[A case of pulmonary malignant melanoma mimicking lung abscess]

An 84-year-old man was admitted with paresis of the right lower limb. Hemorrhagic lesions were demonstrated in the left frontoparietal lobe and cerebellum by cranial computed tomography (CT) and magnetic resonance imaging (MRI). Chest CT revealed an ill-defined mass measuring 4 x 6 cm in the left lower lobe of the lung, although bronchoscopic examination failed to obtain pathological diagnosis. Clinical diagnosis of primary lung cancer with multiple brain metastases was made, and he underwent whole brain radiotherapy. The pulmonary and cerebral lesions mimicked abscesses during his clinical course, and he died of respiratory failure due to bilateral pneumonia three months after admission. Autopsy revealed that both the pulmonary and brain lesions were malignant melanomas, but no other melanoma lesions could be identified despite meticulous investigation. Although malignant melanoma with an unknown primary site is rare in Japan, careful evaluation of the CT and MRI findings might be the key to correct diagnosis in this case.