The distinctive pattern used to identify ALM may be absent in 20% of cases, according to new research.
A fifth of acral lentiginous melanomas lack the distinctive parallel ridge pattern associated with them, a US research team has found.
The systematic review and meta-analysis, involving 8845 acral nevi and 801 acral lentiginous melanomas (ALMs) across 41 studies over 16 countries, highlighted the limitation of relying on this feature for diagnosis.
ALM is the fourth most common subtype of malignant melanoma but distinguishing it from benign acral nevi can be challenging, the authors said.
The parallel ridge pattern is known to be the most sensitive and specific feature for ALM, appearing in less than 1% of nevi in the analysis. However, as it only appeared in 79.6% of melanomas, researchers stressed that its absence should not be used to exclude cancer.
They found that multicomponent structures were significantly associated with ALM (45.8% vs 5% in nevi), as acral nevi usually showed a single, organised pattern.
Other dermoscopic features were more common in nevi than ALM, such as the parallel furrow pattern (51.8% in nevi vs 8.9% in ALM) and latticelike pattern (13% vs 2.7%, respectively).
Fibrillar patterns appeared slightly more often in melanoma than in benign lesions, but the difference wasn’t statistically significant, and homogeneous, globular, reticular and nontypical patterns showed non-significant associations or associations that were inconsistent across the studies (heterogeneity was above 75%).
“While the parallel ridge and parallel furrow patterns remain key diagnostic indicators of ALM and benign acral lesions, this systematic review and meta-analysis establishes the additional diagnostic significance of multicomponent and latticelike features,” the authors wrote.
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The prevalence of melanoma increased by approximately 258% between 1990 and 2018, the researchers noted, and ALM accounts for 2-3% of all melanomas. Prognosis is poor in advanced-stage disease, making early detection a priority.
Previous studies have identified the parallel ridge pattern as highly specific for melanoma, while parallel furrow, latticelike and fibrillar patterns are generally associated with benign acral nevi.
However, this evidence has been largely derived from single-centre studies with limited sample sizes.
To address this gap, the analysis pooled data from diverse populations and applied validated quality assessment tools to minimise bias.
The findings supported and refined existing diagnostic frameworks, including the expanded three-step algorithm, the researchers concluded. They also highlighted multicomponent structures as an additional indicator of malignancy, which demonstrated high specificity and moderate sensitivity.
Of the lesions diagnoses as benign, only 25% were proven by biopsy, leaving a large portion that were diagnosed clinically.
Around 8% of benign lesions were congenital melanocytic nevi, which showed similar dermoscopic features but sometimes demonstrated unique markers, such as the crista-dot pattern.
“Lesions demonstrating parallel ridge and multicomponent features should be prioritised for biopsy specimens, whereas lesions displaying parallel furrow and latticelike features may be more appropriate candidates for clinical monitoring unless additional concerning features are present,” the authors wrote.
