Giant melanoacanthoma mimicking malignant melanoma.

Melanoacanthoma denotes a rare variant of pigmented seborrheic keratosis. A 65-year-old male farmer had pigmented, verrucous, itchy, highly painful, progressively growing irregularly oval plaque on left side of lower back for the past five years. The indurated lesion, measuring maximum diameter 10 cm × 5 cm, had no discharge, bleeding, ulceration, or associated lymphadenopathy. Dermoscopy showed regular pigmentary network and cribiform pattern of ridges without any feature of malignant melanoma. Histopathology showed well-defined islands of basaloid cells interspersed with large and richly dendritic melanocytes. The lesion was totally excised followed by skin grafting. Our patient was unique in its massive size and clinical resemblance with malignant melanoma. The diagnosis was confirmed by dermoscopy and skin biopsy.

Introduction

Melanoacanthoma, a very rare lesion,[1] was originally described by Bloch in 1927 as “non-nevoid melanoepithelioma type1.”[2] The term “melanoacanthoma” was first coined by Mishima and Pinkus in 1960, who considered this benign pigmented lesion being composed of both melanocytes and keratinocytes.[3] This neoplasm represents a slowly growing, usually solitary, growth of head and neck, or trunk, of older people.[4-6] Melanoacanthoma clinically simulate seborrheic keratosis or a melanoma and may attain a diameter of 3 cm or more.[7] According to the current view, melanoacanthoma denotes a rare variant of seborrheic keratosis rather than a distinct entity,[2] that differs from the usual type of pigmented seborrheic keratosis by marked increase in the concentration of melanocytes. The melanocytes are scattered throughout the tumor lobules instead of confining only to the basal layer.[8] These melanocytes are large and highly dendritic and they contain varying amounts of melanin. Melanoacanthoma has also been reported to origin from mucous membrane, which are rare reactive type of mucosal melanotic macule, unrelated to seborrheic keratosis.[9] An Indian report of multiple genital and perianal melanoacanthomas up to size of 6 cm has been published.[10]

The present case of melanoacanthoma is being reported due to paucity of report from this part of the globe as well as for some unique features.

Case Report

A 65-year-old male presented with painful, localized plaque on left side of lower back. The patient had a small indistinct pigmented patch at the same site, which first appeared during his childhood. Five years back the present lesion appeared on the previous patch, then gradually enlarged and became itchy and rose above the skin surface. Initially the lesion was dry, scaly, not painful and but in recent years it turned out to be highly painful. By profession, he was a farmer belonging to lower socioeconomic category. He, as a farmer, had been exposed to scorching sunlight on his back for almost the whole day. He had the history of addiction to smoking, tobacco chewing, and alcohol intake. He had been suffering from tuberculosis of chest in the past and presently from bronchial asthma and mild depression. On systemic examination no relevant abnormality was found. Dermatological examination revealed that the oval lesion [Figure 1] was of maximum diameter 10 cm × 5 cm, markedly tender, having brown to blackish pigmentation, well-defined margin, and verrucous surface. On palpation, the lesion was indurated. No lymphadenopathy was detected in neighboring feeding sites. Laboratory investigations including routine hematological and biochemical comprising hepatic and renal profiles and urine examination were all within normal limit. Chest X-ray PA view, ultrasonography of whole abdomen did not reveal any abnormal lymph node or visceral change.

Figure 1

Oval lesion of maximum transverse diameter 10 cm having brown to blackish pigmentation, well-defined margin and verrucous surface

Dermoscopy showed regular pigmentary network [Figure 2] without any suggestive features of malignant melanoma like brown globules, black dots, pseudopods, or depigmentation. Instead, cribiform pattern of ridges i.e., dark brown furrows between ridges typifying a brain-like appearance [Figure 2] which is characteristic of seborrhoic keratosis was seen. Skin biopsy showed histopathological features [Figure 3] of acanthotic, slightly verrucous epidermis comprising of both basaloid and spinous cells. The basaloid cells formed well-defined islands and numerous melanocytes were scattered throughout the lesion. The lesion was excised totally with adequate margin followed by skin grafting.

Figure 2

Dermoscopy showing regular pigmentary network without any suggestive features of malignant melanoma like brown globules, black dots, pseudopods, or depigmentation. Cribriform patterns of ridges are seen.

Figure 3

The basaloid cells formed well-defined islands and numerous melanocytes were scattered throughout the lesion (H&E stain: 400×)

Discussion

Cutaneous melanoacanthomas manifest as pigmented papules, plaques, cutaneous horns or nodules. Melanoacanthomas have been described more frequently in white patients. They develop in the middle-aged and the elderly like any other type of seborrheic keratosis. They are found mainly on the trunk or head, often on the lip or eyelid. Melanoacanthoma, a benign mixed tumor of melanocytes and keratinocytes,[3] although rare, may clinically mimic pigmented seborrheic keratosis and malignant melanoma.[7] However characteristic histopathological features will differentiate the above three lesions. Two histologic types of melanoacanthomas are described: a diffuse type in which melanocytes are unevenly scattered throughout the lesion and a clonal type in which melanocytes and keratinocytes are clustered in small nests. There was partial or complete disturbance of transfer of melanin from these highly dendritic melanocytes to neighboring keratinocytes. Immunofluorescent studies and an immunoprecipitin assay have shown that melanoacanthomas are not related to malignant melanoma, and hence, removal by simple excision, curettage or cryotherapy should be curative.[10]

In our case, presence of large number of melanocytes even deep into the tumor mass instead of restricting to the basal layer [Figure 3] has excluded pigmented seborrheic keratosis[8] from the diagnosis. Huge size, marked tenderness, preliminary presence of pigmented patch from childhood at the site of the lesion also led us to think a provisional diagnosis of melanoma in the present case. Dermoscopic features [Figure 2] did not support the diagnosis of malignant melanoma Cribriform pattern of ridges, characteristic of seborrhoeic keratosis was noticed. This confirms the current view that melanoacanthoma represents a variant of seborrheic keratosis rather than a distinct entity.[2]

Histopathological features [Figure 3] depicting no evidence of cellular atypia or pagetoid upward extension of tumor cells, however, did not also speak in favor of malignant melanoma.

The only Indian case report[10] had shown multiple lesions of maximum diameter 6 cm on the lower abdomen, inner thighs, external genitals, and perianal areas for the past 10 years. Our case, as compared, was giant measuring of maximum diameter 10 cm × 5 cm, solitary, and site was lower back.

We were prompted to publish this case report for: 1) giant size of melanoacanthoma, which has not much been reported in the earlier literature; 2) clinical dilemma with malignant melanoma; 3) importance of dermoscopy and histopathologcal examination in the exclusion of malignant melanoma from melanoacanthoma.