Glucose consumption increases in benign and malignant focal pulmonary lesions. This may yield both false positive and false negative results in positron emission tomography (PET) with 18F-labeled fluorodeoxyglucose (FDG). A 48- year-old male patient underwent radical (left) and modified (right) neck dissections and total thyroidectomy with the diagnosis of medullary thyroid carcinoma (MTC). Upon detection of elevated calcitonin levels in the postoperative period, he was evaluated with PET, which showed metastatic lesions in the anterior upper mediastinum and in the upper lobe of the left lung. Plain chest radiography and bone scintigraphy were normal. The patient underwent a median sternotomy. Millimetric nodular lesions were detected in the apical segment of the left upper lobe. There were three nodules in the left lower lobe, and two nodules in the right middle lobe, each measuring 1 mm, which PET failed to detect. The lesions were removed with wedge resection. Frozensection analysis of the specimens showed necrotizing granulomatous inflammation in the left lung, and necrotic nodules and neuroendocrine cell hyperplasia in the right middle lobe. Histopathological examination showed necrotizing granulomatous inflammatory process in specimens obtained from the apical segment of the left upper lobe, and multifocal MTC foci in all the specimens from lung parenchymal tissue and in mediastinal dissection material. It was interesting that, in the same patient, PET yielded a false positive result for tuberculous granulation tissue, and a false negative result for metastatic nodules in both lungs.

Keywords : Lung neoplasms/diagnosis; thyroid neoplasms/radiography; tomography, emission-computed; tuberculosis, pulmonary