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Giant clear cell acanthoma with keratoacanthoma-like changes: A case report

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Giant clear cell acanthoma with keratoacanthoma-like changes: A case report
Muammar Arida MD1, Joseph C English III MD1, Thaddeus W Mully MD2
Dermatology Online Journal 12 (4): 11

1. Department of Dermatology, University of Pittsburgh.
2. Departments of Dermatology and Pathology, University of Pittsburgh


Clear cell acanthoma is a benign epidermal lesion with a variable clinical appearance and distinct histopathology features. Although, it is considered an entirely benign entity, few case reports describe unusual or atypical variants of clear cell acanthoma. We observed a case of a large clear cell acanthoma that also has features of a keratoacanthoma.


Clear cell acanthoma (CCA), first described by Degos [1] in 1962, presents as a well-demarcated, solitary red nodule or plaque several millimeters to 2 cm in diameter. Histologically, it is characterized by psoriasiform epidermal hyperplasia comprised of pale keratinocytes that contain abundant glycogen in their cytoplasm. This glycogen can be highlighted with a PAS stain and digested with addition of diastase. Neutrophils may also be seen in the epidermis, and the blood vessels of the dermal papillae may be dilated. These histopathologic changes are usually well demarcated from the uninvolved adjacent epidermis.

The majority of reported cases have this typical appearance. Several authors have reported morphologic variants of CCA that show additional features, including giant, polypoid, pigmented, and cystic forms [2-9]. Other authors reported cytologic atypia and increased mitotic figures in cases of CCA [10, 11]. In this report we describe a giant clear cell acanthoma with changes analogous to those seen in keratoacanthoma.

Clinical synopsis

Figure 1
Figure 1. Large reddish polypoid tumor with a smooth moist surface on the left groin of a 45-year-old man

A 45-year-old man presented to our dermatology clinic complaining of a tumor on his left groin that had been growing slowly for many years. Clinical examination revealed a 3 × 2 × 1.5 cm reddish polypoid lesion (Fig. 1). On close inspection, the lesion had a smooth, moist surface with multiple deep invaginations. Based on a clinical differential diagnosis that included basal cell carcinoma, squamous cell carcinoma, and condyloma, the lesion was excised.

Figure 2Figure 3
Figure 2. The tumor is composed of an exophytic component as well a central endophytic component with cystic change.
Figure 3. At its periphery, the tumor displayed typical features of clear cell acanthoma, namely, psoriasiform epidermal hyperplasia with pale keratinocytes. The pale area is clearly demarcated from adjacent epidermis with numerous infiltrating neutrophils.

Histopathologic examination revealed an exoendophytic proliferation of keratinocytes forming broad-based rete ridges (Fig. 2). At the periphery of the lesion, the epidermis showed psoriasiform hyperplasia and was comprised of keratinocytes with abundant pale-staining cytoplasm (Fig. 3). These pale areas were clearly demarcated from the adjacent epidermis and had numerous infiltrating neutrophils.

Figure 4Figure 5
Figure 4. Cytoplasm of the pale cells is positive with periodic-acid-Schiff (PAS) stain
Figure 5. The staining is removed by diastase digestion (PAS-D)

The cytoplasm of the pale cells was positive for periodic-acid-Schiff (PAS) (Fig. 4) and the staining was removed by diastase digestion (PAS-D) (Fig. 5) indicating that these cells were rich in glycogen.

Centrally, an endophytic component extending deep into the dermis displayed cystic change with formation of a crater-like structure filled with parakeratotic debris (Fig. 6). The epidermis of this deeper part of the tumor was more irregularly hyperplastic, and comprised of keratinocytes with slightly enlarged nuclei with small nucleoli and eosinophilic (glassy) cytoplasm (Fig. 7). Scattered squamous eddies surrounding small parakeratotic plugs were also observed (Fig. 8). The deep atypical areas of this lesion showed less intense reactivity with a PAS stain (Fig. 9). This endophytic portion of the lesion was surrounded by broad zones of dermal fibrosis, a chronic inflammatory cell infiltrate and scattered siderophages.

Figure 6Figure 7
Figure 6. Centrally, the tumor has an endophytic component that extends deep into the epidermis and displays cystic change with formation of a crater-like structure filled with parakeratotic debris.
Figure 7. The epidermis of the deep part is more irregularly hyperplastic and has slightly enlarged keratinocytes with small nucleoli and eosinophilic glassy cytoplasm.

Figure 8Figure 9
Figure 8. The deep part had scattered squamous eddies surrounding small parakeratotic plugs.
Figure 9. The deep atypical areas show less intense reactivity with a PAS stain

In summary, this lesion has two components—a peripheral exophytic component with features of clear cell acanthoma and a central endophytic component with features similar to those described in classic keratoacanthoma [12]. Therefore, we rendered the following diagnosis for this case: An exoendophytic proliferation of keratinocytes with features of a keratoacanthoma and a clear cell acanthoma.


Degos was the first to describe CCA in 1962 and in 1970, he published an article summarizing an 8 year experience with this entity based on more than 140 cases recorded in the literature up to that time [1]. Most of the cases of CCA occur between the ages 50 and 70. The majority are solitary and occur on the lower limb. The average diameter ranges between 10 and 15 mm. The clinical appearance is variable. The lesions may be clinically interpreted as keratoses, warts, eccrine poromas, hemangiomas or carcinomas. The typical histology is that of a psoriasiform epidermal hyperplasia composed of keratinocytes with pale cytoplasm. These pale keratinocytes contain abundant glycogen, and this lesion shows a clear demarcation from the adjacent epidermis. There may be some spongiosis, a parakeratotic scale and an inflammatory infiltrate in dermis.

Many cases that deviate from this classic description of CCA have been reported including giant [2, 8], polypoid [4, 6, 7, 9], pigmented [5], and cystic [3] morphologic variants.

In addition to these clearly benign variants, a few cases have been reported that describe cytologic atypia and increased mitotic figures in what otherwise looks like a classic CCA. One case report described a lesion that had an area of epidermal cytologic atypia spanning the full thickness of the epidermis. The remainder of that lesion showed changes of classic CCA. That case was diagnosed as a squamous cell carcinoma in-situ arising within a CCA [11]. In another paper, cytologic atypia and atypical mitotic figures were noted in two cases of CCA. For these two cases, the authors preferred to use the term atypical CCA rather than malignant CCA because no recurrence or metastasis had been observed during 4 and 5 year followups [10].

Here we report a case of a large polypoid tumor (3 cm) from the groin of a 45-year-old man. Clinically it was suspected to be a carcinoma or a giant condyloma. Microscopic examination revealed an exophytic component with features of CCA. Centrally, however, the tumor had an endophytic component that exhibited features of a keratoacanthoma with a crater-like cystic space and a surrounding jagged epidermal border comprised of large keratinocytes with glassy eosinophilic cytoplasm, and squamous eddies, and surrounded by an inflammatory infiltrate and fibrosis. We did not observe marked cytologic atypia or atypical mitotic figures in this lesion.

In conclusion, our findings are those of keratinocytic proliferation that has grown for years to a very large size. In some areas this lesion has the appearance of a classic CCA. In other areas there are cytoarchitectural features of a keratoacanthoma. Our interpretation is that this lesion represents a large CCA that contains a central area of malignant degeneration.


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