47IN PULMONARY NODULE: DIFFERENTIAL DIAGNOSIS AND BAYESIAN APPROACH

A solitary pulmonary nodule (defined as <3 cm lesion within and surrounded by pulmonary parenchyma) is a common clinical problem. It is usually detected incidentally. Common causes of malignant SPN include primary lung cancer (mainly adenocarcinoma), carcinoid tumours and metastatic cancer. Benign aetiologies include infectious granulomas (mycobacteria, histoplasmosis, coccidiomycosis) other infections, benign tumour (hamartomas . . . ) and less often cysts, vascular abnormalities . . . . The estimated frequency of each aetiology varies among the population, the methodology used and the age of the study. The prevalence of solitary pulmonary nodule (SPN) in the lung cancer screening study varies from 8% to 51% (with a prevalence of malignant nodule from 1% to 12%). The major question is whether the lesion is malignant or not. The Bayesian approach can help us to identify promptly malignant nodule in order to treat them surgically and to avoid surgery for benign nodules. Therefore, it is needed to estimate the probability of cancer (Pca) in the SPN. Likelihood ratio (LR) for overall prevalence of malignancy and for different clinical and radiological information (age, smoking exposure, asbestos exposure, symptoms, cancer history, nodule size, spiculation, calcification, location, growth, . . . ) can be obtained from the literature. The odds of cancer-malignancy (Odds ca) can be calculated by multiplying all of these LRs together. The Pca = odds ca/(1 + odds ca). Using this Bayeasian approach, the probability of cancer based on an abnormal or normal fluoro-2deoxy-D-glucose-positron emission tomography (FDG-PET) scan has been estimated. Sensitivity, specificity, positive predictive value and negative predictive value of PET scan are respectively about 90%, 80%, 90% and 90%. Moreover, the LR for malignancy are higher with an abnormal PET scan when compared to most clinical and radiological LRs. Today, the Bayesian approach of SPN must include PET scan. Surgical excision of the nodule is indicated if the LR of malignancy according to clinical and radiographic characteristics is high, a FDG-PET is positive or the nodule has been proven to be malignant by a sampling procedure.