Parotid incidentalomas on positron emission tomography: what is their clinical significance?

The increased use of PET/CT in cancer staging has resulted in more incidental findings in unexpected locations, making this a challenge for the radiologist interpreting the study. Our aim was to determine the frequency of parotid incidentalomas and to assess the ability of PET/CT to characterize them. At the Norfolk and Norwich University Trust, in between October 2010 to October 2015, 4044 patients had a PET/CT examination. The issued clinical reports that contained the word 'parotid' were traced and all patients selected for this study had no known or suspected parotid disease before the PET/CT scan. The prevalence of parotid incidentalomas was 1.73%; higher than expected as was the mean age of our study group. Ten per cent of patients had no focal lesion identified on subsequent ultrasound. Thirty-two per cent of patients had a biopsy which showed that the majority of these findings were benign and 13% had metastatic deposits, with the primary carcinoma in these cases being neuroendocrine colon carcinoma, non-Hodgkin's lymphoma and melanoma. The most common benign salivary tumour in our study was Warthin's tumour and the majority of these patients had a primary lung carcinoma. Given that the main predisposing factor for both pathologies is smoking, potentially this suggests an association between the incidence of primary lung carcinoma and Warthin's tumour. Ten (14%) patients in our study had a standardized uptake value of below 3.2 (using the VUE Point HD algorithm). These patients had either no identifiable lesion on ultrasound or no malignant features on histology; therefore, we propose that a cut-off of 3.2 on PET/CT could be used to differentiate between physiological or benign uptake from malignant fluorine-18-fluorodeoxyglucose uptake in the parotid gland.