Hemosiderotic clear-cell acanthoma: a pigmented mimicker.

The authors report on a case of a 65-year-old man with pigmented clear-cell acanthoma located on the right thigh. Dermoscopy disclosed a peculiar picture consisting of diffuse black pigmentation with a superficial greyish veil in the central portion, dotted-to-globular dark red-black structures mainly located at the periphery with a homogenous regular reticular arrangement; peripheral translucid desquamation. Dermoscopic features are correlated with the histology, where hemosiderin deposits present in a sheet-like arrangement in the perivascular papillary dermis and in a band-like disposition in the reticular dermis at the base of the lesion can account for the pigmented picture. The lesion arose on a trauma-prone skin site; thus the authors believe that traumatic irritation may be responsible for the clinical and dermoscopic pictures, giving rise to a reaction similar in a way to the Auspitz's sign provocated by trauma for psoriasis. Red blood cells extravasation from extremely superficialized capillaries may have led to hemosiderin deposition in the papillary and the reticular dermis.

Introduction

Clear-cell acanthoma (CCA) is a solitary benign epidermal tumor mainly occurring as an asymptomatic nodule on the lower limbs of older individuals. A number of studies have investigated the dermoscopic appearance of CCA and found that its common features include pinpoint-like/dotted vessels having a homogeneous/bunch-like, reticular, pearl-like or net-like distribution.[1-4] Translucent collarette scaling is an additional finding.[23] Red dots histopathologically correlate with dilated tortuous capillaries of middle reticular dermis progressing to the top of the papillae. They are often a dermoscopic finding of all types of hypopigmented melanocytic tumors, sometimes of seborrheic keratoses, Bowen's disease and rarely of basal cell carcinomas; but in these cases, they do not show a regular distribution over the entire surface with a reticular pattern.[5] The dotted vascular pattern is 100% present in the psoriatic plaque, with a homogenous, though neither fully reticular nor annular, arrangementas displayed on CCA. Their significance in the angiogenetic progression of the psoriatic plaque has been partially established by videocapillaroscopic studies.[6]

We report a peculiar case of pigmented CCA mimicking a melanocytic lesion, where pigmentation was correlated with red blood cell extravasation and hemosiderin deposition.

Case Report

A 65-year-old man presented for the treatment of sudden development of an asymptomatic pigmented lesion on the external skin surface of the right thigh. Clinically the lesion was a 0.5-cm round nonhomogenously pigmented macule with a warty surface [Figure 1a]. Conventional dermoscopy showed a diffuse black pigmentation with a superficial greyish veil in the central portion, dotted to globular dark red-black structures mainly located at the periphery with a homogenous regular reticular arrangement; peripheral translucid desquamation was also present [Figure 1b]. The lesion was excised, and histology showed a sharply demarcated epithelial tumor composed of markedly acanthotic pale squamous epithelium with psoriasiform appearance with suprapapillary thinning, dilatation and tortuosity of the papillary capillaries and superficial crusting filled with neutrophils. Hemosiderin deposits were present in a sheet-like arrangement in the perivascular papillary dermis and in a band-like distribution in the reticular dermis at the base of the lesion [Figure 2].

Figure 1

(a) Clinical picture: Pigmented macule with a desquamative surface. (b) Dermoscopy (×20): Diffuse black/gray pigmentation accompanied by dotted-globular dark red-black structures distributed in a regular reticular arrangement; peripheral translucid desquamation

Figure 2

Histologic picture (hematoxylin-eosin, ×10): Acanthotic epidermis composed of large, pale cells; suprapapillary thinning, tortuous dilated papillary capillary surrounded by hemosiderin deposits

Discussion

CCA is clinically described as a pink, blanchable, dome-shaped papule or nodule. Usually brown or black pigmentation is not a feature. In the case presented, the red dotted dermoscopic structures well correlate with dilated tortuous capillary loops perpendicularly oriented to the skin surface, deeply penetrating the dermal papilla, just underneath a thinning of epithelium. The black color is mainly linked to the abundance of dermal hemosiderin deposition. We assume that the presence of heavy band-like hemosiderin clusters at the base of the lesion may account for the homogenous black/gray pigmentation of the central portion.

The observed dermoscopic features may be highly misleading in the differential diagnosis with other pigmented skin lesions. The lesion arose on a trauma-prone skin site; thus we believe that traumatic irritation may be responsible for the clinical and dermoscopic pictures, giving rise to a reaction similar in a way to the Auspitz's sign induced by trauma in psoriasis. Red blood cells extravasation from extremely superficialized capillaries may have led to hemosiderin deposition in the papillary and the reticular dermis. Skin lesions with dermoscopic hemosiderotic/vascular appearance should include a diagnosis of irritated CCA.