Collagen Vascular Disease–Associated Interstitial Pneumonia With Accumulation of 18F-fluoroestradiol

18F-fluoroestradiol (18F-FES) is a PET radiotracer that binds to the estrogen receptor (ER) in the nucleus of ER-expressing cells and is increasingly used clinically in breast cancer patients to visualize ER expression in lesions. Although 18F-FES accumulation is generally specific, radiation-induced pneumonitis can produce false-positive results, but the underlying mechanisms remain largely unclear. Here, we report the case of a 72-year-old woman with known collagen vascular disease-associated interstitial pneumonia and the accumulation of 18F-FES on 18F-FES-PET/CT, which was conducted to investigate suspected recurrent breast cancer.