Melanoacanthoma: Uncommon presentation of an uncommon condition

Abstract

Melanoacanthoma is very rare variant of seborrheic keratosis presenting as a deeply pigmented benign proliferation of melanocytes and keratinocytes usually presenting over the head, neck and trunk of elderly people. A sixty-two-years-old male was presented with a solitary slow growing asymptomatic hyperpigmented verrucous outgrowth with cerebriform surface measuring 15 cm by 8 cm present over the left inguinal region extending on to the scrotum since past 8 years. There was no associated lymphadenopathy. The histopathology revealed hyperkeratosis, papillomatosis, acanthosis with presence of melanocytes at all levels of epidermis with abundant melanin giving the diagnosis of melanoacanthoma. The patient further underwent surgical excision of the lesion. The case is being reported for its rarity, unusual location, massive size and clinical resemblance to a verrucous carcinoma.

INTRODUCTION

Melanoacanthoma is a benign, deeply pigmented, proliferation of keratinocytes and melanocytes. It is a very rare variant of seborrheic keratosis[1] rather than a distinct entity. The term melanoacanthoma was coined by Mishima and Pinkus in 1960.[2] It was described by Bloch[3] in 1927 as nevoid epithelioma type 1, different from the melanoepithelioma type 2 or the ordinary pigmented variant of seborrheic keratosis. The condition is more commonly seen in light skinned and manifests as pigmented papules, plaques, cutaneous horns or nodules.[4] Patients are generally asymptomatic and may wait for decades before they seek treatment.

CASE REPORT

A 62-year-old male was presented with a blackish verrucous outgrowth over the left inguinal region since the past 8 years. It started with a pigmented elevated lesion at the same site that gradually increased to the current size. Other than occasional pruritus at the sites of occlusion and the feeling of heaviness while walking, the lesion had remained largely asymptomatic. There was no bleeding from the lesion. The patient was a chronic smoker since the past 30 years. There were no concomitant comorbidities or medications.

The systemic examination of the patient was normal. Dermatological examination revealed a single well-defined sessile deeply pigmented verrucous mass measuring 15 cm by 8 cm over the left inguinal region extending onto the medial aspect of the scrotum [Figure 1]. There were areas of depigmentation and maceration at the sites of friction. The mass was non-tender and there was no associated lymphadenopathy. Mucus membrane examination was normal.

Figure 1.

Well-defined deeply pigmented verrucous mass measuring 15 cm by 8 cm over the left inguinal region with areas depigmentation and maceration at the sites of friction

A clinical differential diagnosis of melanoacathotic variant of seborrheic keratosis, pigmented variant of seborrheic keratosis, giant condyloma acuminata and verrucous carcinoma were thought of.

Laboratory investigations including routine hematological and biochemical investigations comprising hepatic and renal profiles and urine examination were all within normal limits. VDRL and HIV testing of the patient were non-reactive.

Histopathological examination of the biopsy specimen revealed hyperkeratosis, papillomatosis, acanthosis, with melanocytes spanning all layers of the epidermis with abundant melanin. The base of the tumor was lying in a straight line with the surrounding normal epidermis. Small horn cysts were seen. Pigment incontinence was seen in the dermis. High power microscopy revealed cells with ovoid nuclei and cleared cytoplasm with abundance of melanin [Figures 2 and 3]. Dendritic melanocytes were demonstrated by the Masson Fontana silver impregnation stain [Figure 4]. All these findings confirmed the diagnosis of melanoacanthoma.

Figure 2.

At ×4 magnification, H and E staining epidermis showing hyperkeratosis, papillomatosis, acanthosis, with melanocytes spanning all layers of the epidermis with abundant melanin and small horn cysts. Base of the tumor is seen lying in a straight line with the surrounding normal epidermis

Figure 3.

×40 magnification, H and E staining epidermis showing cells with ovoid nuclei and cleared cytoplasm with abundance of melanin. Pigment incontinence seen in the dermis

Figure 4.

Masson Fontana silver impregnation stain showing dendritic melanocytes

Patient was further referred to the department of plastic surgery where complete excision with adequate margin and suturing was done [Figure 5].

Figure 5.

The appearance post-excision and suturing

DISCUSSION

Melanoacanthoma is a rare cutaneous tumor reported more commonly in light skinned persons. The lesions are usually solitary and common sites are the head, neck, trunk, often on the lip or the eyelid. Some have suggested that it could be a localized phenomenon induced by trauma.

Melanoacanthoma is a proliferation of keratinocytes and melanocytes localized to the epidermis. The presence of large number of melanocytes even deep into the tumor mass instead of restricting itself to the basal layer differentiates it from pigmented variant of seborrheic keratosis.

In this case, the lack of koilocytes ruled out condyloma acuminata and absence of cellular atypicality did not favor verrucous carcinoma.

Two histological types of melanoacanthoma have been described: (a) diffuse type where melanocytes are scattered unevenly throughout the lesion as was seen in this case and (b) clonal type where melanocytes and keratinocytes are clustered in small nests.[4] Electron microscopic studies have revealed that there is a defect in the transfer of the melanin from these highly dendritc melanocytes to the keratinocytes. Immunoflourescent and Immunoprecipitin studies have shown that melanoacanthomas are not related to malignant melanomas and hence simple excision or cryotherapy is curative.[5]

Oral melanoacanthomas are rare reactive mucosal lesions that demonstrate hyperplasia of spinous keratinocytes and melanocytes and are unrelated to the seborrheic keratosis.[6]

There have been very few reports of atypical presentations of melanoacanthoma from this part of the globe. Shankar, et al. reported a case of giant melanoacanthoma occurring in a 65-year-old farmer presenting as a painful verrucous itchy plaque, 10 cm by 5 cm on the left side of the lower back of 5 years duration, mimicking melanoma.[7] The only other Indian report by Shenoy, et al. showed multiple lesions of maximum diameter 6 cm on the lower back, abdomen, inner thighs and external genitals.[8]

What prompted to publish this report was (1) the rarity of the condition (2) the unusual site of occurrence (3) the massive dimensions and (4) the clinical resemblance to verrucous carcinoma.