Oral Melanoma

Etiology

The etiology, risk factors, and pathogenesis of oral melanoma remain poorly understood. Unlike its skin counterpart, it is not related to sun exposure as the oral mucosa is protected from ultraviolet light.[8] Primary oral mucosa melanomas mainly originate de novo, but up to 37% are preceded by pigmented lesions, lasting for months to years.[7] Denture irritation, infection, and tobacco smoking have been listed as possible risk factors, but a direct relationship is not substantiated.[2]

Epidemiology

Oral melanoma is a very rare and aggressive carcinoma that accounts for 0.2% to 8% of all melanomas [1] and 1% to 2% of all oral malignancies.[2] Interestingly, unlike its skin counterpart, its incidence remained steady over the last years.[7]

Oral melanoma prevalence increases with age.[2] It develops between the 4th and 7th decade of life, [1] with an average age of 60.[7] The gender difference is not significant; some studies report female predominance, others male prevalence, or do not report any discrepancy.[2] Regarding ethnicity, oral melanoma is uncommon in white individuals [7] and more frequent in Japanese,[2] black, and Indian populations.[2]

Pathophysiology

Melanoma develops from a malignant transformation of melanocytes. Even though the exact pathophysiology of mucosal melanomas is yet to be determined, some pathways have been identified.

Mutations in c-KIT

Recent data indicates that c-KIT (CD117) is overexpressed in many mucosal melanoma cases. This pathway is important and common in acral and mucosal melanoma, melanomas unrelated to sun exposure.[9] KIT is a transmembrane tyrosine kinase receptor expressed on hematopoietic progenitor cells, melanocytes, mast cells, primordial germ cells, and interstitial cells of Cajal. Activating mutations and amplifications cause activation of growth and proliferation pathways. Drugs that work on the tyrosine kinase activities, such as imatinib, show some promise in treating mucosal melanomas expressing the c-kit protein, but treatment failure has also been reported.[9]

Mutations in BRAF

Bioinformatics Resources and Applications Facility (BRAF) protein mutations are uncommon in mucosal melanoma and found in less than 10% of cases. However, in cutaneous melanoma, BRAF mutations are found in up to 80% of cases. Dabrafenib and vemurafenib are used for patients exhibiting BRAF mutation-positive melanoma.[2]

Histopathology

The anatomy of the mouth differs from the skin. Due to the lack of histological landmarks analogous to the papillary, reticular dermis, and muscle bundles, a pathological leveling system, and description cannot be appropriately applied for mucosal melanomas. Therefore, Clark’s levels, commonly used in cutaneous melanoma, cannot be implemented. Many melanomas in the mouth have a histologic similarity with lentigo maligna melanoma in a radial growth phase. 

Mucosal melanomas can show three principal patterns. An in situ melanoma is in the epithelium and does not cross the epithelial-connective tissue interface.[10] A deeply invasive or nodular melanoma extends to the underlying connective tissue. A combined pattern is characterized by an in situ or radially growing pattern combined with a nodular component.[10]

The characteristic histopathological features of mucosal melanomas include atypical melanocytes (hyperchromatism and nuclear pleomorphism) in the epithelium and connective tissue junction. This, combined with positive S-100 and HMB-45 markers, is confirmatory of mucosal melanoma.[11]

Approximately 15% of cases of oral melanoma are in situ mucosal lesions, 30% of cases are invasive lesions, and 55% of cases have a combined pattern of invasive with in situ components. Most advanced lesions have a combination pattern of invasive melanoma with an in situ component.

Immunohistochemistry

Although not specific, S-100 and HMB-45 are helpful markers that are positive in almost all oral melanomas (94%).[2] Fatty acid synthase (FASN) can help to differentiate oral melanomas from oral melanocytic nevi since this marker is strongly expressed in melanomas of the mucosal surfaces.[2]

History and Physical

Oral melanomas show an aggressive nature and worse prognosis than cutaneous melanoma. Almost a third of patients already have signs of lymph node metastasis at the moment of diagnosis,[7] probably because this malignancy goes unnoticed due to a lack of symptoms in the early stages.[12] Pain, bleeding, and ulceration are uncommon until late in the disease.

Primary oral melanomas have a strong preference for the maxilla, mainly developing in the hard palate, gingiva, or alveolar ridge.[7][1] Secondary oral melanomas (even rarer than primary tumors) commonly occur in the tongue.[1] Most oral melanomas arise de novo in an apparently normal oral mucosa, but up to a third are preceded by melanosis.[2] This pigmented area is believed to represent the radial expansion phase before the deep invasion of the underlying tissue.[2]

The clinical aspect of oral melanomas is indeed diverse. They can present as macules, patches, or nodules, showing various colors: brown, black, grey, red, or purple shades and even depigmentation.[7] The lesions are asymmetric, with an irregular outline, and can be sometimes multiple.[2] Satellite foci surrounding the tumor have been reported.[2][7] Up to a third of oral melanomas are ulcerated.[7]

The most typical presentation comprises three particular features: a) a brown-black pigmented plaque that is flat or slightly raised, b) a light brown macular feature, and c) a central area of nodular aspect.[2][13]

Amelanotic melanomas are not uncommon, accounting for around a third of oral melanomas.[2] These lesions pose a diagnostic challenge since they may be misdiagnosed as benign tumors, like epulis, or as squamous cell carcinoma.[2]

Evaluation

Early diagnosis is essential since oral melanomas show a very aggressive evolution with a poor prognosis. However, there are still no criteria to aid the diagnosis in the oral cavity. Clinical examination can be supported by dermoscopy, but the complex oral mucosa anatomy and irregular surface represent a technical limitation.[7] Biopsy remains the gold standard to diagnose oral melanomas. Incisional biopsy from the thickest part of a large lesion and excisional biopsy of small lesions are indicated.[2] Ultrasound or CT scan of the head and neck and thoracoabdominal regions is required to stage the tumor correctly.[7]

Dermoscopy

The most common dermoscopic features include diffuse and irregular pigmentation with pseudo-network, regression structures, and a blue-white veil.[2] Other characteristics are asymmetrical structures, irregular margins, and the abrupt interruption of the reticular pattern.[7] Atypical vessels, dots, and globules can also be seen.[7]

Treatment / Management

Surgery is the mainstay of treatment in oral malignant melanoma.[11] Radical excision with disease-free margins is the first goal in surgical management. Adjuvant therapy includes radiation, chemotherapy, and immunotherapy.[2](B3)

Even though melanoma is classically non-radiosensitive,[2] some authors have seen improvement after radiotherapy, especially better local control and survival.[11](B3)

Chemotherapy with platinum analog, nitrosoureas, dacarbazine, and immunotherapy with IL-2 have shown low response.[2] Metastatic melanomas with a c-kit mutation have benefited from imatinib therapy.[9] Dabrafenib and vemurafenib are used for patients exhibiting a positive BRAF mutation melanoma.[2](B3)

Differential Diagnosis

The differential diagnosis of oral melanoma is extensive; it needs to be differentiated from focal and diffuse oral pigmentations. Amalgam tattoos most commonly mimic oral melanomas, and dermoscopy may aid in differentiating them.[2]

The following are included in the differential diagnosis of an oral melanoma:[7]

Focal Oral Pigmentations

• Amalgam tattoo (exogenous pigment)
• Melanoacanthoma
• Melanotic macules
• Melanocytic nevi

Diffuse Oral Pigmentations

• Physiological/racial pigmentations
• Smoker’s melanosis
• Drug-induced hyperpigmentation
• Postinflammatory hyperpigmentation

Systemic Diseases

• Peutz–Jeghers syndrome
• Laugier–Hunzikerdisease
• Leopard syndrome
• Carney complex syndrome
• McCune-Albright syndrome
• Adrenal gland diseases

Staging

Tumor, T[14]

There is no T1 or T2 in mucosal melanoma.

• T3: Tumors limited to the mucosa and immediately underlying soft tissue, regardless of thickness or greatest dimension; for example, polypoid nasal disease, pigmented or non-pigmented lesions of the oral cavity, pharynx, or larynx
• T4: Moderately advanced or very advanced
• T4a: Moderately advanced disease. Tumor involving deep soft tissue, cartilage, bone, or overlying skin
• T4b: Very advanced disease. Tumor involving the brain, dura, skull base, lower cranial nerves (IX, X, XI, XII), masticator space, carotid artery, prevertebral space, or mediastinal structures

Lymph Nodes, N

• NX: Regional lymph nodes cannot be assessed
• N0: No regional lymph node metastases
• N1: Regional lymph node metastases present

Distant Metastases, M

• M0: No distance metastasis
• M1: Distant metastasis

Prognosis

Patients with oral melanomas are often diagnosed at an advanced stage of the disease and have a very poor prognosis.[15] The five-year survival rate is 25,5% for mucosal melanomas in the head and neck.[2] Around a third of patients present lymph node metastasis at the moment of diagnosis.[7] This is believed to be partly because the region is highly vascularized and also due to its lymphatic drainage anatomy.[7] The expression of bcl-2 is linked to a better prognosis of mucosal melanomas.[16] Aberrant expression of p53 protein and loss of expression of p16 protein are associated with a poor prognosis.[16]

After complete surgical excision, relapse rates have been reported to be about 20%.[11] Recurrence has been noted even up to 11 years following surgery. Metastases in the lymph nodes of the neck, lungs, liver, and brain can be seen.

Complications

The complications are related to the surgical excision of the melanoma, including loss of tissue, prolonged healing, grafting, and prothesis need. Also, xerostomia, candidiasis, and hypernasal speech are side effects of the surgery.