Melanoacanthoma is acquired, almost always solitary, pigmented lesions of the skin that appear in adulthood. In most patients, they are asymptomatic and progressively enlarge slowly.[9][10][11] [14] However, albeit uncommon, they can grow rapidly, itch, be tender, or become irritated.[2][14][12][13]

The duration that the melanoacanthoma was present prior to biopsy-confirmation of the diagnosis was provided for 29 individuals. It ranged from 5 weeks to 40 years. The investigators for two of the individuals provided descriptive durations: "as long as he could remember" in a 40-year-old man with multiple melanoacanthomas on his left upper eyelid and "since childhood" in a 56-year-old woman with a melanoacanthoma on her abdomen.[2][4][8][9][10][11][13][14][15][16]

The skin lesion was present for more than 10 years in 28% (8 of 29) of the patients, and between 6 to 10 years for 24% (7 of 29) of the patients. For 35% (11 of 29) of the patients, it was only present for less than 3 years. Overall, the median duration of the melanoacanthoma was present before establishing its diagnosis was 6 years.

The size of the melanoacanthoma was described for 36 patients. The greatest diameter ranged from minute to 2 millimeters to 15 centimeters. Five of the lesions (14%) were greater than or equal to 10 centimeters; they were characterized as being 'giant.'[2][4][8][9] Eleven of the lesions (31%) were between 2.5 centimeters to 9.9 centimeters; they were referred to as 'large.'[2][4][5][10][14] A third of the melanoacanthoma was between 1 to 2 centimeters, and only 22% (8 of 36) were less than 1 centimeter. The median size was 2 centimeters.

The color of the melanoacanthoma can mimic that of melanoma; it was specified for 28 patients. Nine of the patients' melanoacanthoma were composed of 2 colors. In addition, it was described as either pigmented or hyperpigmented for an additional 8 individuals.[4][9]

Half of the lesions (14 of 28) were black.[2][13][14][16] The next most common colors were either both black and brown (21%, 6 of 28) or brown (18%, 5 of 28).[2][4][8][10][11][12] The melanoacanthoma was either black and gray (7%, 2 of 28) or blue and brown (4%, 1 of 28) in the remaining patients.[2][4]

The melanoacanthoma color in one study was not able to accurately be interpreted based on the information provided. The 8 melanoacanthomas evaluated had either 1, 2, or more colors. The most common colors were either light brown (5 patients) or dark brown (4 patients); black, blue-gray, and pink melanoacanthoma were each observed in 1 patient.[5]

The appearance of a melanoacanthoma often mimics that of melanoma.[2][3][8][9][10][12][13][14][16] It typically presents as a solitary papule, nodule, or plaque.[4][8][10][12][13][14] However, its surface can be lobulated or verrucous.[2][4][14][8][13][15]

The melanoacanthoma location was specified for 52 patients. The head and neck (46%, 24 of 52) or the trunk (30%, 16 of 52) were the most common sites. Most of the remaining 24% of melanoacanthomas were located either on an extremity (12%, 6 of 52) or the genitals (6%, 3 of 52: either inguinal or penis or scrotum in 1 patient) or the buttock and hip (4%, 2 of 52).

The head and neck melanoacanthoma were most commonly located on the scalp (28%, 6 of 21). Other sites included the ear (19%, 4 of 21), the eyelid (19%, 4 of 21), the face (19%, 3 of 21 on the preauricular area and 1 of 21 on the nose) and the neck (15%, 3 of 21). The specific location on the head and neck was not stated in 3 patients.

A 58-year-old woman had multiple slowly growing melanoacanthomas of 10-years duration (2%, 1 of 52). Her lesions were located on the neck, trunk (chest, abdomen, and back), extremities (proximal arms and legs), and buttocks. Several of her lesions (particularly those on her left buttock and periumbilical region) were horn-like cutaneous nodules. Also, one of the melanoacanthomas on her trunk became ulcerated.[9]

Jain et al. also summarized the features of another patient (described by Shenoy et al.) with multiple genital and perianal melanoacanthomas; the man's lesions were located not only on his trunk (lower abdomen) and extremity (left inner thigh) but also on his penile shaft and perianal skin. Multiple melanoacanthomas were also reported on the left upper eyelid of a 40-year-old man. In contrast to patients with cutaneous melanoacanthoma in whom multiple tumors are seldom observed, multiple lesions are not uncommon in individuals with oral melanoacanthoma.[1][9]

Rarely, a melanoacanthoma presents as a cutaneous horn, mimicking a verruca or a squamous cell carcinoma. The buttock and periumbilical lesions on the woman with multiple melanoacanthomas morphologically appeared as cutaneous horns. Also, the melanoacanthoma on the left upper eyelid of a 45-year-old man developed into a horn-like cutaneous nodule that grew downwards and extended to below his lower eyelid.[9]

Biopsy, partial or excisional, is the most appropriate approach for the evaluation of a lesion in which the diagnosis of melanoacanthoma is being considered. An adequate biopsy specimen should be obtained so that the pathologist has sufficient tissue to examine and thereby be able to provide an accurate diagnosis.

The clinical features of a melanoacanthoma mimic those of melanoma. Therefore, the clinician often cannot establish the diagnosis of melanoacanthoma based solely on the lesion's morphologic appearance. Thus, the evaluation of a suspected melanoacanthoma may include dermoscopy and reflectance confocal microscopy. However, biopsy for microscopic examination is usually necessary to establish the diagnosis.

Dermoscopy

Dermoscopy is a non-invasive in vivo examination technique to evaluate the surface of the skin. Subsurface skin structure in the epidermis can be visualized using a hand-held dermatoscope. Dermoscopy, most often used to evaluate pigmented skin lesions, has been done by four groups of investigators to study melanoacanthoma.

Dermoscopic features of melanoacanthoma, which mimicked those of a pigmented Spitz nevus, were initially described by Rossiello et al. in 2006. They examined a raised 4 millimeter verrucous pigmented left hip lesion and viewed a starburst pattern consisting of symmetrically distributed pigmented streaks on the periphery; this pattern has previously been considered diagnostic for a pigmented Spitz nevus. Also, in the center, they noted other features of a pigmented Spitz nevus, which can also occasionally be found in a melanoma: (1) gray to whitish hyperkeratotic areas and (2) multiple, black to brown, dots and (3) multiple globules.[15] 

Shankar et al., in 2010, evaluated a giant 10 cm by 5 cm melanoacanthoma. Dermoscopic examination showed a regular pigmentary network. A cribriform pattern of ridges and fissures characteristic for a seborrheic keratosis was also observed. There were no melanoma dermoscopic features.[8]

Subsequently, in 2015, Chung et al. studied the lesions from eight patients with melanoacanthomas with dermoscopy. They noted features of seborrheic keratoses (comedo-like openings, hairpin vessels, milia-like cysts, moth-eaten border, and sharp demarcations) in all of the lesions. However, they also observed features specific for melanoma features (atypical dots, blue-white veil, granularity, and polymorphous vessels) on six of the lesions.[15]

More recently, in 2016, Shahriari et al.'s dermoscopic examination of melanoacanthoma which presented as a 7 mm black papule on the left submandibular area showed a lesion with a verrucous contour, scale, and features that prompted the investigators to consider a pigmented nodular melanoma: irregular blue-white and black clods.[13] 

Reflectance Confocal Microscopy

Reflectance confocal microscopy is a novel non-invasive imaging technique. It allows in vivo imaging of the skin. It allows visualization of the epidermis and superficial dermis at nearly histologic resolution.

Reflectance confocal microscopy of melanoacanthoma was initially described by Shahriari et al. in 2016 of a left mandibular black papule. A well-circumscribed lesion whose overall architecture showed keratotic debris-filled surface depressions and ridges was observed at the level of the stratum corneum of the epidermis; this correlated with the verrucous surface of the lesion. Numerous tangled bright dendritic cells were noted in the suprabasal spinous and granular layers of the hyperplastic epidermis; this feature was suggestive of that seen in melanoma with a pagetoid distribution of melanocytes throughout the epidermis. Since a melanoma was suspected based on the reflectance confocal microscopy findings, the lesion was excised.[13]

Porto et al., in 2019, also used reflectance confocal microscopy to evaluate a melanoacanthoma presenting as a 9 mm dark brown patch on the scalp. At the spinous-granular layer, a typical honeycomb pattern was present; also, there were not only epidermal projections and keratin-filled invaginations but also widespread dendritic pagetoid cells. At the dermo-epidermal junction, between the dermal papillae, there was also infiltration of dendritic cells. Melanophages were noted at the level of the papillary dermis. The presence of suprabasal dendritic cells can be observed in squamous cell carcinoma in situ, and the pagetoid spread of dendritic melanocytes can be noted in melanoma; therefore, the lesion was excised.[12]

In summary, several investigators have described the dermoscopic features and the reflectance confocal microscopic features of melanoacanthoma. Although dermoscopic findings such as a starburst pattern, a regular pigmentary network, and a cribriform pattern of ridges and fissures are typically observed, other features suggestive of melanoma are also frequently present. Similarly, features observed on reflectance confocal microscopy mimic those of melanoma. Hence, dermoscopy and reflectance confocal microscopy alone are not reliable for establishing the diagnosis of melanoacanthoma.

Sometimes confused for melanoma on visual examination, most melanoacanthomas were surgically removed either at the time of biopsy or subsequently by simple excision. Partial removal during the biopsy of a melanoacanthoma has resulted in the persistence of the preauricular lesion in one patient.[14] Also, the recurrence of a nasal melanoacanthoma was observed 6 months after it had been removed.[2]

Melanoacanthoma is benign epithelial neoplasms. Hence, other therapeutic interventional modalities that result in the removal of the epidermis should effectively treat the lesion. Therefore, cryotherapy with liquid nitrogen, particularly for smaller lesions and curettage, are potential options that might be considered.[8]

Topical 5-fluorouracil 5% cream was used to treat smaller lesions in a 58-year-old woman with multiple melanoacanthomas. She applied the medication twice daily for four weeks. However, the cream was discontinued since it caused a burning sensation at the application sites, and there was not any significant improvement of the melanoacanthomas.[9]

• Actinic keratosis (pigmented)
• Basal cell carcinoma (pigmented)
• Condyloma accuminata
• Lentigo maligna
• Melanocytic nevus
• Melanoma
• Pigmented spindle cell (Spitz) nevus
• Seborrheic keratosis
• Solar lentigo
• Squamous cell carcinoma (pigmented)
• Verrucous carcinoma
• Verruca vulgaris

The morphologic features of a melanoacanthoma can mimic those of other skin lesions. Melanoacanthomas most commonly resemble other pigmented cutaneous lesions such as actinic keratosis, a basal cell carcinoma, a lentigo, a melanoma, a nevus, or a seborrheic keratosis. However, microscopic evaluation of the lesion enables the diagnosis of melanoacanthoma to be established.[5][10][12]

The pathologic differential diagnosis is a pigmented seborrheic keratosis. However, in seborrheic keratosis, the pigment and melanocytes are predominantly present and restricted to the basal layer or lower layers of the epidermis. In contrast, in a melanoacanthoma, the pigment results from the dendritic melanocytes that are present throughout not only the lower layers but also the upper layers of the epidermis. The absence of cellular atypia and numerous mitoses excludes squamous cell carcinoma. The absence of koilocytes excludes the diagnosis of verrucae.

Actinic keratoses are premalignant skin lesions. They typically present as scaly red plaques on sun-exposed areas. However, pigmented actinic keratoses, usually in patients with darker skin color, can present as a flat dark brown or black hyperpigmented plaque that mimics other pigmented lesions such as melanoma or seborrheic keratosis. Although the diagnosis may be suspected clinically, it is often established after the lesion has been biopsied. Management is similar to non-pigmented actinic keratoses, including destructive modalities (such as liquid nitrogen cryotherapy or curettage or superficial chemical peels), topical treatments (with either 5-fluorouracil, imiquimod or ingenol mebutate) or photodynamic therapy.

Basal cell carcinoma is the most common skin cancer. It typically occurs on sun-exposed sites; however, the tumor can occur on areas that do not regularly get exposed to the sun, such as the areola of the breast, the axilla, the perianal region, and the umbilicus. The morphology of the cancer is variable. Superficial basal cell carcinomas present as flat red scaly plaques. Nodular basal cell carcinomas appear as flesh-colored to translucent papules or nodules with overlying and/or adjacent telangiectasias; some of the lesions may ulcerate. Sclerosing basal cell carcinomas mimic scars. However, an uncommon variant of basal cell carcinoma is pigmented and presents as a black or brown papule, plaque or nodule and may masquerade as either a melanoma. Microscopically, the pigment can be found in the basaloid tumor cells in macrophages in the dermis or both locations. Treatment of a pigmented basal cell carcinoma is similar to that of the non-pigmented tumor—often, the tumor is removed by either Mohs microscopically controlled surgery or simple excision.

Lentigo microscopically presents as hyperpigmentation of cells (including both the keratinocytes and melanocytes) in the basal layer of the epidermis; also, there may be a slight increase in the number of melanocytes. When this occurs in a sun-exposed location, the lesion is referred to as a solar lentigo; at other sites, the lesion is denoted as a lentigo simplex or just a lentigo. Clinically, the benign pigmented lesion is flat.

Melanoma is a malignant melanocytic neoplasm. The lesion may be flat (such as a lentigo maligna melanoma, which is typically found on sun-exposed sites such as the face and superficial spreading malignant melanoma) or a raised nodule (such as a nodular melanoma). Less frequently, melanomas are amelanotic and present as non-pigmented flesh-colored nodules. However, more commonly, melanomas are darkly pigmented; indeed, the individual tumors can be multicolored with black, brown, red, and even blue areas. Melanomas may spontaneously bleed or ulcerate or both.

A melanocytic nevus is a benign lesion. Common subtypes include junctional, intradermal, and compound. Whereas the intradermal nevus is flesh-colored and only contains nests of melanocytes in the dermis, the junctional nevus (which is a flat lesion with melanocyte nests restricted to the dermo-epidermal junction) and compound nevus (which is a raised lesion with melanocyte nests not only found along the dermo-epidermal junction but also in the dermis) are pigmented. 

Seborrheic keratoses are benign pigmented plaques that are usually acquired in adulthood. Although an individual may have only one or a few lesions, many patients have numerous lesions. Their morphologic appearance is either flat (macular) or raised (plaque) with a ‘stuck on’ appearance. Rarely, they may be the cutaneous harbinger of an occult visceral malignancy when the new onset of multiple lesions suddenly appears; this has been referred to as the ‘sign of Leser-Trelat.’ Microscopically, seborrheic keratoses can share similar to melanoacanthoma; however, seborrheic keratoses only consist of keratinocytes, whereas melanoacanthomas are comprised of both keratinocytes and melanocytes throughout the entire lesion.

Squamous cell carcinoma and squamous cell carcinoma in situ both typically present as red plaques or nodules. In contrast, pigmented squamous cell carcinoma and pigmented squamous cell carcinoma in situ present as brown tumors. They are often located in sun-exposed locations of the body. However, the pigmented lesions are often mistaken to be either benign or malignant melanocytic lesions.

The prognosis for an individual with a melanoacanthoma is excellent. The skin lesion is non-cancerous. There are no systemic manifestations associated with melanoacanthoma.

Most patients have had their melanoacanthoma removed either during the biopsy or after the diagnosis has been established. Several of these individuals have had clinical follow-up after their melanoacanthoma has been excision. There was no recurrence of the benign neoplasm in nearly all of these individuals.[11][14][16]

However, the recurrence or persistence of melanoacanthoma has been described in three individuals. A 54-year-old White woman presented with a 13 by 16 millimeter, black lesion on the bridge of her nose of 18 months duration. The clinical diagnosis of a seborrheic keratosis was considered, and the lesion was removed. Within 6 months after the removal of the melanoacanthoma, the lesion reappeared.[2] 

A 65-year-old African-American woman presented with a 2 by 1 centimeter, black plaque on her right preauricular area. An intradermal nevus was suspected clinically, and a superficial shave biopsy was performed. Microscopic examination showed a melanoacanthoma that extended to the deep margin of the specimen. The woman declined further treatment; yet, at follow up several months later, the melanoacanthoma had not only persisted but also increased in size. The residual lesion was completely removed after a deep shave biopsy. There has been no subsequent recurrence after the biopsy site healed.[14] 

The melanoacanthoma is a benign epithelial lesion. To date, there has not been any observation of malignant transformation. Similarly, in contrast to seborrheic keratoses, which can occur adjacent to or above a skin cancer, there has been no report of a cutaneous collision tumor consisting of a melanoacanthoma and a malignant skin neoplasm. 

Although most melanoacanthomas grow slowly, they can become giant, measuring over 10 cm in greatest diameter. Also, albeit less common, some melanoacanthoma has been noted to grow rapidly. In either of these settings, the lesion can become irritated and tender from rubbing against the adjacent skin or the overlying clothing.

Rarely, a larger melanoacanthoma may spontaneously ulcerate. In one woman, the melanoacanthoma on her back ulcerated and thick yellow-colored pus drained from the lesion. She had difficulty lying down, and the situation did not resolve after multiple courses of antibiotics. Subsequently, excision and skin grafting was planned; however, she declined the surgery.[9] 

A melanoacanthoma is a benign skin lesion. However, its clinical morphology mimics not only other benign epithelial tumors such as a seborrheic keratosis but also malignant neoplasms such as melanoma. Therefore, it is essential that older individuals, particularly those 60 years of age and older -who either discover a new black skin lesion or witness a pigmented cutaneous lesion that may have been present for several years but have continued to enlarge to seek additional evaluation.

Non-invasive methods of examination--including clinical inspection, dermoscopy, and reflectance confocal microscopy—can be performed. However, it is difficult to establish the diagnosis of melanoacanthoma using these modalities since features of melanoma can be observed with each of these methods of investigation. Therefore, when the possibility of a melanoacanthoma is considered, an appropriate biopsy of the lesion should be performed to confirm the diagnosis and to exclude other conditions.

Melanoacanthoma is a rare benign lesion of the epidermis; however, its actual incidence remains to be established. Melanoacanthoma is seldom the diagnosis considered by the clinician who evaluates the patient; most often, when a biopsy specimen is submitted, either melanoma or a seborrheic keratosis or both was the diagnosis suggested. Complete removal of the melanoacanthoma is recommended since the lesion may persist following partial removal.

The clinical morphology of a melanoacanthoma can mimic that of another benign skin tumor, a seborrheic keratosis. However, for many of the patients in whom this epithelial neoplasm has been described, the initial impression was a melanoma. Therefore, the patient’s primary health care professional, either physician (such as a family practice doctor or internist) or nurse practitioner, should consider performing a complete skin check of the patient, especially those over 60 years of age.

The detection of a new or progressively enlarging asymptomatic black lesion should prompt the primary health care professional to refer the patient to a dermatologist or surgeon for evaluation, including biopsy, of the lesion.

Appropriate postoperative care, after the biopsy has been done, should be initiated to promote healing of the wound without infection. This management may involve either the personnel in the office where the biopsy was performed or visiting nurses or both.

Periodic follow up by the clinician to confirm healing and ensure that there is no persistence or recurrence of the melanoacanthoma. [Level 5]