Significance of incidental parotid gland 18F-FDG uptake on PET/CT and its impact on patient management

Objectives To evaluate the incidence and clinical significance of unexpected focal uptake of 18F-FDG in the parotid glands on PET/CT in patients with known or suspected non-head and neck primary malignancy.

Methods A retrospective review was conducted on 7252 consecutive patients who underwent FDG PET/CT scans for various oncologic indications over the course of 48 months. Patients with known or suspected head and neck cancer (HN) were excluded from the study. FDG uptake in the parotid glands was quantified with maximum standardized uptake value (SUVmax) corrected for body weight. Outcome was assessed by histological correlation when available, or by clinical and imaging follow up.

Results Incidental parotid FDG uptake was identified in 27 of 7252 patients (0.4%). Twenty-five had available histology and/or follow-up. One of 25 patients (4.0%) had histologically confirmed malignancy, with SUVmax of 7.4. The patient was known for follicular lymphoma and was found to have lymphomatous involvement of the left parotid gland following the PET/CT scan. In the remaining 24 patients considered to have benign uptake by histological (n=9) and/or clinical (n=15) correlation, mean SUVmax was 6.5 (SD +/- 3.6, range: 1.9 - 18.3).

Conclusions As the largest Western study of FDG-avid parotid incidentalomas, we found 0.4% of oncologic patients had unexpected focal parotid uptake in whom 4% had proven malignancy. Previously reported malignancy rates in FDG-avid parotid incidentalomas vary widely from 0-29%. In this study, one of 4 lymphoma patients had unexpected disease involvement of the parotid gland. Interestingly, two previous studies both detected lymphoma in 1 of 2 malignant cases in their respective populations. Consequently, special attention should be given to parotid gland FDG uptake in patients with known or suspected lymphoma as they appear to have a higher predilection for parotid gland malignancy than other oncologic patients