Once in a blue moon: A rare, metastatic malignant blue nevus

Background: Malignant blue nevus is an uncommon yet highly aggressive melanoma variant that frequently emerges from cellular blue nevi, though it may also arise de novo.The present case highlights the diagnostic challenges associated with malignant blue nevus, due to its rarity and the lack of distinctive clinical and dermoscopic features.Methods: We report the case of a 68-year-old male who presented with a pulmonary mass, which was confirmed via biopsy as a metastasis from melanoma.In an effort to identify the primary tumor, total body photography and digital dermoscopy were performed.The only notable finding was a blue, nodular lesion on the dorsum of his left foot, which the patient reported having for many years.The lesion exhibited uniform blue pigmentation with focal scar-like structures but lacked malignant features, such as color heterogeneity or ulceration.A biopsy was performed to determine its nature.Results: Histopathological analysis revealed a malignant blue nevus, characterized by spindle cells arranged in dense fascicular formations, marked cytologic atypia, increased mitotic activity, and the presence of atypical mitoses.Immunohistochemical evaluation confirmed the diagnosis, showing positive staining for SOX-10, HMB-45, and PRAME.Conclusions: The absence of definitive clinical or dermoscopic criteria presents a significant challenge in diagnosing malignant blue nevus.This case emphasizes the importance of dermatologists maintaining vigilance for this rare entity, especially when assessing long-standing or progressively evolving lesions.It also highlights the need for further research to identify specific dermoscopic features that could enable earlier detection and improve patient outcomes.