Imaging for malignant pleural effusions—still no routine role of positron emission tomography

J Thorac Dis 2019;11(4):1079-1081 jtd.amegroups. com Malignant pleural effusions (MPEs) are a common diagnostic challenge for the respiratory physician. Their incidence is increasing with an estimated 150,000 new cases in the USA annually (1). As well as an increasing incidence, patients with MPE are presenting older due to ageing population and improved survival from primary cancer. The survival from diagnosis of all-cause MPEs is poor at around 5 months (2). Therefore, a clear and minimally invasive pathway for investigating these patients is essential. Currently, the mainstay of early investigation is computed tomography (CT) scanning and pleural fluid sampling for cytological analysis. Computed tomography is recommended on all patients with an undiagnosed unilateral exudative pleural effusion as a quick and non-invasive method of imaging the pleura (3). Various studies have assessed the sensitivity of CT for detecting malignant pleural disease. Hallifax et al. in 2015 performed a retrospective review of 370 thoracoscopy procedures where the patient had a CT scan prior to the procedure (4). This analysis involved interpreting the radiologists reporting of the scan as to their certainty of malignancy. The sensitivity and specificity of CT was calculated using the thoracoscopic biopsy results as gold standard and was 68% and 78% respectively. Several further analyses have been performed and apart from a known false negative rate, especially for certain tumours (e.g., mesothelioma), the utility of CT scanning is unequivocal. Pleural fluid cytology is another cornerstone in the investigation of malignant pleural disease and again is recommended in all patients with undiagnosed unilateral pleural effusions. The sensitivity of cytology to detect malignant disease varies depending on the population studied and the immunohistochemical techniques used. A recent UK based prospective study found the overall sensitivity for detecting malignancy was 46% (95% CI, 42– 58%) with a considerable number of mesothelioma cases, in whom cytological yield was just 6% (5). When this analysis was limited to just lung carcinomas, sensitivity rose to 56%, and higher still in lung adenocarcinomas (82%). The downsides of pleural fluid cytology include the time taken for immunohistochemical analysis, up to 5–7 days, during which the patient may be symptomatic without definitive management of their effusion. Additionally, it is still an invasive test and may not be immediately possible in those on anticoagulation or with only a small amount of pleural fluid. Positron emission tomography (PET) scanning is becoming increasingly used in malignant disease given improved availability and evidence base. It has been full incorporated in the diagnostic and treatment pathway of lung cancer but without a clear translation to MPE investigation currently. In MPE most studies have focused on the ability of PET to diagnose and stage mesothelioma (6,7). The drawback of PET in all-cause effusions suspicious of malignancy are the false positives due to other causes of pleural inflammation including inflammatory pleuritis, parapneumonic effusions or previous pleurodesis. Additionally, the availability of PET scanners varies dramatically worldwide so any guidelines should take this into account. A meta-analysis of diagnostic accuracy studies focusing Editorial