Zosteriform Cutaneous Metastasis of Lung Adenocarcinoma Mimicking Herpes Zoster: A Diagnostic Pitfall.

Zosteriform cutaneous metastasis from lung cancer is rare, but dermatomal distribution can lead to diagnostic anchoring on herpes zoster. A 65-year-old man with metastatic lung adenocarcinoma presented with a one-month history of painless, nonvesicular papules confined to the left T7-T9 dermatomes. Herpes zoster was initially suspected because of the striking unilateral dermatomal distribution and the patient's oncologic treatment-related immunosuppression, despite the absence of pain, vesiculation, and crusting. Oral valacyclovir was prescribed. The patient did not attend the planned one-week follow-up and next presented approximately six months later, when the eruption had progressed to extensive infiltrated violaceous papules and plaques spanning T2-T10. Histopathologic examination showed dermal infiltration by malignant gland-forming epithelial cells with lymphovascular invasion, and nuclear thyroid transcription factor-1 positivity supported pulmonary origin. The patient died approximately seven months following the diagnosis of cutaneous metastasis. This case documents the progression of initially limited zosteriform lesions to extensive infiltrative cutaneous metastases. In patients with malignancy, dermatomal distribution should not outweigh discordant morphology or clinical behavior; painless, nonvesicular, persistent, or progressive eruptions warrant early biopsy.