Pale cell acanthoma, more commonly called clear cell acanthoma, is a rare benign skin tumor made up of keratinocytes packed with glycogen, which gives them their characteristic pale or “clear” appearance under the microscope. First described by Degos and colleagues in 1962, it typically shows up as a slow-growing, reddish, dome-shaped nodule on the lower legs of middle-aged and older adults. Despite being harmless, it regularly gets mistaken for more worrisome conditions, and that diagnostic confusion is arguably the most important thing to understand about it.
What It Looks Like on the Skin
Pale cell acanthoma most often affects people between the ages of 50 and 70, with no strong preference for either sex. The classic presentation is a solitary, well-defined plaque or nodule, slightly raised, measuring anywhere from about 3 to 20 millimeters across. It grows slowly and is usually painless. The surface tends to look wet or shiny, and the color ranges from pink to reddish-brown, though a vivid red is probably the most common shade. A thin crust or a collar of scale around the edge is a frequent finding.1PubMed Central. Giant clear cell acanthoma: a case report and a review of the literature
The lower legs are the favorite site by a wide margin, but pale cell acanthoma has been documented virtually everywhere on the body, including the trunk, arms, face, areola, palate, and even the umbilicus. Most people have a single lesion, though a “multiple eruptive” variant exists in which dozens of spots can appear at once. The eruptive form is uncommon and tends to be more challenging to manage simply because of the number of lesions involved.
The Dermoscopy Clue That Sets It Apart
Under a dermatoscope, pale cell acanthoma has a signature pattern that experienced clinicians learn to recognize: tiny dotted or coiled vessels lined up in parallel rows, creating what dermatologists call a “string of pearls” arrangement. The vessels sit along the ridges of the thickened epidermis, giving the surface a distinctive pinstriped look.2PubMed Central. The string of pearls pattern in dermoscopy of a chest papule A surrounding collarette of fine scale often accompanies this vascular pattern.3Journal of the American Academy of Dermatology. Multiple clear-cell acanthomas
That string-of-pearls pattern is useful because it is quite specific to this tumor. Other reddish nodules on the legs, like pyogenic granulomas or amelanotic melanomas, show different vascular architectures. Still, the pattern is not always perfectly developed, and in atypical cases dermoscopy alone is not enough to rule out malignancy. When there is any real doubt, a biopsy remains the standard.
What Makes the Cells “Pale”
The hallmark under the microscope is a sharply demarcated zone of thickened epidermis whose keratinocytes are swollen and pale. That paleness is caused by large amounts of glycogen stuffed into the cells’ cytoplasm. On routine staining, the glycogen dissolves during tissue processing, leaving the cells looking washed out or empty. A special histochemical stain called PAS (periodic acid–Schiff) will light up the glycogen a bright magenta; when the tissue is then treated with diastase, an enzyme that digests glycogen, the magenta color disappears, confirming that the stored material really is glycogen.4PubMed Central. Pale (Clear) Cell Acanthoma of the Palate
One of the most striking features is the abrupt border between the tumor and the surrounding normal skin. On a biopsy slide, you can draw a line where the pale, glycogenated cells stop and normal-looking keratinocytes begin. The tumor epidermis also shows psoriasiform acanthosis, meaning the rete ridges are elongated and club-shaped in a pattern reminiscent of psoriasis, along with dilated capillaries in the upper dermis and a scattering of inflammatory cells, particularly neutrophils, migrating through the spinous layer.5PubMed Central. Clear Cell Acanthoma with Malignant Cytologic Features: A Case Report and Review of the Literature
Is It Really a Tumor, or a Reactive Process?
This question has simmered in dermatopathology circles for decades, and it still does not have a clean answer. The traditional view, dating back to Degos, treats pale cell acanthoma as a benign epidermal neoplasm, a true growth with its own distinct biology. But a competing school of thought argues it is actually a localized inflammatory reaction pattern, essentially a spot of psoriasis-like skin change rather than a genuine tumor.
Evidence supporting the reactive interpretation is surprisingly strong. In one study, 12 of 14 cases were found sitting on top of or adjacent to underlying chronic inflammatory conditions, mainly scars and stasis dermatitis, and the protein-expression profile of the pale cells closely paralleled that of psoriasis.6PubMed. Clear-cell acanthoma versus acanthosis: a psoriasiform reaction pattern lacking tricholemmal differentiation Other conditions linked to the emergence of pale cell acanthoma include varicose veins, seborrheic keratosis, bacterial and viral infections, ichthyosis, atopic dermatitis, and even insect bites.7PubMed Central. Clear Cell Acanthoma with Atypical Location -Case Report and Literature Review The fact that so many of these are inflammatory or irritative states lends weight to the idea that pale cell acanthoma is a reaction to local inflammation rather than a self-driven growth.
On the other hand, the sharp demarcation from surrounding epidermis, the consistent glycogen-storage pattern, and the fact that most lesions are solitary and persistent argue for something more organized than a simple reaction. Immunohistochemical studies have shown that the staining behavior of pale cell acanthoma keratinocytes is nearly identical to that of normal epidermis, but with some subtle differences, including lower cell-proliferation markers compared to psoriasis and squamous cell carcinoma.8PubMed. Immunohistochemical characterization of keratin expression in clear cell acanthoma9Korean Journal of Dermatology. The expressions of cytokeratin 16, involucrin and PCNA in clear cell acanthoma on areola That lower proliferation rate distinguishes it from truly hyperproliferative conditions like psoriasis and suggests the cells are not dividing rapidly but instead just accumulating glycogen in an unusual way. The debate continues, and it is entirely possible that “pale cell acanthoma” as currently defined captures a spectrum, some lesions being truly neoplastic and others reactive.
Why It Gets Confused with Dangerous Lesions
The clinical appearance of pale cell acanthoma overlaps uncomfortably with several malignant and premalignant skin conditions. A shiny, reddish nodule on the leg that occasionally bleeds or crusts can look an awful lot like an amelanotic melanoma, a basal cell carcinoma, or a squamous cell carcinoma. Benign vascular growths like pyogenic granuloma also share the reddish color and tendency to bleed.10PubMed Central. The complexity of clear cell acanthoma diagnosis based on the literature review Even dermatologists with extensive experience may feel uncomfortable making the diagnosis on clinical grounds alone, particularly when the lesion is in an atypical location or lacks the classic dermoscopic pattern.
Another layer of complexity shows up under the microscope. Bowen’s disease, a form of squamous cell carcinoma in situ, can occasionally develop prominent clear cell changes that mimic the glycogen-rich keratinocytes of pale cell acanthoma.11PubMed Central. Lobulated Bowen’s Disease with a Clear Cell Change The PAS-with-diastase stain and careful assessment of cellular atypia are critical for telling the two apart, since Bowen’s disease carries a risk of progression to invasive cancer and pale cell acanthoma does not. Rare reports have also described pale cell acanthomas with atypical-looking cells under the microscope, further muddying the waters and sometimes requiring additional immunostaining or expert consultation to confirm the benign diagnosis.12PubMed Central. Clear Cell Acanthoma with Malignant Cytologic Features: A Case Report and Review of the Literature
The practical takeaway for patients is straightforward: if a clinician suspects pale cell acanthoma but has even modest uncertainty, a biopsy is warranted. The benign diagnosis is reassuring only once it has been confirmed histologically, and the cost of missing a melanoma or squamous cell carcinoma is too high to gamble on a clinical guess.
Treatment Options
Because pale cell acanthoma is benign and often asymptomatic, treatment is not strictly necessary unless the lesion is cosmetically bothersome, prone to catching on clothing, or diagnostically uncertain. When treatment is desired, several approaches work:
- Surgical excision: Cutting the lesion out has traditionally been the first-line approach. It provides a specimen for histological examination, which can be especially valuable when the clinical diagnosis is uncertain.
- Cryotherapy: Liquid nitrogen freezing can destroy the lesion but often requires three to four treatment sessions.
- COâ‚‚ laser ablation: Laser treatment has emerged as an attractive alternative, particularly for patients with multiple lesions, lesions near joints, lesions that have not responded to cryotherapy, patients on blood-thinning medications, or people who are especially sensitive to the pain of freezing.
- Pulsed dye laser: For lesions in cosmetically sensitive areas like the face, a vascular-targeting pulsed dye laser can selectively destroy the dilated blood vessels feeding the tumor. This approach has shown good cosmetic outcomes with minimal scarring.
COâ‚‚ laser ablation has the advantage of usually requiring only a single session, a relatively bloodless field, short procedure time, and less postoperative pain compared to repeated cryotherapy.13PubMed. Clear cell acanthoma successfully treated with a carbon dioxide laser For the multiple eruptive variant, where dozens of lesions make excision impractical, COâ‚‚ laser ablation across multiple spots in one sitting is a viable strategy.14PubMed. Multiple eruptive clear cell acanthomas successfully treated with CO2 laser ablation
When the lesion sits on exposed skin like the face, the cosmetic stakes shift the calculus. One reported case on the face was treated with a 595 nm pulsed dye laser after the diagnosis was made clinically and dermoscopically, without an initial biopsy, to avoid scarring. Multiple laser sessions achieved clearance with an excellent cosmetic result and no recurrence during follow-up.15PubMed Central. Non-Invasive Diagnosis of a Facial Tumor Using Dermoscopy and Successful Treatment with 595 nm Pulsed Dye Laser That kind of biopsy-sparing approach requires high clinical confidence in the diagnosis, which brings us back to the importance of good dermoscopy skills and, increasingly, newer imaging technologies.
Recurrence and Long-Term Outlook
The prognosis for pale cell acanthoma is excellent. It does not transform into cancer, and malignant potential has never been convincingly demonstrated. In the largest published case series, covering 70 patients, only a single local recurrence was documented, and that was after cryotherapy rather than excision.16Actas Dermo-Sifiliográficas. [Translated article] Clear Cell Acanthoma: A Series of 70 Cases Because the condition is benign, long-term clinical follow-up after removal is generally not required. If a lesion does recur, a second treatment round typically resolves it without difficulty.
Unusual Presentations and Atypical Locations
While the lower legs remain the textbook location, pale cell acanthoma has been reported in places that can catch clinicians off guard. Cases have been documented on the palate inside the mouth, where the diagnosis would not be high on anyone’s initial list.17PubMed Central. Pale (Clear) Cell Acanthoma of the Palate It has also appeared on the areola of the breast, the chest wall, and the scalp.
Age is another axis of surprise. Although this is overwhelmingly a condition of middle-aged and older adults, the youngest reported case involved an eight-month-old infant who developed a weeping nodule on the umbilicus that had been present since the newborn period. The lesion was initially mistaken for a urachal duct remnant, a completely different embryological problem, before biopsy confirmed pale cell acanthoma. Three rounds of cryotherapy cleared it.18PubMed. Urachal duct remnant-like umbilical clear cell acanthoma in an infant: an unusual presentation and pitfall in clinical practice Cases like this underscore that rarity is not impossibility, and that histological confirmation matters even in demographic groups where a diagnosis seems implausible.
Size can be atypical too. Although the standard range is under two centimeters, “giant” variants exceeding several centimeters have been reported, and these large lesions are even more likely to raise alarm about malignancy on first inspection.19PubMed Central. Giant clear cell acanthoma: a case report and a review of the literature
Non-Invasive Diagnosis with Confocal Microscopy
One of the more interesting recent developments is the use of reflectance confocal microscopy, a technology that creates near-histological-resolution images of the skin without cutting into it. The device uses a low-power laser to scan the skin and produce real-time images of cellular architecture at a depth of a few hundred micrometers, roughly reaching the upper dermis.
In studies of pale cell acanthoma, confocal microscopy has been able to identify the key features visible on traditional biopsy: well-demarcated lesions edged by a hyperkeratotic collarette, large pale keratinocytes, elongated rete ridges, and dilated capillaries forming glomeruloid shapes in the upper dermis.20PubMed Central. Reflectance confocal microscopy of clear cell acanthoma: A novel insight to avoid invasive procedures The technology holds promise for confirming the diagnosis in cosmetically sensitive areas where a biopsy scar would be unwelcome, or in elderly patients for whom even a minor procedure carries disproportionate inconvenience. It is not yet widely available outside academic centers, but the trajectory points toward broader adoption as the devices become more accessible and training becomes more standardized.
The Psoriasis Connection
The histological resemblance between pale cell acanthoma and psoriasis deserves its own attention because it is one of the most debated aspects of the condition’s biology. Both entities show psoriasiform acanthosis with elongated, club-shaped rete ridges. Both feature neutrophil infiltration through the epidermis. Both show dilated capillaries in the dermal papillae. The main difference is the glycogen accumulation, which is characteristic of pale cell acanthoma but absent in ordinary psoriasis.
The emergence of pale cell acanthoma directly within active psoriatic plaques has been documented, bolstering the argument that the two conditions share mechanistic ground.21PubMed Central. Clear Cell Acanthoma with Atypical Location -Case Report and Literature Review The immunohistochemical overlap between the two is substantial: cytokeratin 16 and involucrin, markers associated with abnormal keratinocyte maturation, are expressed at similar levels in both conditions. Where they diverge is in proliferation rate. PCNA, a marker for dividing cells, is significantly higher in psoriasis and squamous cell carcinoma than in pale cell acanthoma, suggesting that pale cell acanthoma keratinocytes are metabolically altered rather than rapidly dividing.22Korean Journal of Dermatology. The expressions of cytokeratin 16, involucrin and PCNA in clear cell acanthoma on areola
What drives the glycogen hoarding remains unclear. Glycogen normally exists in small quantities in keratinocytes as part of the skin’s energy metabolism, but something in pale cell acanthoma causes it to accumulate to the point where it dominates the cell’s appearance. Whether this reflects a defect in glycogen breakdown, an increase in synthesis, or a maturational arrest in the keratinocyte lifecycle has not been definitively established. It is the kind of basic-science question that rarely attracts large research funding, given that the condition is benign and uncommon, so the answer may be a long time coming.