Clonal seborrheic keratosis: a rare skin tumor

Seborrheic keratosis is a benign epidermal neoplasm, representing one of the most common skin tumors. Clonal seborrheic keratosis is one of the histological subtypes of this entity. It is an uncommon lesion which may resemble other benign or malignant lesion. We report a case of a 60-year-old woman presented with a 7 year history of a gradually growing, cutaneous lesion on her left arm. On physical examination, the lesion was elevated, well-circumscribed, measuring 5 cm in maximum diameter. The tumor was biopsied. Histopathological examination revealed the presence of well-defined nests of clear-looking or basaloid keratinocytes within an acanthotic epidermis, corresponding to the Borst-Jadassohn phenomenon. The tumor cells were small and monomorphic. We diagnosed this tumor as clonal seborrheic keratosis. Although surgical excision was recommended, our patient refused an operation. This case of clonal seborrheic keratosis is presented for its rarity and for differential diagnosis.


Introduction
Seborrheic keratosis (SK) is one of the most common benign tumors of the skin. The clinical diagnosis of this entity is often simple. Clonal SK is an atypical variant which may resemble other benign or malignant lesion. We herein report a patient who clinically presented with reddish, focally dark-brown lesion that histopathologically showed a benign lesion mainly composed of clonal ("nested") seborrheic keratosis. Through this report, we will highlight the clinical and histopathological features of this lesion and review the main differential diagnoses.

Patient and observation
A 60-year-old woman presented with a 7 year history of a gradually growing, asymptomatic, cutaneous lesion on her left arm. Her medical and family history was unremarkable; especially there was no personal or family history of skin malignancy. On physical examination, the lesion was elevated, well-circumscribed, irregularly shaped, reddish, focally dark-brown, measuring 5 cm in maximum diameter ( Figure 1).
No regional lymph nodes were palpable. The patient did not feel any pruritus or pain at the lesion site. Dermoscopy was not performed.

Discussion
Seborrheic keratosis is a benign epidermal neoplasm [1,2]. It represents one of the more common skin tumors seen by dermatologists in clinical practice [1]. The etiology of SK is still unclear. Although, advanced age and ultraviolet irradiation may explain its development [2]. Clonal SK is one of the histological subtypes of this entity. It is an uncommon lesion, seen in adult and middle-aged patients [3]. It may be found anywhere on the body (excluding palms and soles) and it is especially common on the face, chest and back. Clinically, clonal SK cannot be differentiated from ordinary SK [4]. It appears as a sharply demarcated gray-brown to black lesion, which is slightly raised, covered sometimes with greasy scales. However, the clinical diagnosis of clonal SK may be challenging, miming both premalignant and malignant lesions.
Dermoscopically, it reveals the presence of variously sized, blue-gray globular-like structures that are aggregated to form short lines or irregularly distributed within the lesion [5,6]. It can reveal other features suggestive of SK, including demarcated borders, milia-like cysts, comedo-like openings, and the jelly sign [5,6].
Dermoscopy does not reach 100% diagnostic accuracy and clonal SK may represent a dermoscopic trap, simulating either a melanocytic lesion or a basal cell carcinoma [1,4,5]. Histopathological examination should constantly be performed in cases in which dermoscopy exhibits confounding features that do not allow an accurate diagnosis [6]. Histologically, clonal SK is characterized by the presence of intraepithelial, well defined nests of basaloid or pale cells which correspond to the globular structures seen on dermoscopy examination [2,5]. This intra-epidermal proliferation is known as Borst-Jadassohn phenomenon [1,2]. In the histological differential diagnosis of clonal SK, we have to consider other benign tumor such as Hidroacanthoma Simplex (HS), epidermal nevus; and also malignant neoplasm such as Bowen's disease and superficial Basal Cell Carcinoma. HS is a benign, uncommon skin tumor described as verrucous keratotic plaque clinically misdiagnosed as a seborrheic keratosis or in situ squamous cell carcinoma [3]. Histologically, HS is characterized by discrete intraepidermal circumscribed nests of basaloid and pale-staining cells within an irregularly acanthotic epidermis, miming clonal SK [3][4][5]. Therefore, it is difficult to differentiate it especially when the ductal structure and cystic space are absent [7]. In the literature, the differential diagnoses between HS and clonal SK was controversial because both tumors revealed very comparable patterns of cytokeratin expression [7,8]. Currently, Takayama et al. reported that lumican is a powerful differential diagnostic marker that discriminates these two neoplasms [7,9]. The author demonstrated that in 78.6% of HS cases, lumican was positive in poroid cells, whereas the tumor cells in most cases of clonal SK were negative [9]. Epidermal nevus may resemble clinically SK.

Conclusion
In conclusion, although clonal SK is a benign disorder, there are various indications for the removal of lesions especially when it has a suspicious clinical presentation. Figure 1: elevated well-circumscribed lesion which is irregularly shaped, reddish, and focally dark-brown