Diagnosis: Pigmented epithelioid melanocytoma
Pigmented epithelioid melanocytoma (PEM) is a term proposed by Zembowicz et al1 in 2004 to describe heavily pigmented melanocytic tumors ranging from epithelioid blue nevi to animal-type melanoma. PEM is considered by some a low-grade type of melanoma that has an indolent course and rare visceral metastatis.1 Histologically, there is no characteristic to differentiate metastasizing vs nonmetastasizing PEMs.1,2 PEM shows no ethnic or gender predilection.1 Involvement of areas such as genitals and mucosa suggest that sun exposure is not a risk factor in the pathogenesis of PEM.1
PEM most commonly presents as a slow growing, blue to black nodule on an extremity. However, they also commonly appear on the trunk, head, and neck.3 PEMs typically arise sporadically but have also been reported in association with preexisting dermal or congenital nevi that expanded in later years.1,3 PEM has a predilection for children, adolescents, and young adults and has a more indolent course in younger patients.1,4 The median age is 27 years, though reports of presentation range from newborn to age 92 years.3 It has a high tendency to involve regional lymph nodes and rarely results in distant metastases or death.1,4
Dermoscopic examination of PEM reveals a combination of homogeneous blue pigment, black and blue pigment, brown comedo-like openings, white pigment representing crystalline structures, and blue pigment extending into lymphatic vessels.5
Histopathologically, PEM is a dermal melanocytic proliferation of pigmented epithelioid and spindled melanocytes.3,4 The overlying epidermis is often hyperplastic and a Grenz zone may be present. Mitoses and cytological atypia are variable and necrosis is not seen. Immunohistochemistry will demonstrate a positive S-100, vimentin, HMB-45, and Melan-A.
Please refer to the Table2,3,6-12 for the differential diagnosis of PEM.
PEM is a low-grade melanocytic tumor that usually has a favorable long-term clinical outcome.13 In a 67-month study of 26 patients who underwent complete local excision and were subsequently monitored, none of them had adverse outcomes, including those with sentinel lymph node metastasis.13 There is no histologic criteria to differentiate metastasizing vs nonmetastasizing PEMs.1 Treatment of PEM includes excising the lesion with clear margins to reduce the possibility of recurrence.13 Sentinel lymph node biopsy may not be necessary because the presence of lymph node metastasis does not appear to affect the prognosis. Given the favorable clinical course of PEM, it is not justified to treat patients with biologic or chemotherapeutic agents.13
The grey-black hyperpigmented nodule found on our patient’s anterior scalp was asymptomatic (Figure 1). The lesion was biopsied for diagnosis and definitively excised with 1-mm margins. Punch biopsy of this patient’s lesion revealed a dermal proliferation of plump epithelioid cells with cytoplasmic pigmentation, mild nuclear atypia, and no mitosis (Figures 2, 3). The epithelioid cells were distributed between collagen bundles and along adnexal structures with extension into the subcutaneous fat. Dermoscopy demonstrated a homogenous bluish grey and black pigmentation with no crystalline structures or irregular pigmentation networks. The patient showed no signs of recurrence at 1-month follow up.
We present a case of PEM on the scalp. It most commonly presents as a black or blue nodule on the extremity of a young adult. The differential diagnosis for a black or blue nodule is broad, the most concerning being nodular melanoma, therefore diagnosis is typically confirmed by biopsy. Treatment includes local excision with clear margins to reduce the likelihood of recurrence. Although rare, visceral metastasis can occur, making it essential for patients to undergo complete excision and close follow-up.