Solitary pulmonary nodules: Tc99m-MIBI to discriminate between malignant and benign lesions in a region with a high prevalence of tuberculosis

Full text: A solitary pulmonary nodule (SPN) is defined as a single circumscribed lung lesion surrounded by aerated lung without atelectasis, satellite lesions or cavity. The generally accepted definition of such nodules includes lesions up to 3 cm in diameter. Some studies include larger lesions of up to 6 cm. Most SPN are due to primary bronchogenic carcinoma or benign granulomas. South Africa has a high incidence of tuberculosis, which often presents as SPN. Assessment of pulmonary nodules is important to differentiate benign and malignant lesions. Radiological imaging is often inconclusive in determining the presence of malignancy. F-18 FDG PET is useful in the evaluation but is expensive and not available in South Africa. Tc-99m MIBI has been used in the evaluation of various malignancies, including lung cancers for diagnosis and staging. The aim of our study was to evaluate Tc-99m MIBI scintigraphy as a non-invasive diagnostic tool to differentiate whether the SPN is malignant or benign. The study group included thirty patients, 19 men and 11 women, with a mean age of 53 years (range 19-90 years). The nodule size ranged between 1 and 5 cm in diameter, with a mean size of 2 cm. All the patients were referred from the Lung Unit of Tygerberg Hospital. The inclusion criteria were a recent chest X-ray and a planned or available CT scan of the chest. Pregnant patients were not included in the study. Imaging was performed after the administration of Tc-99m MIBI intravenously in the arm opposite to the SPN. Planar views and SPECT of the chest area were obtained. Final diagnosis was obtained by means of: bronchoscopy with histology, cytology, microbiology or thoracotomy, fine needle aspiration cytology (image guided) or serial CXR follow-up (documenting 2 year lesion stability). Two experienced nuclear physicians who were blinded to the aetiology of the SPN evaluated MIBI scans. All-malignant lesions showed uptake of MIBI. There were three false positive scans in the benign group, which included one patient each with active tuberculosis, silicosis and aspergilloma. The sensitivity and specificity for the detection of malignant lesions were 100% and 86% respectively, and the positive and negative predictive values 75% and 100%, respectively. MIBI scan is useful to discriminate between malignant and benign SPN in the majority of patients A negative scan excludes malignancy (NPV=100%) and a positive scan may lead to some unnecessary resections (PPV= 75%). Based on these preliminary findings we conclude that Tc99m-MIBI is a useful and inexpensive diagnostic tool to differentiate non-invasively between benign and malignant lesions in an area with high tuberculosis prevalence. This modality may be especially useful in developing countries with a high prevalence of benign nodules when F-18 FDG PET is not available. (author)