PATIENT HISTORY AND NEUROIMAGING:
A 5-month old boy presented with a hard, non-tender swelling behind the left ear, first noticed when he was 2 months old. He was otherwise well with a normal neurological examination. Over the following 3 months the lesion increased in size to 5cm diameter and the overlaying skin became fixed. The bony lesion was excised in two stages with an ellipse of overlaying skin. The underlying dura was not involved and the defect was repaired with a pedicled, pericranial graft and a rotational scalp flap.
T1-weighted magnetic resonance (Fig. 1) imaging showed a 3x4x7cm contrast-enhancing lesion in the left occipital bone, with marked intracranial extension but not penetrating the dura. The overlaying subcutaneous tissue was involved. Fig. 1 is an axial T1 weighted, post-contrast MRI showing intracranial extension of occipital bone mass indenting underlying dura (arrows)
GROSS AND MICROSCOPIC DESCRIPTION
Resected specimen showed a dark pigmented surface (Fig 2) and consisted of an ellipse of overlaying skin and subaponeurotic tissue together with multiple bony fragments. The subaponeurotic fascia was replaced by dense fibrocollagenous connective tissue which showed extensive infiltration and replacement by clusters of cells forming two distinct populations. Large cuboidal to columnar pigmented cells were arranged into tubular structures. These cells did not show any mitotic activity (Fig 3 open arrow, x 125). A Masson Fontana stain for melanin was positive. The tubules were surrounded by fibrocollagenous tissue. Additionally, many nests composed of cells having deeply basophilic small nuclei were randomly located throughout the connective tissue. These cells had inconspicuous cytoplasm, were not pigmented and did not show mitotic activity (Fig 3 black arrow, x 125). Immunohistochemistry revealed that the large cells, but not the small cells, were positive for CAM 5.2 (Fig 4, x 125) and HMB 45 (Fig 5 , x 125). Both cell types were positive for neuron specific enolase. Ultrastructural examination of the tumor demonstrated atypical premelanosomes (Fig 6 x 25,000; bar=200 nm) in the large cells. A single deposit of tumor tissue was present in the superficial dermis but the resection margins were free of tumor.