Solitary Pulmonary Lung Nodules: New Paths to Diagnosis Become Clear
ABSTRACT & COMMENTARY
Synopsis: In a comparison of two (F-18) Fluoro-2-deoxy-D glucose positron emission tomographic (FDG-PET) scans to standard clinical (Bayesean), analysis indicated that a positive scan differentiated benign from malignant nodules with a high degree of clinical certainty.
Source: Dewan NA, et al. Chest 1997;112:416-422.
The approach to the newly discovered solitary pulmonary nodule (SPN) on chest radiograph has seemingly changed little over the past 25 years. Comparisons to previous chest radiographs and rate of growth, while helpful, were frequently unavailable in the newly presenting patients. A clinical approach often developed in which background likelihood of malignancy was used to determine the appropriate diagnostic strategy. Young non-smokers without occupational history of asbestos were observed over a two-year period for progression, while older smokers were resected (COPY OUT HERE?) While size greater than 2 cm, indistinct borders, and cavitation were more likely to be seen in malignant lesions, only central calcification has proved to be a reliable sign of a benign process. Evaluation of nodule density by CT scanning proved to be disappointing and was not widely implemented. Fine needle aspiration was primarily reserved for patients who were not surgical candidates. Recently, the use of PET scanning has been introduced, and the high uptake of labeled glucose has been found to characterize most lung cancers.1
Dewan et al now add to this growing body of knowledge supporting FDG-PET scanning as a practical approach to evaluating the SPN. Fifty-two patients (mean age, 63 years) with noncalcified nodules less than 3 cm were evaluated. Thirty-seven were found to be malignant, and 15 had a benign diagnosis confirmed histologically in all but one patient. FDG PET scanning was abnormal in 35 of 37 malignant nodules and normal in 13 of 15 benign nodules. Sensitivity was 95% and specificity 87% overall. Likelihood ratios were approximately 7 with an abnormal scan and 0.06 for a normal scan. The use of standard criteria (both clinical and radiographic) in combination was less likely to provide a correct diagnosis than PET scanning alone. Patients with both benign and malignant nodules had a calculated probability of malignancy that varied from 5% to 95%.
COMMENT BY ALAN M. FEIN, MD
The present study confirms that PET scanning evaluating radiolabelled glucose uptake is a highly effective modality for establishing whether an SPN is benign or malignant. As we can see from the study of Dewan et al, standard clinical and radiologic criteria were decidedly unreliable in determining the nature of these lesions. The probability of cancer using these criteria varied quite widely. With high calculated clinical probability, specificity was high, but sensitivity was low. In contrast, a probability less than 5% had a high sensitivity but very low specificity. The results of the PET scan were a decided improvement. Although sensitivity was lower in nodules below 1.5 cm, other studies have suggested that accuracy of PET scanning is acceptable even in these smaller lesions.2 This information adds weight to an innovative paradigm for evaluating the newly presenting SPN. First, stability should be established by evaluating old chest radiographs, if available. A nodule that is unchanged in size (remember that volume, not diameter, is key) over two years may be considered benign. Unfortunately, except for central (target) calcification, very little individual or combined clinical or radiographic characteristics provide enough diagnostic accuracy to establish malignancy. Therefore, in nodules whose stability is unknown, PET scanning should be ordered. A normal scan allows continued follow-up, while increased FDG uptake mandates resection. While there may be overlap with inflammatory lesions, this technique represents a major diagnostic advance. Although PET scanning is not widely available, information like that provided by Dewan and colleagues will provide the evidence to remedy this situation.
References
1. Libby DM, et al. Solitary pulmonary nodule: Update 1995. Am J Med 1995;99:491-496.
2. Coleman RF. Multicenter prospective study of FDG-PET in the evaluation of indeterminate solitary pulmonary nodules. J Nuclear Med 1995;36:94 P.
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