A 4‐YEAR‐OLD GIRL WITH MASS IN THE FRONTOTEMPORAL BONE

CLINICAL HISTORY

A 4‐month‐old female infant presented with a growing extracranial mass at the left frontotemporal region. Covered with intact skin, the lesion measured 3.5 × 3 × 3 cm; it was well‐defined, bony hard in consistency, and fixed to the underlying skull. There were no other abnormal symptoms, and routine laboratory tests were within normal limits. The mass appeared sclerotic on skull film. Computerized tomography scan of the brain (Figure 1A) showed a homogeneously enhancing expansile bone tumor. The lesion caused widening of the lower part of the left coronal suture, just above the orbital roof. The patient underwent left frontotemporal craniotomy, and the mass was found to involve the frontal and temporal bones, around the inferior limb of the coronal suture, adherent to the underlying dura. The orbital roof and apex were also affected. The lesion was totally resected. The immediate postoperative course was uneventful.

Figure 1.

On gross inspection of the fresh specimen, the mass was hard and showed gray tan cut surfaces (Figure 1B).

MICROSCOPIC FINDINGS

Intraoperative smears and imprints disclosed bimodal population of large melanin‐containing epithelial cells and smaller neuroblast‐like cells (Figure 2). The former possessed abundant pale eosinophilic cytoplasm, whereas the latter appeared as round naked nuclei with finely dispersed chromatin pattern. Thick clusters of fibrovascular tissue were also evident. An intraoperative diagnosis was rendered. Permanent sections subsequently confirmed the diagnosis by demonstrating lobules of monotonous small round cells (Figure 3A), and pigmented epithelial cells with glandular formation (Figure 3B). Mitotic figures were rarely observed, 0–1/10 high‐power field (HPF). Immunohistochemically, the small round tumor cells expressed synaptophysin (Figure 4A), while the pigmented epithelial cells were reactive for cytokeratin (Figure 4B) and HMB‐45 (Figure 4C).

Figure 2.

Figure 3.

Figure 4.

DIAGNOSIS AND DISCUSSION

Diagnosis.  Melanotic neuroectodermal tumor of infancy (MNTI).

Discussion.  MNTI is a rare, locally invasive neoplasm; typically arising in the cranial bones, especially the maxilla (2). Several synonyms have been applied for this distinctive lesion, including melanotic progonoma, congenital melanoma, retinal anlage tumor, congenital melanocarcinoma, and pigmented epulis of infancy (8). Infants of less than 1 year of age are commonly affected, with no gender preference 4, 8. Since the first description in 1918, approximately 200 MNTIs have been reported (4).

On gross examination, MNTI is a non‐ulcerative lesion, with gray‐brown cut surfaces. Expansion of the surrounding bone and invasion of the adjacent tissue are characteristic. Cytologic diagnosis is feasible, as demonstrated in our case. The dual populations of small neuroblast‐like cells and large melanin‐containing cells are the diagnostic hallmarks 4, 7. The presence of thick clusters of hyperplastic blood vessels also points toward the neuronal derivation, but, in order to identify the diagnostic cells, care should be taken to look for them in the background of smear. Analog to the cytologic appearances, two cellular components are demonstrable on the permanent section 2, 8. One consists of small round neoplastic cells, forming lobules intervened by fibrovascular septa. These round cells are embedded within a fine fibrillary matrix. The other is the melanin‐containing large epithelial cell, with glandular or tubular arrangement. As in our case, mitotic activity is generally low, less than 2/10 high‐power field (1). The neuronal nature of small cell population is supported immunohistochemically and ultrastructurally by synaptophysin reactivity and the presence of neurosecretory granules, respectively 3, 6. The pigmented epithelial cells are stained positively with HMB‐45 and contain cytoplasmic melanosomes, features supporting their melanocytic derivative 3, 6.

Because of its rarity, MNTI may be missed for other more common lesions. Differential diagnostic traps include infantile small round cell tumors, particularly embryonal rhabdomyosarcoma, neuroblastoma/primitive neuroectodermal tumor (PNET), and malignant lymphoma (4). The possibility of melanocytic lesions may also entertain given the presence of pigmented cells in MNTI (4). Nevertheless, as the histologic features of MNTI are highly characteristic, a correct diagnosis can be made even with the cytologic preparation provided the pathologist is aware of the entity.

Most MNTI is locally invasive, and usually controlled by surgical extirpation 2, 8. The aggressive melanotic medulloblastoma or other pigmented embryonal neuroepithelial tumors should not be regarded as MNTI (2). In support to this, a recent genetic study has failed to link MNTI with malignant small cell tumors such as neuroblastoma, peripheral PNET/Ewing’s sarcoma, and desmoplastic small round cell tumor (5).