Dermoscopic Features of Acral Lentiginous Melanoma in Situ
Acral lentiginous melanoma (ALM) is a subtype of melanoma that predominantly occurs on the palms, soles, and nail beds. It is the most common type of cutaneous melanoma in Asians, often presenting diagnostic challenges due to its unique anatomical location and subtle early features. Early detection is crucial for improving patient outcomes, as delayed diagnosis can lead to advanced disease and increased mortality. Dermoscopy, a non-invasive imaging technique, has significantly enhanced the diagnostic accuracy of ALM, particularly in its early stages, including melanoma in situ. This article explores the dermoscopic features of ALM in situ, as illustrated in a case study, and discusses their clinical significance.
Case Presentation
A 57-year-old woman presented with a pigmented patch on the left foot that had appeared five months earlier and progressively enlarged. The lesion was asymptomatic, with no associated pain or discomfort. Physical examination revealed a light brown patch measuring 10 mm × 8 mm, characterized by an irregular shape and multiple colors. The lesion’s clinical appearance raised suspicion for melanoma, prompting further evaluation with dermoscopy.
Dermoscopic Findings
Dermoscopic examination of the lesion revealed several key features indicative of ALM in situ. The most prominent finding was the parallel ridge pattern, which is considered the hallmark of early ALM. This pattern is characterized by pigmentation that follows the ridges of the skin, reflecting the proliferation of atypical melanocytes in the basal layer of the epidermis. The parallel ridge pattern has a specificity of 99% and a positive predictive value of 84% for ALM, making it a highly reliable diagnostic marker.
In addition to the parallel ridge pattern, the lesion exhibited irregular diffuse pigmentation, consisting of blotches of varying shades of brown. This pattern is the second most common dermoscopic feature of ALM, with a specificity of 96.6%. Other notable findings included peripheral dots and globules, which represent clusters of melanocytes, and regression structures, indicative of areas where the tumor has partially regressed. The lesion also displayed a milk-red area and superficial erosion, features more commonly associated with invasive melanoma but occasionally observed in early-stage ALM.
The dermoscopic image further highlighted the presence of eccrine pores, recognized as whitish dots regularly distributed within the band-like pigmentation. These structures are a normal anatomical feature of acral skin but can aid in the identification of pathological changes when their distribution or appearance is altered.
Histopathological and Immunohistochemical Confirmation
The lesion was surgically excised, and histopathological analysis confirmed the diagnosis of ALM in situ. Microscopic examination revealed confluent nests of atypical melanocytes with large hyperchromatic nuclei scattered along the basal layer of the epidermis. Importantly, there was no evidence of dermal invasion, consistent with the in situ nature of the lesion.
Immunohistochemical staining provided further support for the diagnosis. The atypical melanocytes were positive for HMB-45, a marker commonly expressed in melanoma cells, but negative for S-100, another melanocytic marker. This staining pattern is consistent with ALM and helps differentiate it from other pigmented lesions.
Clinical Significance of Dermoscopy in ALM
The early detection of melanoma remains a significant challenge for dermatologists and patients alike. Misdiagnosis or delayed diagnosis can lead to advanced disease, increased treatment complexity, and higher mortality rates. Dermoscopy has emerged as a valuable tool for improving diagnostic accuracy, particularly for melanomas in anatomically challenging locations such as the acral regions.
The majority of dermoscopic criteria for melanoma were established based on invasive lesions, making the identification of melanoma in situ particularly challenging. However, the parallel ridge pattern and irregular diffuse pigmentation pattern are highly specific for early ALM, enabling clinicians to detect these lesions at a curable stage.
In the presented case, the dermoscopic findings not only included the classic features of ALM in situ but also exhibited characteristics typically associated with invasive melanoma, such as the milk-red area and superficial erosion. This underscores the importance of considering a broad range of dermoscopic features when evaluating acral lesions, as the presence of atypical findings may indicate a more advanced or aggressive tumor.
Broader Implications for Dermatological Practice
The case highlights the critical role of dermoscopy in the early diagnosis of ALM, particularly in populations where this subtype is prevalent, such as in Asian countries. The parallel ridge pattern, with its high specificity and positive predictive value, should be a key diagnostic criterion for clinicians evaluating pigmented lesions on acral skin. Additionally, the presence of irregular diffuse pigmentation, peripheral dots and globules, and regression structures can further support the diagnosis of ALM in situ.
However, the observation of features such as the milk-red area and superficial erosion in this case suggests that the dermoscopic spectrum of ALM in situ may be broader than previously recognized. Further studies are needed to refine the dermoscopic criteria for early ALM, particularly in Asian populations, where this subtype is most common.
Conclusion
Acral lentiginous melanoma in situ presents unique diagnostic challenges due to its subtle clinical and dermoscopic features. The parallel ridge pattern and irregular diffuse pigmentation are the most specific dermoscopic findings for early ALM, enabling clinicians to detect these lesions at a curable stage. However, the presence of atypical features, such as the milk-red area and superficial erosion, highlights the need for a comprehensive approach to dermoscopic evaluation. As dermoscopy continues to evolve, it will play an increasingly important role in the early diagnosis and management of ALM, ultimately reducing the mortality and economic burden associated with this aggressive subtype of melanoma.
doi.org/10.1097/CM9.0000000000000386
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