*2.1.3 Positron emission tomography scan*

Positron emission tomography scan can be a useful tool to detect the extent of the disease, identify multisystem disease such as cardiac sarcoidosis, and may help to identify a desirable site for biopsy [5]. Moreover, it could be invaluable in the decision to initiate immunosuppression and assess the efficacy of treatment [6, 7]. Furthermore, it may help in predicting relapse in pulmonary sarcoidosis [8].

A retrospective study by Teirstein et al. showed that a combination of diagnostic modalities such as 18F-fluorodeoxyglucose (FDG-PET) and CT scan is more sensitive than PET-only imaging [9]. Whole-body FDG-PET was found to be significantly better in identifying occult and reversible granulomas. Moreover, a positive PET scan in isolation should not be considered as an indication for treatment. In another study by Yu et al., the sensitivity and specificity for benign and malignant disease were 94.2% and 73.8%, respectively [10]. It was, however, noted that maximum standard uptake value (SUVMax) as semiquantitative measurement alone could not be used to differentiate benign vs. malignant lesions.

The FDG-PET scan has a cumulative effect in cardiac sarcoidosis. PET scan has also been evaluated in predicting supraventricular arrhythmias, and it was noted that patients with left atrial enlargement were associated with increased likelihood of supraventricular arrhythmias [11]. Smedema et al. reported that biventricular late gadolinium enhancement was the strongest predictor of adverse outcome, and an asymptomatic myocardial scar of less than 8% in the left ventricular mass was associated with a favorable outcome in patients with pulmonary sarcoidosis [12].

On the basis of current available evidence, the role of PET-CT is limited in routine clinical care of patients with pulmonary sarcoidosis. However, it may be a useful imaging modality in multisystem sarcoidosis, in particular when the clinical suspicion for cardiac involvement is high and the diagnostic techniques such as echocardiography and or cardiac MRI have unequivocal results. Moreover, PET-CT may become a useful adjunct to assess the response to immunosuppression with corticosteroids and/or antimetabolites and may guide us to an appropriate biopsy site to sample suspected multisystem disease.
