Amelanotic / hypomelanotic melanoma
Amelanotic/hypomelanotic melanoma (AHM) represents 2-8% of all melanomas, but its real incidence is difficult to estimate because at the beginning it is often misdiagnosed and/or confused with inflammatory diseases, benign tumors and other malignant tumors (i.e. actinic keratosis, Bowen’s disease, basal cell carcinoma, keratoacanthoma, Merkel cell carcinoma, pyogenic granuloma, haemangioma, wart, etc). Definitely, if singly evaluated, the real prevalence of amelanotic melanoma is very low, since many of these tumors present a residual pigmentation that can be seen mostly at the periphery and should be considered hypomelanotic rather than amelanotic. The low or absent melanin production and the presence of regression are the two reasons why melanoma could be amelanotic or hypomelanotic; both these phenomena may even occur within the same lesion.
AHM has been classified in several variants:
- True amelanotic melanoma (melanoma not producing any trace of pigment)
- Hypomelanotic melanoma (with low production of melanin that may extend to the entire lesion)
- Partially pigmented melanoma(with a pigmentation occupying less than 25% of the lesion)
- Regressive melanoma (in advanced stages of regression)
AHM often occurs in sun-exposed skin of older people and appears as a flat lesion, but more frequently as an elevated tumor, firm on palpation, and typified by rapid growth. This three characteristics (EFG: Elevation, Firmness, Growth) represent a precious clinical guide in melanomas without the classical ABCD clinical criteria.
By dermoscopy, the diagnostic clues for AHM are the vascular structures, often also difficult to detect because require a minimal pressure of the dermatoscope on the lesion to avoid the compression of the blood vessels. As general rule, six main categories of vascular morphologies can be seen in tumoral lesions: comma vessels (in intradermal nevi); dotted vessels (in melanocytic lesions, especially Spitz nevi and melanoma); linear–irregular vessels (in melanoma and other skin malignancies); arborizing vessels (in basal cell carcinoma); hairpin vessels (in keratinizing tumors, especially if surrounded by a whitish halo, and melanoma); and glomerular vessels (in Bowen’s disease). Moreover, also the following three specific global morphologies can be identified: crown vessels surrounding a white center, strawberry pattern and milky-red areas/globules . (Fig.1)
The most common vascular structures in AHM are dotted vessels, linear-irregular vessels, hairpin-irregular vessels, serpentine vessels or a combination of them (polymorphic vessels); also milky-red areas can be frequently visualized.Morphology of vessels modifies during tumor growth. In fact, in early stages, the vessels often appear short and homogenous in shape, while in the advanced tumors, they appear longer and irregular. In thin melanoma (less than 0.5 mm Breslow thickness) the vessels are usually point-like with a rather regular arrangement. Tumors of 0.5–2mm thickness show both point-like and hairpin vessels, again with a regular arrangement. When thickness is over 2 mm the hairpin loops are more twisted, splintered and irregularly distributed, while melanomas of more than 3 mm of thickness develop polymorphic vessels. A multicenter retrospective study from Menzies et al. evaluated several amelanotic and hypomelanotic melanomas to determine the diagnostic accuracy of various dermoscopic features. Positive predictors included blue-white veil, scar-like depigmentation, multiple blue-gray dots, irregularly shaped depigmentation, irregular brown dots or globules, 5 to 6 colors within the same lesion and peripheral light brown structureless areas covering more than 10% of the lesion. Among vascular features, the positive predictors were predominant central vessels, milky red-areas, more than one shade of pink, and a combination of dotted and linear irregular vessels. The most significant negative predictors of melanoma were multiple (>3) milia-like cysts, the predominance of comma vessels with a regular arrangement, a symmetrical pigmentation, and regular and multiple blue-gray globules (Fig.2, 3,4,5,6,7).
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