Nodular melanomas (NMs) are defined as invasive melanomas without a radial growth phase. Specifically, they lack an in situ component beyond three rete ridges of the invasive vertical growth phase. While they represent only 14% of invasive melanomas, whereas they represent the majority of thick melanomas (>3 mm Breslow thickness).
Clinically, NMs may lack the “ABCD” features more typical of superficial spreading melanomas (SSMs) since they appear as a symmetric lesion.
The classic melanoma specific criteria are predominantly seen in SSMs but much less frequently in NMs.
Especially the features correlating with the radial growth of melanoma and pagetoid spread (pseudopods, radial streaming) as well as those criteria found in thin melanoma (such as atypical broadened network and the early regression feature of multiple blue-gray fine dots or peppering) are often lacking in NMs.
However, pigmented NMs more frequently have those dermoscopy features associated with thick melanoma :
- blue-white veil
- multiple (5–6) colors
- crystalline structures (seen only with polarized light)
- atypical vascular patterns
In a more recent publication using multivariate analysis the following criteria were found to be highly significant for nodular melanoma
- asymmetric pigmentation
- blue–black pigmented areas
- homogeneous disorganized pattern
- combination of polymorphous vessels and milky-red globules/areas
- combination of polymorphous vessels and red homogeneous areas
Melanomas tend to have an asymmetric pigmentation pattern under dermoscopy. While most NM are asymmetric, 6% have a symmetric pattern  . Nevertheless, the majority of pigmented NMs can be diagnosed using standard dermoscopy criteria.
What is diagnostically problematic is the fact that a significant number (more than a third) of NMs are hypomelanotic or amelanotic. In these cases, atypical vascular patterns may be the only clues for the diagnosis of such lesions . The most significant of these patterns are the combination of linear irregular and dotted vessels or linear irregular vessels as the predominant vessel type, greater than one shade of pink, milky red-pink areas and or globules, and hairpin vessels.