Pigmented hidroacanthoma simplex: A diagnostic pitfall

Abstract

Hidroacanthoma simplex (HAS) is a rare skin appendage tumor and is recognized as an intraepidermal variant of poroma. Pigmented HAS is an extremely rare variant with dendritic melanocytes and/or melanin pigment in the tumor cells and a few cases of pigmented HAS have been reported in the English literature. We herein report a case of pigmented HAS and discuss the clinicopathological features of pigmented HAS and the pitfall in the diagnosis of black lesions.

Key Words: Pigmented hidroacanthoma simplex, Seborrheic keratosis, Porocarcinoma, Skin appendage tumor, Clinicopathological features

INTRODUCTION

Hidroacanthoma simplex (HAS) is a rare benign skin appendage tumor and is recognized as an intraepidermal variant of poroma[1]. Pigmented HAS is an extremely rare variant of HAS and a few cases of pigmented HAS have been reported in the English literature[2-6]. Herein, we report an additional case of pigmented HAS in a 73 years old Japanese woman and we discuss the clinicopathological features of pigmented HAS and the pitfall in the diagnosis of black lesions.

CASE REPORT

A 73 years old Japanese woman presented with a nodule on the anterior region of her right lower leg, which had first been noticed one year earlier. The lesion was a brown to black scaly dome-shaped nodule, measuring 12 mm × 8 mm in diameter, and no clinical symptoms, such as erosion and pain, were present. Macroscopic inspection led to the initial clinical diagnosis of seborrheic keratosis and total tumor resection was performed.

The histopathological study of the tumor revealed irregular elongation of the rete ridges, hyperkeratosis and intraepidermal proliferation of small-sized basaloid cells, which exhibited the so-called “Jadassohn phenomenon” (Figure 1A). The nuclei of the basaloid cells were round to oval and smaller than those of neighboring epidermal keratinocytes, although these cells had a small nucleolus (Figure 1B). Mitotic figures were rarely seen (< 1/10 high power fields). Dendritic melanocytes were scattered in the tumor nests but melanin pigment was not present in the cytoplasm of the basaloid cells (Figure 1B). Within the tumor nests, a few ductal structures were observed (Figure 1C). No invasive neoplastic growth was observed.

Figure 1 Histopathological findings of the right lower leg tumor (HE stain). A: Intraepidermal proliferation (so-called “Jadassohn phenomenon”) of small-sized basaloid cells (original magnification, × 40); B: The nuclei of the basaloid cells are smaller than those of neighboring epidermal keratinocytes and dendritic melanocytes are scattered within the tumor nests (original magnification, × 200); C: Within the tumor nests, a few ductal structures are observed (arrow) (original magnification, × 600).

Immunohistochemical study showed that epithelial membrane antigen was expressed in approximately half of the tumor cells but no carcinoembryonic antigen-positive cells were found. Overexpression of p53 protein was not observed in the tumor cells. Melan-A was expressed in dendritic melanocytes.

Accordingly, the final diagnosis of pigmented HAS was made.

DISCUSSION

HAS is a rare benign skin appendage tumor, first described by Coburn et al[7] in 1956. HAS is recognized as an intraepidermal variant of poroma and thought to originate from the outer cells of the acrosyringium[1]. According to the report by Anzai et al[8] in which 70 Japanese cases of HAS were analyzed, the most common sites of HAS were the trunk (42.8%) and lower extremities (45.7%) and most lesions were clinically diagnosed as seborrheic keratosis or Bowen’s disease, whereas none were clinically diagnosed as HAS[8]. These results suggest that clinical diagnosis of HAS by macroscopic inspection is extremely difficult.

The most peculiar finding in the present case is the presence of dendritic melanocytes in the intraepidermal tumor nests. Although various benign and malignant cutaneous tumors often accompany non-neoplastic melanocytes, described as “melanocytic colonization”, poroma and HAS usually lack melanocytes and melanin pigment. However, a few cases of pigmented HAS, an extremely rare variant with dendritic melanocytes and/or melanin pigment in the tumor cells, have been reported in the English literature[2-6]. In the present case, intraepidermal proliferation of bland basaloid cells exhibiting the “Jadassohn phenomenon” was observed and the nuclei of these basaloid cells were smaller than those of neighboring epidermal keratinocytes. Further dendritic melanocytes were scattered within the tumor nests. These findings are similar to the characteristics of clonal type of seborrheic keratosis because HAS and clonal type of seborrheic keratosis exhibit an intraepidermal proliferation of small basaloid cells. However, a few ductal structures were observed; therefore, the clonal type of seborrheic keratosis was excluded and a final diagnosis of pigmented HAS made.

It is well known that HAS is often associated with porocarcinoma and that porocarcinoma shows aggressive clinical behavior. Snow et al[9] reported that approximately 20% of porocarcinoma recur after excision and regional lymph node metastases occurred in 20% of the cases, while 12% also showed distant metastases. The mortality rate exceeds 65% when regional lymph nodes are involved and the survival for patients with distant metastases is reported to be between 5 to 24 mo[9]. In addition, most of the reported cases of pigmented HAS had a malignant component[2-6] and we previously reported a case of porocarcinoma arising in pigmented HAS, which led to death from multiple lymph node, bone and liver metastases[6]. Lee et al[10] recommended early wide local excision and careful attention in the case of HAS due to its potential risk of malignant change.

Dermoscopic findings of pigmented HAS have been reported[11]. Pigmented poroma, including HAS, mimics a number of skin tumors, including basal cell carcinoma, seborrheic keratosis and malignant melanoma on dermoscopy[11]. Therefore, distinction between pigmented HAS and seborrheic keratosis may be challenging on dermoscopy.

In conclusion, it is important for dermatologists to take pigmented HAS into consideration in a differential diagnosis of black lesions because HAS, especially pigmented HAS, clinically resembles seborrheic keratosis and Bowen’s disease. In addition, it is important to completely resect HAS before malignant transformation and rapid progression to invasive porocarcinoma.