Superficial spreading melanoma and blue naevus within naevus spilus--ultrastructural assessment of giant pigment granules.

Background: Abnormal melanosomes occur in naevus spilus (NS).

Objective: To determine whether melanoma arising in NS contains abnormal melanosomes.

Methods: Light and electron microscopy of melanoma within NS and congenital naevus (Case 1), and of blue naevus within NS (Case 2). Light microscopy of additional naevi and melanomas.

Results: The melanoma and NS (Case 1) both had large numbers of giant pigment granules (GPGs), demonstrated ultrastructurally to be melanosome macrocomplexes. These were not observed in a congenital naevus (Case 1), or in the blue naevus within NS (Case 2). Small numbers of GPGs were observed in 0/2 NS, 8/16 benign naevi and 11/16 malignant melanomas (MMs).

Conclusions: NS may be associated with other melanocytic tumours including MM and blue naevi; abnormal melanosomes in NS may also be present in the associated tumour. The prognostic importance of these is unknown, but occasional GPGs in melanocytic lesions are a frequent finding.