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Objective:
(18) F-fluorodeoxyglucose (FDG) positron emission tomography (PET)-computed tomography (CT) is routinely used for staging and treatment planning of a variety of malignancies. As with other imaging modalities, 18F-FDG PET is prone to discovering incidental findings with variable clinical significance.1 One such location is the breast, where incidental hypermetabolic lesions have been identified in 0.4 – 1% of 18F-FDG PET/CT scans. These incidental findings represent underlying primary breast cancer (PBC) in 29.7 – 71.5% of cases. To our knowledge, no prior study has characterized these incidental PET/CT findings in a large urban safety net hospital population. We aimed to conduct a retrospective cohort study evaluating the incidence of incidental breast findings on 18F-FDG PET scans performed for nonbreast indications.

Materials and Methods:
The Institutional Review Board (IRB) at Boston University Medical Campus approved this retrospective study and waived the requirement for informed consent. From January 1, 2016, to December 31, 2021, 5,934 18F-FDG PET/CT imaging studies were conducted, of which 1,115 were excluded due to prior or current history of breast cancer resulting in a study cohort of 4,819 18F-FDG PET/CT studies. Fifty-six exams in 50 patients had an 18F-FDG avid lesion within the breast. Twenty-four patients with incomplete follow-up were excluded, leaving a final cohort of 26 patients. Outcomes for these 26 patients were obtained from imaging and the electronic medical record review. Mann–Whitney U test was conducted to test for differences in continuous variables between malignant and benign findings.

Results:
18F-FDG avid lesions were identified within the breast in 56 of 4,819 (1.2%) studies. In the 26 patients with complete follow-up, 9 (34.6%) were malignant, and 17 (65.4%) were benign based on biopsy, imaging characteristics, or > 2-year follow-up with stability. Biopsy revealed 7 invasive ductal carcinomas and 2 B-cell lymphomas. Older age was significantly associated with patients having invasive ductal carcinoma (median = 62.5; IQR = 12) than with a benign incidental finding (median = 58; IQR = 15) (p = 0.001). SUVmax was higher in patients with primary breast cancer (median = 4.9; IQR = 8.1) than in those with benign findings (median = 3.45; IQR = 1.0), but this was not statistically significant (p = 0.135).

Conclusion:
In a safety net population undergoing 18F-FDG PET/CT imaging for a nonbreast indication, focal uptake was found in 1.2% of studies, which is higher than the 0.4 – 1.0% reported in the literature. As 34.6% of these incidental PET/CT findings proved to be malignant, it is critical that these underserved patients undergo prompt diagnostic imaging evaluation to minimize the chance of a more advanced stage at diagnosis.