The study patients were identified from our database of a prospective patient cohort who underwent [18F]FES PET/CT for the assessment of ER status because of recurrent or metastatic breast cancer lesions . The patients’ medical histories and [18F]FDG PET/CT studies performed to identify unsuspected regional nodal disease and/or distant metastases were retrospectively reviewed. Our institutional review board approved this study protocol and waived the requirement for informed consent. This study was conducted in accordance with the Declaration of Helsinki and our institutional guidelines.
All eligible patients had ER-positive primary breast cancer with suspected first recurrence at presentation, were enrolled in a previous prospective cohort study to assess [18F]FES PET/CT, and underwent [18F]FDG PET/CT within 60 days before needle biopsy or surgery. Patients were excluded if they had ER-negative or ER-unknown primary breast cancer. The complete selection criteria, setting, location, and dates of the prospective cohort study were previously reported . The patient population used in this study represented a consecutive sample of eligible patients.
[18F]FES and [18F]FDG PET/CT procedure
The production of [18F]FES was described previously . PET/CT imaging was performed from the skull base-to-upper thigh using a Discovery PET/CT 690 or 710 scanner (GE Healthcare), 80–100 minutes after intravenous injection of 111–222 MBq (3–6 mCi) of [18F]FES . PET/CT images were reconstructed using the manufacturer-provided iterative algorithm with 4 iterations and 18 subsets.
[18F]FDG PET/CT images were obtained from the skull base-to-upper thigh using one of several different PET/CT scanners (Biograph Sensation 16 or Biograph TruePoint 40, Siemens Healthineers; or Discovery PET/CT 690, 690 Elite, or 710, GE Healthcare), 50–70 minutes after intravenous injection of 5.2–7.4 MBq/kg (0.14–0.2 mCi/kg) of [18F]FDG as described in a previous report .
Experienced breast pathologists who were blinded to the results of the [18F]FES PET/CT reviewed core-needle biopsy and surgical samples in accordance with routine standard procedures. The recurrent or metastatic specimens taken from the breast, liver, ovary, uterus, or bone were excluded from the analysis. Clinical information and other laboratory test results were available to the assessors of the histological examinations. Immunostaining for ER was performed using the 6F11 mouse monoclonal antibody (NCL-ER-6F11, Novocastra Laboratories), as previously described . Semi-quantitative ER receptor expression was evaluated according to the Allred score . ER status was considered positive if Allred scores were 3 or higher.
Analysis of PET/CT imaging
The recurrent lesion from which a sample was taken for histological examination was selected as the reference lesion for the sensitivity analysis, as reported previously . Three board-certified nuclear medicine physicians (S.B, J.Y.K, and S.J.N) who were blinded to all patient information including histological diagnosis, independently assessed the [18F]FES PET/CT images in a random sequence. The physicians qualitatively assessed the [18F]FES uptake of reference lesions, as well as up to five additional non-reference lesions in the order of highest intensity relative to background activity [25, 37]. Lesions were interpreted as ER-positive breast cancer when the majority of [18F]FES assessments were positive.
Two board-certified nuclear medicine physicians (H.J.S and D.Y.L) interpreted the [18F]FDG PET/CT images in consensus using a syngo.via image display system (VB20A, Siemens Healthineers), with the images displayed in a random sequence. These two physicians were also blinded to all patient information and the [18F]FES PET/CT images; they only had knowledge of the location of the biopsy sites for [18F]FDG PET/CT image evaluation. The lesions were classified as positive, equivocal, or negative lesions by qualitative analysis. An [18F]FDG focus was interpreted as negative when it was related to a non-malignant process or the physiological biodistribution of [18F]FDG. Focally abnormal [18F]FDG uptake higher than that of surrounding tissues and that could not be related to non-malignant or physiological uptake was interpreted as positive. [18F]FDG uptake that could not be characterized was defined as equivocal .
For quantitative analysis, the readers calculated the maximal standardized uptake value (SUV) as SUV = activity (Bq/g)/[injected activity (Bq)/body weight (g)]. The [18F]FDG SUV results were harmonized across different PET/CT scanners by matching the recovery coefficient profiles without partial volume correction, as described previously .
All data are presented as median values with interquartile range (IQR) for continuous variables, and numbers (%) for categorical variables. A two-sided P-value of less than 0.05 was considered as the threshold for significance. Quantitative parameters were compared using the Mann-Whitney U test or Kruskal-Wallis H test and categorical data were compared using the McNemar test. All statistical tests were conducted using SPSS Statistics for Windows (version 21, IBM Company). The sensitivity of PET/CT was defined as the probability that a PET/CT result would show positive uptake when the patient had histologically proven recurrent breast cancer. Estimates are presented with 95% Clopper-Pearson exact confidence intervals (CI).