Sinonasal mucosal melanoma
Decreased visual acuity in right eye; 3rd nerve palsy.
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Occupying/replacing the sphenoid sinus and nasopharynx is a 42 x 44 x 49mm midline mass lesion which remodels the dorsum sella, medial orbital walls and anterior sella, whose anterior extent abuts and remodels the posterior nasal septum.
The cavernous sinuses appear anteriorly effaced rather than invaded; carotid flow voids are maintained. Pituitary gland appears separate although compressed posterior to the mass. Bony remodelling narrows the orbital apices, particularly on the right, and likely compresses the right optic nerve at the level of the optic canal.
The mass consists of a rind of mildly enhancing T1 and T2 isointense material about a non-enhancing heterogeneous internal region with low T2 rim, mixed internal T2 signal, and prominent internal T1 hyperintensity.
The smooth dural enhancement along the anterior cranial floor is probably reactive; there is no convincing evidence of an intracranial component.
Remainder of the intracranial structure are unremarkable aside from supratentorial T2 high signal foci, within acceptable limits for age.
Large mass is favoured to have arisen within the sphenoid sinus, which has slowly expanded with gross bony remodelling, obliteration of the nasopharynx, and mass effect upon the orbital apices particularly on the right.
The unusual morphology, with a central non-enhancing heterogeneous component surrounded by a rind of mildly enhancing soft tissue suggests either chronic sinus infection or intrasinus obstructing malignancy with mucocoele formation. The former is favoured due to the presence of remodelling rather than destruction suggesting slow growth.
Pituitary macroadenoma, meningioma and esthesioneuroblastoma are felt unlikely due to location and morphology.
Biopsy was undertaken.
MICROSCOPIC DESCRIPTION: The sections show fragments of cellular tumour infiltrating bone, cartilage and sinonasal mucosa. The tumour consists of sheets of moderately discohesive cells with a plasmacytoid appearance, large markedly pleomorphic nuclei with coarse chromatin and prominent nucleoli. There is frequent binucleation and a small amount of cytoplasmic brown pigment. Mitoses including atypical forms are frequent. By immunohistochemistry the tumour cells are Melan-A positive, HMB45 positive (focal), S100 negative, cytokeratin AE-13 negative, CAM 5.2 negative, Cd45 negative.
DIAGNOSIS: 1. Nasal mass, biopsy: Malignant melanoma.
Sinonasal mucosal melanoma is a distinct subtype of malignant melanoma. They are rare lesions accounting for <5% of sinonasal tumours and <1% of malignant melanomas.
They arise in the nasal cavity more commonly than in the paranasal sinuses (with maxillary sinus being the most common).