PET should not be used to “fish” for a diagnosis. On the other hand, when a pathological diagnosis is already established, PET can be useful for follow-up, staging and reassessment after therapy. PET scanning is an excellent modality to assess tumor size and metabolic activity and it is coming into wider use as supporting data becomes avaialble for various tumor types. It is a resonable technique to distinguish benign from malignant lung lesions over 1 cm in size.
PET has been studied extensively in the evaluation of indeterminate lung lesions. It has been found to be able to defferentiate benign from malignant lesions as small as 1 cm. An overall sensitivity is 96% (range, 83%–100%), specificity of 79% (range, 52%–100%), and accuracy of 91% (range, 86%–100%).
False-negative results can occur in lesions smaller than 1 cm because a critical mass of metabolically active malignant cells is required for PET diagnosis. In lesions smaller than 1 cm, only high FDG uptake will be of diagnostic relevance. False negatives can also occur in tumors with a low metabolism, like carcinoid tumors and bronchioloalveolar cell carcinomas. False-positive FDG uptake is seen in inflammatory conditions such as bacterial pneumonia; pyogenic abscesses; aspergillosis; and granulomatous diseases such as tuberculosis, sarcoidosis, histoplasmosis, Wegener’s granulomatosis, and coal miner’s lung. In these lesions the FDG uptake has been attributed to granulocyte and/or macrophage activity.
In general, when the index of suspcion is low for the processes that can cause false positives, PET can distinguish benign lesions from metastases of a known cancer.
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Raghunandan Vikram et al, Utility of PET/CT in Differentiating Benign from Malignant Adrenal Nodules in Patients with Cancer November 2008, Volume 191, Number 5