Clear Cell Mesenchymal Neoplasm Pathology

Author: Dr. Harriet Cheng, Dermatopathologist

Reviewed: 2025

Introduction

Clear cell mesenchymal neoplasm, also known as

distinctive dermal clear cell mesenchymal neoplasm (DDCCMN)

, represents a rare and recently described entity in dermatopathology. First reported in 2004, this tumor is characterized by a proliferation of large cells with abundant optically clear cytoplasm situated predominantly in the reticular dermis. It typically presents as a smooth cutaneous nodule on the lower extremities of adults, with a benign clinical course following local excision. Recognition of this neoplasm is crucial to distinguish it from more aggressive clear cell tumors, such as metastatic clear cell renal cell carcinoma or clear cell melanoma, which share cytological similarities but differ in prognosis and management.

The histogenesis of DDCCMN remains uncertain, but its mesenchymal nature is supported by immunohistochemical profiles showing positivity for vimentin and NKI-C3, with negativity for epithelial, melanocytic, and lymphoid markers. Over the past two decades, additional cases have expanded the clinical spectrum, including rare occurrences on the scalp and eruptive forms, though the core pathological features remain consistent. This article provides a comprehensive review of its clinical presentation, histopathological findings, diagnostic criteria, and differential diagnosis, drawing from peer-reviewed case series and pathology resources.

Clinical features

DDCCMN typically affects adults with a median age of 45 years (range: 38–70 years), showing a slight male predominance (3:2 male-to-female ratio in initial series). Nearly all cases arise on the

lower limbs

, presenting as solitary, smooth-surfaced dermal nodules measuring 0.5–2.5 cm in diameter. Lesions are often present for weeks to 5 years before excision, clinically described as non-ulcerated, mobile subcutaneous masses without overlying skin changes.

Rare variants include scalp involvement in elderly patients and eruptive multiple lesions, as reported in exceptional cases. Patients are generally asymptomatic, though tenderness or slow growth may prompt biopsy. No systemic associations or predisposing factors, such as sun exposure or trauma, have been identified. Complete surgical excision is curative, with no recurrences reported in follow-up periods up to 11 years.

Histopathology

Microscopic description

At low magnification, DDCCMN appears as a well-circumscribed nodular proliferation confined to the reticular dermis, often extending into the superficial subcutis while sparing the papillary dermis. The tumor entraps adnexal structures and thin collagen fibers, imparting a lacelike or sieve-like architecture.

Cytologically, the neoplasm is composed of uniform

large clear cells

with abundant optically clear to pale cytoplasm, distinct cell borders, and vesicular nuclei with inconspicuous nucleoli. Nuclear pleomorphism is minimal in typical cases, and mitoses are rare (<1 per 25 high-power fields). Rare atypical variants exhibit nuclear enlargement, hyperchromasia, and increased mitotic activity, raising concerns for uncertain malignant potential.

No necrosis, vascular invasion, or significant stromal desmoplasia is observed. The clear cytoplasm results from glycogen accumulation or lipid content, though special stains for these are not routinely performed.

Microscopic images

• Low power: Nodular dermal proliferation with entrapment of collagen and adnexa.
• Medium power: Sheets of clear cells with lacelike pattern.
• High power: Bland vesicular nuclei in clear cytoplasm; rare mitoses.

Cytology descriptions

Cytological preparations, if available, show dispersed large cells with fragile, clear cytoplasm that may artifactually rupture, mimicking lipidized histiocytes. Nuclei remain bland and vesicular. Fine-needle aspiration is rarely performed due to the superficial location.

Osmium stain

Osmium tetroxide staining highlights lipid content in the clear cytoplasm, supporting a mesenchymal or histiocyte-like differentiation. However, this is not a standard diagnostic tool and is reserved for research settings.

Immunohistochemistry

DDCCMN displays a limited immunoprofile consistent with mesenchymal origin:

Marker	Result	Notes
NKI-C3	Positive (5/5)	Strong cytoplasmic staining; melanoma-associated antigen.
CD68	Focal positive (2/5)	Histiocytic marker; nonspecific.
Vimentin	Positive (2/3)	Mesenchymal marker.
S100, Melan-A, HMB45	Negative	Excludes melanoma.
Cytokeratins (AE1/AE3, CAM5.2)	Negative	Excludes carcinoma.
LCA (CD45)	Negative	Excludes lymphoid lesions.
EMA, SMA, Desmin	Negative	Further supports mesenchymal nature.

NKI-C3 positivity is the most consistent finding, aiding distinction from mimics. Molecular studies are lacking, with no recurrent fusions identified to date.

Ultrastructure

Electron microscopy reveals abundant cytoplasmic glycogen granules and rough endoplasmic reticulum, without melanosomes, tonofilaments, or desmosomes, reinforcing non-melanocytic, non-epithelial differentiation.

Differential diagnosis

Clear cell tumors in dermis require a broad differential. Key discriminators include location, IHC, and behavior:

Diagnosis	Key Features	IHC	Distinguishing Notes
Clear cell hidradenoma	Adnexal differentiation, ductal structures	CK+, EMA+, CEA+	Epidermal connection; PAS+ diastase-resistant material.
Balloon cell nevus	Melanocytic, junctional component	S100+, HMB45+	Melanosomes on EM.
Metastatic clear cell RCC	Deeper invasion, vascular spaces	CK+, CD10+, PAX8+	Systemic workup positive.
Xanthoma	Multinucleated touton giant cells	CD68+, lipid stains +	Hyperlipidemia history.
Clear cell sarcoma	Deep soft tissue, nested growth	S100+, SOX10+, melanoma markers	TYR gene rearrangement.
PEComa	Perivascular arrangement	HMB45+, melan-A+	Smooth muscle actin +.

DDCCMN is favored by reticular dermal location, NKI-C3+, and negativity for lineage-specific markers.

Clinicopathologic variants

• Atypical DDCCMN: Nuclear pleomorphism, increased mitoses; uncertain potential; requires wide re-excision ± radiation.
• Eruptive form: Multiple lesions; desmoplastic stroma.
• Scalp variant: Older patients; identical histology.

Management and prognosis

Local excision suffices for typical cases, with no recurrences in long-term follow-up. Atypical lesions warrant wider margins and close surveillance. No role for adjuvant therapy in benign cases.

Frequently Asked Questions (FAQs)

Q: What is the most common site for clear cell mesenchymal neoplasm?

A: The lower extremities, presenting as smooth nodules.

Q: Is DDCCMN malignant?

A: Typically benign; rare atypical cases have uncertain potential but no metastases reported.

Q: What is the key immunohistochemical marker?

A: NKI-C3 positivity with negativity for melanocytic and epithelial markers.

Q: How does it differ from clear cell renal cell carcinoma metastasis?

A: Negative cytokeratins and PAX8; no vascular invasion or systemic disease.

Q: What is the treatment of choice?

A: Complete surgical excision; excellent prognosis.