Halo melanoma of the nail unit: A diagnostic and surgical pitfall

Abstract

Nail unit melanoma (NUM) is a rare but aggressive form of melanoma that often masquerades as a benign nail disorder, leading to diagnostic delay. We report a 60-year-old Indian male who presented with a progressive left thumb nail lesion that evolved from longitudinal melanonychia to destructive nail dystrophy with a friable exophytic mass, together with surrounding periungual depigmentation and focal pigmentation. Onychoscopy revealed an irregular pigment network and black-brown globules over a depigmented background. Positron emission tomography-computed tomography demonstrated a fluorodeoxyglucose-avid primary lesion involving the thumb and a metabolically active ipsilateral axillary lymph node. Histopathology of the amputated specimen confirmed malignant melanoma. Microsections from the depigmented periungual skin showed loss of epidermal melanocytes with dermal melanophages and pigment incontinence, along with scattered atypical melanocytes containing fine granular melanin in continuity with the primary tumour, indicating lateral lentiginous tumour spread within the depigmented field. This case underscores the diagnostic challenges of NUM, in which destructive nail changes may obscure early recognition and periungual depigmentation may mask subclinical lateral tumour spread. Onychoscopy and metabolic imaging are valuable adjuncts for diagnosis and staging; whereas histopathology remains the gold standard. Early suspicion and prompt biopsy are crucial for improving outcomes.