Case Author(s): Lisa Oakley, M.D. and Mark Mintun, M.D. , 09/19/97 . Rating: #D3, #Q3

Diagnosis: Tuberculosis

Brief history:

73 year old man with a history of prostate cancer, presents with an indeterminant left upper lobe nodule.



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Full history/Diagnosis is available below

Diagnosis: Tuberculosis

Full history:

73 year old man with a history of prostate cancer, found to have an incidental left upper lobe nodule on routine chest radiograph. Follow up computed tomography of the chest showed the nodule to be inderminant. PET imaging was requested to further characterize this lesion and to evaluate for additional foci of disease.


F-18 fluorodeoxyglucose (FDG)


PET images show a single focus of intense activity in the left upper lobe which correlates with the nodule seen on CT scan. The standard uptake value is high (5.7), making this lesion worrisome for malignancy. No additional foci of abnormal tracer uptake are seen. Specifically, there is no evidence of metastatic disease or mediastinal adenopathy.

CT of the chest shows a 2 x 3 x 1.5 cm left upper lobe nodule with straight margins (atypical for malignancy), but no definite benign-appearing calcification. Additional axial slices (not included) showed calcified mediastinal lymph nodes consistent with old granulomatous disease.


The incidence of solitary pulmonary nodules detected by CXR is 130,000 new cases each year. 50-60% of these nodules are benign and yet 20-40% of these lesions go on to invasive diagnostic nodule biopsy or resection. This is because few criteria can reliably differentiate benign from malignant lesions.

PET is useful for the further characterization of indeterminant lung nodules in that it helps stratify patients into those that need further work-up and biopsy and those who can be managed with watchful waiting. Ideally, the nodule should be greater than 8-10 mm in size due to limitations in PET resolution.

Nodules showing relative increased tracer uptake (ie. increased metabolic activity) are highly sensitive (93-100%) and slightly less specific (78-90%) for malignancy. Semi-quantitative analysis using the standardized uptake value (SUV) is an objective measure of the lesion's metabolic activity when compared with the average concentration of FDG within the body. When evaluating lung nodules, any SUV greater than 2.5 is considered positive and malignancy is found in > 83%. If the nodule has an SUV < 2.5, then malignancy is seen in < 5%.

As in this case, most false positives for malignancy are due to active granulomatous disease.


Biopsy of the lung nodule was positive for tuberculosis.

Major teaching point(s):

While this case represents a false positive study, the PET results appropriately indicated the need of a definitive diagnosis and, in this patient, led to an important change in management

In the evaluation of indeterminant lung nodules, many patients with benign disease can be spared the morbidity of biopsy if initial work-up includes a negative PET scan.

If the nodule is less than 8-10 mm in size, then all bets are off.

Differential Diagnosis List

Malignancy (ie. primary lung cancer or metastatic disease)

Granulomatous disease

ACR Codes and Keywords:

References and General Discussion of PET Tumor Imaging Studies (Anatomic field:Lung, Mediastinum, and Pleura, Category:Inflammation,Infection)

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Case number: pt015

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