Unknown: Two adjacent nodules on the leg
Heleen D de Koning MD, H Jorn Bovenschen MD PhD
Dermatology Online Journal 16 (6): 13

Abstract

Poroma is a rare benign neoplasm (derived from the intraepidermal part of the eccrine or apocrine duct), which may clinically mimic malignant tumors such as (amelanotic) malignant melanoma and porocarcinoma. Histopathological examination is the key to the correct diagnosis, which is illustrated in the present case, in which a pigmented basal cell carcinoma and a poroma are in close proximity to each other. Despite a clinical differential diagnosis of melanoma, histopathology showed the typical characteristics of a poroma, a rare but much more favorable tumor. Histopathological features of poroma are discussed.

Answer: Poroma (Arrow) and pigmented nodular basal cell carcinoma (Arrowhead)

Figure 1	Figure 2a
Figure 1. (Arrow) Painful erythematous teleangiectatic nodule (diameter 12 mm), with a nearby (Arrowhead) sharply demarcated hyperpigmented nodule (diameter 10 mm) on the left lower leg Figure 2. Histology of the upper lesion (Arrow) showed a polypoid exophytic tumor with acanthosis of the epidermis and lumina with surrounding cylindrical epithelial cells. Comparable nests of epithelial ‘poroid’ cells with tubular differentiation and lumina were observed in the dermis, presenting as inter-anastomosing cords. There was an abundance of small vessels in and around the tumor, a fibrous eosinophil stroma reaction and a small infiltrate containing lymphocytes and plasma cells. The tumor reached to the level of the sweat glands and there were especially no signs of infiltration. (H&E, 2a: x50 and 2b: x100).

Figure 2b	Figure 3
Figure 3. The lower lesion (Arrowhead) was histopathologically clearly consistent with a pigmented basal cell carcinoma, with atypical basaloid cells, retraction artefact, peripheral palisading basal cells, and an abundance of pigment. (H&E, x100)

A 62-year-old otherwise healthy Caucasian woman presented with a painful, exsudative, easily bleeding erythematous telangiectatic nodule on the medial side of her left lower leg, which had steadily grown over the past three years (Arrow) (Figure 1). Just below she had a tender indurated nodule with hyperpigmentation (Arrowhead). Both lesions were excised and histopathological findings of the lesions are depicted in Figures 2 and 3, respectively.

Histopathological findings

The clinical differential diagnosis in this case included (I) (amelanotic) melanoma, pyogenic granuloma and squamous cell carcinoma, and for (II): melanoma, satellite metastasis, dysplastic nevus, or pigmented basal cell carcinoma. The diagnosis is generally made by means of histopathology.

Histology of the excision of the upper lesion (Arrow) showed a polypoid exophytic tumor with acanthosis of the epidermis with a non-atypical basal cell proliferation (Figure 2A). In between, lumina with surrounding cylindrical epithelial cells were detected. Comparable nests of epithelial “poroid” cells with tubular differentiation and lumina were observed in the dermis, presenting as inter-anastomosing cords. There was an abundance of small vessels in and around the tumor. Some fibrous, eosinophil stroma reaction with some amyloid deposition was seen. The small infiltrate contained lymphocytes and plasma cells. The tumor reached to the level of the sweat glands and there were no signs of infiltration (Figure 2B). There was no nuclear atypia or dysplasia and only few mitotic figures were present. Immunoperoxidase staining of carcinoembryonic antigen (CEA) was positive for several lumina in the tumor, whereas the epithelial membrane antigen (EMA) was less obviously positive. PAS-positive material was observed. It was concluded that the histopathology was typically in line with a poroma. On histopathological examination, the lower (Arrowhead) appeared to be a pigmented nodular basal cell carcinoma (Figure 3).

Discussion

In this case, both tumors presented in close proximity to one another, which raised a strong suspicion of malignant melanoma. Because basal cell carcinoma is a well-established tumor in the differential diagnosis of melanoma, only the poroma will be further discussed here. Poroma is a rare benign neoplasm derived from the intraepidermal part of the eccrine or apocrine duct. Exact prevalence numbers are unknown. Eccrine poromas mainly arise on plantar, palmar, and plantar skin, but in a large series the head was affected in 40 percent of the cases [1, 2]. In fact, any skin area containing sweat glands can be affected. Recent studies have shown that poromas can be of either eccrine or apocrine lineage, the latter being even more common. A keratin expression study found that eccrine poromas are pure periductal sweat gland tumors derived from the basal keratinocytes of the eccrine duct ridge and the lowermost acrosyringium [3]. The etiology is largely unknown, but associations with scarring, radiation and trauma have been reported [4, 5, 6]. It usually appears in middle-aged or elderly people as a solitary lesion, but the tumors may occur in clusters (poromatosis) [1]. Lesions are usually asymptomatic although pain has been reported. Poromas have a heterogenic appearance, but most of them typically present as a sharply demarcated, soft erythematous 2 to 12 mm papule or nodule that may be sessile or pedunculated [7, 8]. In general, malignant (amelanotic) melanoma, porocarcinoma, and non-melanoma skin cancer, such as squamous cell carcinoma and Merkel cell tumor must be excluded [9].

Epiluminescence findings show a considerable heterogeneity ranging from polymorphous vascular patterns with pin-point, hairpin vessels and/or arborizing vessels, to blue-gray ovoid nests and blue-gray dots [8, 10]. Because these findings are commonly observed in skin cancers, they do not discriminate between poroma and malignant (amelanotic) melanoma, non-melanoma skin cancer or porocarcinoma. Therefore, epiluminescent examination of poroma has little diagnostic significance and histological examination is indispensable for making the correct diagnosis [8, 10, 11].

Surgical excision of the entire lesion is warranted if a malignancy is suspected; small biopsy specimens may falsely appear benign in the case of a porocarcinoma, the malignant counterpart of poroma [12]. It is important to distinguish between these two entities because malignant porocarcinoma can metastasize and cause death. Clinical signs of porocarcinoma include bleeding, pain, and pruritus, whereas poromas are mostly asymptomatic. Histologically, cytologic atypia and infiltration of poroid cells distinguish malignant porocarcinoma from eccrine poroma [12]. Differences may be very subtle. Malignant transformation of poroma into porocarcinoma has been described, but is controversial. However, in a series of 69 cases of porocarcinoma, 18 percent appeared to have arisen in continuity with a preexisting poroma [12].

In conclusion, poroma is a rare benign tumor that may clinically mimic malignant tumors such as (amelanotic) malignant melanoma and porocarcinoma. In the present case, the uncommon co-occurrence of poroma and pigmented basal cell carcinoma was a major pitfall. Because the diagnosis is based on specific histopathological criteria surgical excision of the entire lesion is warranted if a malignancy is suspected.

References

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