Incidental lesions detectable with FDG PET/CT

Objectives Various incidental lesions are found during FDG PET/CT study. The aim of this retrospective study was to clarify the types and frequency of malignant and clinically meaningful non-malignant lesions (NMLs) incidentally detectable with PET/CT.

Methods Subjects were 3648 asymptomatic individuals (2228 men, 1420 women, 53.7 ±12.3 years old) who underwent PET/CT studies as a part of cancer screening program at our institution from Aug 2005 to Oct 2011. PET/CT was performed 60 min after injection of 145 to 260 MBq FDG (Discovery ST, GE Healthcare). Findings of PET/CT images were prospectively recorded by 4 readers. The PET findings were compared with final diagnoses obtained by other imaging modalities, laboratory studies, and clinical follow-up for no less than one year.

Results A wide variety of carcinomas (CAs) were found in 42 (1.15%) of the 3648 subjects. Of the 42 MLs, 40 lesions (95.2%) were PET positive, and 2 lung CAs (4.8%) were PET negative and detected on CT images. Most subjects with CA underwent potentially curative treatments. A total of 797 NMLs were found in 712 (19.5%) of the 3648 subjects. Of the 797 NMLs, 415 lesions (52.1%) were PET positive, and 382 lesions (47.9%) were PET negative and detected on CT images. The NMLs were classified into the following 3 sites. 1) Head & neck: 391 lesions were found, including chronic thyroiditis (193), maxillary sinusitis (110) and benign thyroid tumor (43). 2) Thorax: 44 lesions were found, including pneumonia (12), atypical mycobacteriosis of the lung (12), sarcoidosis (7) and emphysema (5). 3) Abdomen & pelvis: 362 lesions were noted, including cholecystolithiasis (170), urinary tract stone (71), adrenal tumor (45), pancreatic cyst (19), colorectal polyp (17) and abdominal aortic aneurysm (12).

Conclusions In our study subjects, a wide variety of MLs and NMLs were found incidentally at a rate of 1.15% and 19.5%, respectively. Cautious image interpretation is required in both PET and CT portions.