatofibroma" and is it dangerous?

Cellular dermatofibroma is a variant with more cells, more mitotic activity, and frequent extension into the subcutaneous fat. It is still classified as benign, but it recurs in 10–33% of cases — far more than classic dermatofibroma — and a small subset can metastasize. Complete excision with clear margins is essential, and follow-up is appropriate. The presence of cellular features on a pathology report is not a cancer diagnosis, but it does change the management plan from "observe" to "treat and watch."

7. My pathology report says "atypical fibrous histiocytoma." Is that cancer?

Atypical fibrous histiocytoma is part of the dermatofibroma family with worrisome histologic features — increased mitotic activity, nuclear atypia, occasional bizarre cells — but without the diagnostic features of a sarcoma. The recurrence and metastasis risks are higher than classic dermatofibroma but lower than a true sarcoma. Wide local excision with clear margins is the standard approach, with regular clinical follow-up. If perineural invasion is present, the surveillance should be more intensive.

8. What is nodular fasciitis, and why might my surgeon recommend not removing it?

Nodular fasciitis is a rapidly growing soft tissue lesion that looks alarming on biopsy and can be mistaken for a sarcoma. Molecular testing has shown that 74–92% of these lesions carry a USP6 gene fusion — a clonal genetic event that drives the rapid growth but also causes the lesion to self-limit and regress spontaneously. Once the diagnosis is confirmed (especially via USP6 testing), many cases can be observed rather than excised. The exceptions are the rare aggressive variants with unusual fusion partners, which have shown malignant behavior in case reports.

9. My dermatofibroma is on my face. Does that change anything?

Yes. Facial dermatofibromas recur in up to 22% of cases — more often than dermatofibromas on the limbs — and frequently turn out to be cellular or subcutaneous on histology, even when the clinical impression was that of a conventional lesion. Wider excision, attention to deep margin clearance, and a clinical follow-up plan are warranted. If reconstruction will be complex, a Mohs surgeon should be considered.

10. My doctor mentioned "atypical fibroxanthoma." What is the difference between AFX and pleomorphic dermal sarcoma?

Both tumors arise on the sun-damaged skin of elderly patients and share the same UV-driven mutational landscape. The difference comes down to a single histologic feature set: PDS shows significant subcutaneous invasion, tumor necrosis, lymphovascular invasion, or perineural invasion. AFX shows none of those. The prognostic implications are very different — AFX has a local recurrence rate of under 5% and exceedingly rare metastasis, while PDS recurs in up to 28% of cases and can metastasize in up to 10%. AFX is typically managed with Mohs micrographic surgery. PDS requires wide excision with at least 2 cm margins.

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