Melanotic neuroectodermal tumor of infancy

Melanotic neuroectodermal tumor of infancy (MNTI) is a rare, benign but locally aggressive neoplasm of neural crest origin.[1][2] Despite its generally benign classification, the tumor exhibits rapid growth, significant local bone destruction, and a notable recurrence rate.[3][4] It arises predominantly in the craniofacial skeleton of infants, with a strong predilection for the anterior maxillary alveolar ridge.[2][5] The pathogenesis is attributed to aberrant proliferation of neural crest–derived progenitor cells capable of both melanocytic and neuroblastic differentiation.[3][6] On dental radiographs, MNTI characteristically presents as a well-defined, expansile osteolytic lesion displacing developing tooth germs.[7] Complete surgical excision is the standard of care.[2][4]

The currently accepted designation is melanotic neuroectodermal tumor of infancy (MNTI), a name established by Borello and Gorlin in 1966.[5] Historical uncertainty regarding the tumor's histogenesis generated numerous earlier names, including:

• Congenital melanocarcinoma (Krompecher, 1918 — the original description)[5]
• Retinal anlage tumor
• Melanotic progonoma
• Pigmented congenital epulis
• Melanotic adamantinoma

These terms are obsolete but appear in older literature and may cause confusion. The designation "melanocarcinoma" is particularly misleading, as the tumor is benign in the vast majority of cases.[2][5] MNTI is classified among neoplasms of neural crest origin; it is not an odontogenic tumor despite its intimate relationship with developing teeth.[3]

• Age: The vast majority of cases present during the first year of life, with peak incidence between 2 and 6 months. Mean age at diagnosis is approximately 5–6.5 months.[2][5][8]
• Sex: Males are slightly more affected than females, with a reported male-to-female ratio of approximately 1.3:1.[5]
• Frequency: MNTI is exceedingly rare; systematic reviews have identified fewer than 500 cases reported since 1918.[5][9]
• Anatomic distribution in the jaws: The maxilla accounts for the overwhelming majority of gnathic cases, with the anterior alveolar ridge being the characteristic site. The mandible is involved in a small minority of jaw cases. Extracranial sites (epididymis, femur, soft tissues) are rare.[2][3][5][10][11]

MNTI typically presents as a rapidly enlarging, painless mass of the anterior maxillary alveolar ridge in an infant under 1 year of age.[1][2][5]

Presentation and examination findings

• The mass is firm, nonmobile, and nonulcerative.[12]
• A characteristic blue-black or dark pigmentation of the overlying mucosa is common, reflecting melanin production by the tumor cells. Occasional cases are nonpigmented, which can delay clinical recognition.[2][13]
• Facial asymmetry and visible swelling develop rapidly over weeks.[1][13]
• The lesion may clinically mimic an eruption cyst, particularly in the early stages.[1]

Tempo and complications

• Growth is rapid, often progressing noticeably over days to weeks.[1][3]
• Displacement or loosening of developing primary teeth is common.[1][7]
• Large tumors can cause nasal obstruction, feeding difficulty, or orbital displacement when the maxilla is extensively involved.[14][15]
• Pathologic fracture of the mandible has been reported in mandibular cases.[2]

Red flags

• Rapid enlargement in an infant warrants urgent evaluation to exclude malignant small round blue cell tumors.[3]
• Elevated urinary vanillylmandelic acid (VMA) levels occur rarely and, when present, raise concern for neuroblastoma or malignant transformation.[2][6]

Location and distribution

MNTI of the jaws arises characteristically in the anterior alveolar process of the maxilla, centered on the region of the developing primary incisors.[2][5][7] Mandibular involvement is uncommon and typically also affects the anterior alveolar ridge.[5][16] The lesion is intraosseous in origin, though an extraosseous soft-tissue component is common in larger tumors.[2][13]

Periphery and shape

The periphery is well defined in the great majority of cases.[7] A systematic review of CT features of 53 gnathic MNTIs found a well-defined border in nearly all cases.[7] The outline is typically smooth or gently lobulated. Despite the well-defined margin, the tumor is locally infiltrative histologically, and the radiographic border may underestimate microscopic extension.[3][7]

Internal architecture

• The internal pattern is predominantly radiolucent (osteolytic).[1][2][7]
• A bilocular radiolucent pattern is common and has been identified as a characteristic feature on CT, present in the majority of cases in a systematic review. This bilocular appearance is created by a radiopaque septum or displaced tooth germ dividing the lesion into two compartments.[7]
• Unilocular and, less commonly, multilocular patterns also occur.[7]
• Internal calcification or osteogenesis within the lesion is occasionally seen on CT.[13]

Effects on teeth and adjacent structures

• Tooth displacement is a hallmark feature, present in the vast majority of cases. Developing tooth germs — most often the primary incisors — are displaced away from their normal position, sometimes entirely enveloped by the tumor.[1][2][7]
• Tooth germs may be found floating within the lesion or pushed to its periphery.
• Root resorption of adjacent teeth is not a prominent feature, distinguishing MNTI from several other aggressive pediatric jaw lesions.
• The lesion may extend into the nasal cavity, maxillary sinus, or orbit in advanced cases.[14][15]

Bone expansion and cortical involvement

• Bone expansion is a universal feature, present in all cases in the systematic CT review.[7]
• Cortical thinning is characteristic; cortical perforation with soft-tissue extension occurs in larger or more aggressive tumors.[2][13]
• Bone erosion, remodeling, and occasionally reactive hyperostosis or periosteal new bone formation can coexist within the same lesion.[13]

Growth pattern

The tumor grows rapidly, and serial imaging over short intervals may demonstrate substantial interval enlargement.[1][3] The expansile, destructive growth pattern with well-defined margins is characteristic of a benign but locally aggressive process.

Recognized variants

• Mandibular MNTI is uncommon and may present with more aggressive local destruction, including pathologic fracture.[2][16]
• Calvarial MNTI produces a lytic defect in the skull vault, often with an extradural intracranial component visible on cross-sectional imaging.[12][13][17]

CT findings

Medical CT is the primary cross-sectional modality for evaluating bony extent. CT demonstrates the expansile osteolytic lesion, cortical thinning or destruction, bone remodeling, and the relationship of the mass to displaced tooth germs.[7][13] Reactive hyperostosis and osteogenesis within the lesion are occasionally identified and are best appreciated on CT.[13] The soft-tissue component typically shows moderate contrast enhancement.[18]

MRI findings

MRI is superior to CT for delineating the soft-tissue extent of the tumor and its relationship to adjacent structures such as the orbit, nasal cavity, and intracranial compartment.[2][13]

• Signal characteristics: MNTI characteristically demonstrates iso- to hypointense signal on both T1- and T2-weighted sequences relative to brain or muscle. This pattern is notable because it does not show the T1 shortening typically expected from melanin-containing tumors, suggesting that factors beyond absolute melanin content influence the MR signal.[13][18]
• Occasional cases show T1 hyperintensity and T2 hypointensity consistent with paramagnetic melanin effects.[19]
• Enhancement: Marked, often inhomogeneous contrast enhancement of the non-ossified soft-tissue component is characteristic.[12][18]
• MRI is particularly valuable for calvarial and skull-base tumors to assess intracranial extension.[13][17][20]

The differential diagnosis of a well-defined, expansile osteolytic lesion of the anterior maxilla or mandible in an infant is narrow but includes entities with substantially different management implications.

The highest-risk diagnostic pitfall is misidentification of MNTI as a malignant small round blue cell tumor (neuroblastoma, rhabdomyosarcoma, or Ewing sarcoma) on small biopsy specimens, because the neuroblastic component of MNTI can dominate and obscure the diagnostic biphasic pattern.[3][8] Adequate tissue sampling and a targeted immunohistochemical panel (HMB-45, cytokeratin, synaptophysin, chromogranin) are essential to avoid unnecessary aggressive chemotherapy.

Diagnostic microscopic features

MNTI has a distinctive biphasic cellular composition:[2][3][8]

• Large epithelioid cells: Polygonal cells with abundant cytoplasm containing melanin pigment, arranged in tubular, alveolar, or tubuloglandular formations.[5][8][10]
• Small neuroblast-like cells: Primitive round cells with scant cytoplasm and hyperchromatic nuclei, forming nests surrounded by the larger pigmented cells.[3][5][8]
• Stroma: Dense fibrocollagenous or desmoplastic background.[3][5]
• Mitoses and necrosis are generally absent in typical cases but may be seen. Rosette formation is uncharacteristic and, when present, should raise concern for neuroblastoma.[5][24]

Immunohistochemical profile

• Large (epithelioid) cells: Consistently positive for cytokeratin, HMB-45, and vimentin; epithelial membrane antigen (EMA) is variably expressed. S-100 protein expression is variable across studies.[2][4][5][8]
• Small (neuroblastic) cells: Positive for synaptophysin and neuron-specific enolase (NSE); negative for chromogranin A — a key discriminator from neuroblastoma.[2][5][8]
• CD56 positivity has been reported in both cell populations.[4]

Radiologic-pathologic correlation

The bilocular radiolucent pattern seen on imaging corresponds to the biphasic tumor architecture, with the radiopaque septum often representing a displaced tooth germ or dense fibrous stroma separating tumor compartments.[7] The well-defined radiographic border correlates with a pushing margin grossly, though microscopic infiltration beyond the apparent capsule is common and accounts for the high recurrence rate after conservative excision.[3][7]

Molecular findings

BRAF V600E mutations have been identified in a subset of MNTIs, with reported positivity rates varying across small series.[4][10][14] Co-expression of BRAF V600E and β-catenin has been observed in some cases, suggesting a potential role for MAPK and Wnt pathway alterations in tumor biology.[4] These findings remain investigational but may eventually inform targeted therapy for aggressive or recurrent cases.[4][14]

Workup and referral

• Urgent referral to oral and maxillofacial surgery or pediatric surgery is indicated for any rapidly growing jaw mass in an infant.[1][2]
• Preoperative imaging with CT (for bony detail) and MRI (for soft-tissue extent) is recommended to guide surgical planning.[7][13]
• Urinary VMA measurement may be obtained preoperatively; elevation is rare but, when present, warrants evaluation for neuroblastoma or malignant MNTI.[1][2][6]
• Incisional biopsy with immunohistochemistry is essential for definitive diagnosis before surgery.[3][16]

Definitive treatment

• Complete surgical excision is the standard first-line treatment. The goal is removal with clear margins to minimize recurrence.[1][2][4][5]
• Treatment approaches range from conservative complete excision (enucleation/curettage) to radical resection (partial or total maxillectomy), depending on tumor size and extent.[5][10][15]
• Immediate reconstruction, including cryopreserved autologous bone grafting, has been described for extensive maxillary defects.[15]

Recurrence and surveillance

• Recurrence rates are commonly reported at 20%–25%, though ranges of 10%–60% appear in the literature, reflecting variability in surgical margins and follow-up methods.[4][14][24]
• Recurrences tend to occur within the first several months after surgery and may exhibit more aggressive behavior than the primary tumor.[4][8][14]
• Younger age at diagnosis has been identified as a prognostic factor for higher recurrence risk.[9]
• Long-term clinical and radiographic follow-up is essential; late recurrence has been reported up to 12 years after initial resection.[12]

Adjuvant therapy

• Chemotherapy (typically neuroblastoma-based protocols) and radiotherapy have been used as neoadjuvant or adjuvant therapy in cases of unresectable, recurrent, or metastatic disease. Neoadjuvant chemotherapy can facilitate delayed complete resection in initially inoperable tumors.[25]
• The role of adjuvant chemotherapy in preventing recurrence after incomplete resection remains unclear.[25]

Prognosis

• The overall prognosis is favorable, with the majority of patients cured by complete surgical excision.[1][2][5]
• Malignant behavior (metastasis to lymph nodes or distant organs) occurs in a small minority of cases. Mandibular location and age older than 12 months at diagnosis have been associated with worse survival outcomes.[2][5][6][24]

Pediatric dental implications

Loss of primary tooth germs during surgical resection is common and expected.[24] Permanent tooth replacement with dental implants is typically deferred until skeletal maturity to avoid ankylosis and restriction of craniofacial growth.[24] Prosthetic rehabilitation planning should be coordinated with pediatric dentistry from an early stage.

Potential for targeted therapy

The identification of BRAF V600E mutations in a subset of MNTIs has raised the possibility of targeted therapy with BRAF inhibitors for aggressive or recurrent cases that are not amenable to further surgery.[4][14] This approach remains investigational, and no clinical trials specific to MNTI have been conducted.