F-18 Fluoroestradiol (FES) PET for ER+ Breast Cancer

F-18 fluoroestradiol (FES) PET/CT is a whole-body, noninvasive map of estrogen receptor expression that complements — rather than duplicates — FDG PET. Approved by the FDA in 2020 as Cerianna, FES binds the estrogen receptor and concentrates in receptor-expressing cells, letting a single scan report receptor status across every site of disease at once. For recurrent or metastatic estrogen receptor (ER)-positive breast cancer, that answers a question a biopsy of one lesion cannot: whether all the disease still expresses a functional receptor worth targeting with endocrine therapy.^{1, 3}

Breast cancer treatment is built on receptor status, and ER status in particular determines whether endocrine therapy is an option. Historically that status came from immunohistochemistry on a biopsy — a single site, at a single time. But receptor expression can differ between metastases and can change over the course of therapy. FES PET was developed to make ER status visible in vivo, everywhere, repeatedly, and without a needle. This guide covers the radiopharmaceutical physics, the clinical role relative to FDG, a worked dosimetry calculation, practical acquisition and quality-control considerations, and the regulatory framework. DRPS supports this work through its PET/CT and nuclear medicine physics and accreditation support services across Florida, Maryland, Virginia, Washington DC, California, and Nevada.

Introduction

FES answers a different question than FDG. Fluorodeoxyglucose (FDG) is a glucose analog that accumulates wherever metabolism is high — tumor, inflammation, brown fat, healing tissue. It is superb at finding active disease but says nothing directly about whether that disease expresses the estrogen receptor. FES (chemically, 16α-18F-fluoro-17β-fluoroestradiol) is a radiolabeled estradiol that binds the ER. Where FDG shows where the cancer is metabolically active, FES shows whether those sites carry a functional target for endocrine therapy.^{1, 3, 5}

That distinction has direct treatment consequences. An ER-positive primary tumor does not guarantee that every metastasis remains ER-positive; receptor expression can be lost — or, occasionally, regained — over time and treatment. A patient whose bone and liver lesions all avidly bind FES is a strong candidate for endocrine therapy. A patient whose dominant lesions show no FES uptake, despite an ER-positive primary, may progress on endocrine therapy and warrant a different strategy. FES makes that heterogeneity visible in one whole-body study.^{5, 6, 7}

FES is not a replacement for FDG, for biopsy, or for standard staging imaging. It is a targeted problem-solver. Understanding where it fits — and how to acquire and quantify it correctly — is what turns a new radiopharmaceutical into reliable clinical value.

Topic Explanation

What is FES and how does it work?

FES is estradiol labeled with fluorine-18 that binds the estrogen receptor and concentrates in ER-expressing cells. After intravenous injection, it distributes through the blood, binds ER in target tissue, and — like all F-18 tracers — decays by positron emission, producing paired 511 keV annihilation photons that a PET scanner detects. The result is a quantitative image of regional ER binding.¹

The physical basis is standard PET. Fluorine-18 has a physical half-life of about 110 minutes and decays predominantly by positron emission; the positron annihilates with a nearby electron to yield two 511 keV photons emitted in nearly opposite directions. That is the same detection physics used for FDG and other F-18 agents, which means FES runs on existing PET/CT hardware with no special detector — the novelty is entirely in the molecule's targeting, not the imaging chain.^{1, 10}

Hepatobiliary handling matters clinically. FES undergoes substantial metabolism and hepatobiliary excretion, so the liver and gallbladder show high physiologic activity. That is why FES is not the tool for evaluating liver lesions on its own — high background can obscure them — and why the reported critical organs include the liver, gallbladder, and uterus.¹

FES versus FDG at a glance

For background on the general SUV framework these tracers share, see our guide to PET SUV quantification and PET uptake time.

The activity, uptake time, and dosimetry values in the FES column are drawn from the prescribing information; the FDG values are shown only for orientation and vary by institutional protocol.^{1, 5}

Key Technical Principles

Dosimetry and administered activity

The recommended FES activity is 222 MBq (6 mCi), within a range of 111 to 222 MBq (3 to 6 mCi), and the estimated effective dose is about 4.9 mSv in a 70 kg adult. Imaging typically begins about 80 minutes after injection, allowing target binding while background clears, with a scan duration of roughly 20 to 30 minutes.¹

A practical point for scheduling and dose calibration is physical decay over that uptake interval. Activity decays as:

With a half-life $T_{1/2}\approx 110$ minutes, the decay constant is $\lambda =\ln 2/110\approx 0.0063{\text{min}}^{-1}$. For an 80-minute uptake period, the fraction of the injected activity remaining from physical decay alone is:

So of a 222 MBq injection, roughly 134 MBq remains at the start of imaging from decay alone — before accounting for biological clearance. This is why FES logistics are tight: the dose must be calibrated for the administration time, and delays between injection and acquisition meaningfully reduce counts. The same math governs how long a prepared dose remains usable and drives coordination between the radiopharmacy, the injection, and the scanner.^{1, 10}

Quantification with SUV

FES uptake is quantified with the standardized uptake value (SUV), the same normalization used across PET:

where $C_t$ is the decay-corrected activity concentration in tissue (for example, kBq/mL), $A_{\text{inj}}$ is the injected activity, and $w$ is the patient's body weight. In a systematic review of FES imaging, the mean SUV in breast cancer lesions was reported near 2.05, and FES uptake thresholds in the range of roughly 1.1 to 1.82 were used to identify ER-positive disease and to characterize receptor heterogeneity. Pooled FES detection rates for breast cancer were about 0.80, similar to FDG.⁵

Because SUV depends on injected activity, uptake time, body weight, blood glucose handling, and scanner calibration, comparability across scans and across sites requires disciplined quality control of the dose calibrator, scanner cross-calibration, and clock synchronization — the same rigor that underpins any quantitative PET program.⁵

Receptor blockade and drug interactions

Endocrine drugs that block the estrogen receptor reduce FES uptake and can hide ER-positive lesions. The prescribing information notes that tamoxifen and fulvestrant can occupy or downregulate the receptor and may reduce FES binding for up to about 8 and 28 weeks, respectively. In contrast, aromatase inhibitors and GnRH agonists lower circulating estrogen but do not block the receptor, so they generally do not require a washout before FES. Knowing which class a patient is on — and how recently — is essential to avoid a false-negative interpretation.^{1, 4}

Clinical Impact

Guiding endocrine therapy decisions

The SNMMI appropriate use criteria concluded that the most appropriate uses of FES PET are to assess ER functionality when endocrine therapy is being considered — either at initial diagnosis of metastatic breast cancer or after progression on endocrine therapy — to determine the ER status of lesions that are difficult or dangerous to biopsy, and to clarify ER status when other tests are inconclusive. In each case, FES converts an unanswered receptor question into an image-based answer that can redirect therapy.^{3, 4}

Revealing receptor heterogeneity

Because FES images the whole body at once, it can show that some metastases bind the tracer while others do not — intrapatient ER heterogeneity that a single biopsy cannot capture. In a validated cohort, patients whose disease was ER-homogeneous on FES (all lesions FES-positive) had longer progression-free survival than those with ER-heterogeneous disease (median about 19.8 versus 15.0 months), supporting the prognostic value of whole-body receptor assessment. FES has also documented the return of ER expression in bone-dominant disease over the course of treatment, restoring a rationale for endocrine therapy that a single earlier biopsy would have missed.^{6, 7}

Staging and recurrence — an evolving evidence base

Beyond the approved problem-solving indications, FES is being studied for staging and suspected recurrence. A single-center phase 2 study found FES PET/CT detected distant metastases and recurrences at rates comparable to standard-of-care imaging, with fewer false positives in that cohort and a signal of particular value in invasive lobular carcinoma — uses that are not part of the current appropriate use criteria and warrant further investigation. The practical message is that FES is a targeted tool with a defined, evidence-based core and an actively expanding research frontier.⁸

Practical Optimization Tips

• Screen endocrine medications before scheduling. Confirm whether the patient is on tamoxifen or fulvestrant and when it was last taken; interpret a negative scan cautiously if a blocking agent is recent. Aromatase inhibitors and GnRH agonists generally do not require interruption.
• Respect the uptake window. Aim for the recommended time to imaging and keep it consistent, because uptake time affects SUV and cross-study comparability.
• Calibrate the dose for the administration time. With a ~110-minute half-life, delays measurably reduce counts; document injection and acquisition times.
• Do not read liver lesions on FES alone. High hepatobiliary background limits sensitivity there; pair with anatomic imaging or FDG as appropriate.
• Maintain quantitative QC. Dose calibrator constancy and linearity, scanner cross-calibration, and clock synchronization keep SUV values trustworthy across time and scanners.
• Interpret FES alongside FDG and anatomy, not in isolation. The clinical power comes from combining receptor status (FES) with disease burden (FDG or standard imaging).
• Standardize acquisition and reconstruction so that SUV thresholds used for ER-positivity remain valid within your program.

Regulatory Considerations

FES is an FDA-approved radioactive drug, so its clinical use sits at the intersection of FDA drug regulation and NRC or Agreement State medical-use rules. The molecule and its labeling are governed by the FDA approval; the possession and medical use of the byproduct material are governed by the facility's radioactive material license.^{1, 9}

Key frameworks to align with:

• FDA prescribing information (Cerianna, 2020) — defines the approved indication, administered activity, dosimetry, drug interactions, and handling.¹
• 10 CFR Part 35, Medical Use of Byproduct Material — governs authorized users, dose calibrator quality control, and radiation safety for diagnostic PET radiopharmaceuticals under the imaging and localization provisions. Agreement States administer equivalent programs.⁹
• SNMMI Appropriate Use Criteria for ER-targeted PET — the clinical-appropriateness framework payers and programs reference for FES.^{3, 4}
• ACR and IAC PET/CT accreditation — scanner performance, dose calibrator QC, and physicist oversight requirements that apply to any PET/CT program running FES.

Across the states DRPS serves, Florida, Maryland, Virginia, California, Nevada, Pennsylvania, New York, and New Jersey are NRC Agreement States that license the medical use of byproduct material under their own radiation-control programs, while Washington, DC and Delaware are regulated directly by the NRC. A facility must confirm which authority issues its license and what authorized-user, QC, and reporting requirements apply. For related PET performance context, see our guides to PET/CT NEMA NU-2 performance testing and Ga-68 DOTATATE PET for neuroendocrine tumors.

Frequently Asked Questions (FAQs)

What is F-18 fluoroestradiol (FES) PET?

FES PET is a positron emission tomography scan using 16-alpha-18F-fluoro-17-beta-fluoroestradiol, a radiolabeled form of estradiol that binds the estrogen receptor. It provides a whole-body, noninvasive map of estrogen receptor expression. It was approved by the FDA in 2020 under the brand name Cerianna as an adjunct to biopsy for detecting estrogen receptor-positive lesions in recurrent or metastatic breast cancer.

How is FES PET different from FDG PET?

FDG images glucose metabolism and marks metabolically active tissue of many types, while FES images estrogen receptor expression specifically. FDG answers where active disease is; FES answers whether those sites express a functional estrogen receptor. The two are complementary, and many patients benefit from the information both provide.

When is FES PET most appropriate?

The SNMMI appropriate use criteria identify the strongest uses as assessing estrogen receptor functionality when endocrine therapy is being considered at metastatic diagnosis or after progression, clarifying receptor status of lesions that are difficult or dangerous to biopsy, and resolving cases where other tests are inconclusive.

Do endocrine drugs interfere with FES PET?

Yes, but selectively. Estrogen receptor-blocking agents reduce FES uptake and can mask receptor-positive lesions: the prescribing information notes that tamoxifen and fulvestrant may block the receptor for up to about 8 and 28 weeks, respectively. Aromatase inhibitors and GnRH agonists lower estrogen levels but do not block the receptor itself, so they generally do not require interruption before FES.

What is the radiation dose from a FES PET scan?

The prescribing information estimates an effective dose of about 4.9 mSv from the recommended 222 MBq (6 mCi) administered activity in a 70 kg adult, with the liver, gallbladder, and uterus among the organs receiving the highest absorbed dose. The added CT for attenuation correction and localization contributes separately.

How much activity is given and when is imaging performed?

The recommended activity is 222 MBq (6 mCi), within a range of 111 to 222 MBq (3 to 6 mCi), given as a single intravenous injection. Image acquisition typically begins about 80 minutes after injection, though start times from 20 to 80 minutes and scan durations of 20 to 30 minutes may be tailored to the equipment and clinical question.

Can FES PET detect receptor heterogeneity between metastases?

Yes, and this is a distinctive strength. Because FES images the whole body in one study, it can show that some metastases are receptor-positive while others are receptor-negative in the same patient. This intrapatient heterogeneity has been associated with worse outcomes and is information a single biopsy cannot provide.

Key Takeaways

• FES images the estrogen receptor, not metabolism. It complements FDG by reporting whether disease is ER-positive and targetable, not just where it is active.
• It is a targeted problem-solver. The strongest indications are assessing ER functionality before or after endocrine therapy, clarifying hard-to-biopsy lesions, and resolving inconclusive results.
• Endocrine blockade matters. Tamoxifen and fulvestrant can suppress FES uptake for weeks; aromatase inhibitors and GnRH agonists generally do not.
• Dosimetry is modest and PET-standard. About 4.9 mSv from 222 MBq, imaged near 80 minutes, on ordinary F-18 PET/CT hardware.
• Whole-body imaging reveals heterogeneity. FES can show ER-positive and ER-negative metastases in the same patient — prognostic information a single biopsy cannot give.
• Quantitative QC underpins trust. Dose calibrator QC, scanner cross-calibration, and consistent uptake timing keep SUV-based ER assessment reliable.

Conclusion

F-18 fluoroestradiol PET added a genuinely new axis to breast cancer imaging: receptor status, whole body, noninvasive, repeatable. It does not replace FDG or biopsy, and it is not a general staging tool — its power is precision. When the clinical question is whether the estrogen receptor is present and functional across all sites of disease, FES answers it in a single scan, and it can surface heterogeneity that a needle in one lesion would never reveal.

Realizing that value depends on physics discipline as much as on interpretation: correct activity and timing, careful attention to receptor-blocking medications, and the quantitative quality control that keeps SUV meaningful. Facilities that treat FES as a quantitative study — not just another F-18 injection — will give their oncology colleagues receptor information they can actually act on.

How DRPS Can Help

Diagnostic Radiation Physics Services supports PET/CT and nuclear medicine facilities adopting FES and other targeted radiopharmaceuticals — with PET/CT and nuclear medicine physics, scanner performance and cross-calibration, dose calibrator quality control, quantitative SUV validation, accreditation support, and radiation safety and licensing guidance. All work is performed by board-certified medical physicists.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware.

A quantitative PET program is not just about producing a picture. It is about producing a number the oncology team can trust.

Related Resources

• F-18 PSMA PET imaging
• Ga-68 DOTATATE PET for neuroendocrine tumors
• PET SUV quantification
• PET uptake time
• PET/CT NEMA NU-2 performance testing
• PET/CT and nuclear medicine physics
• Accreditation support
• Medical physicist consulting

References

1. U.S. Food and Drug Administration. CERIANNA (fluoroestradiol F 18) injection prescribing information. 2020. accessdata.fda.gov
2. U.S. Food and Drug Administration. Drug Trial Snapshot: CERIANNA. 2020. fda.gov
3. Ulaner GA, Mankoff DA, Clark AS, et al. Summary: Appropriate Use Criteria for Estrogen Receptor-Targeted PET Imaging with 16α-18F-Fluoro-17β-Fluoroestradiol. Journal of Nuclear Medicine. 2023;64(3):351-354. doi:10.2967/jnumed.123.265420. PubMed
4. Society of Nuclear Medicine and Molecular Imaging. Appropriate Use Criteria for Estrogen Receptor-Targeted PET Imaging with 16α-18F-Fluoro-17β-Fluoroestradiol. 2022. snmmi.org
5. Huang YT, Chen TW, Chen LY, Huang YY, Lu YS. The Application of 18F-FES PET in Clinical Cancer Care: A Systematic Review. Clinical Nuclear Medicine. 2023;48(9):785-795. doi:10.1097/RLU.0000000000004760. PubMed
6. Currin E, Peterson LM, Schubert EK, et al. Temporal Heterogeneity of Estrogen Receptor Expression in Bone-Dominant Breast Cancer: 18F-Fluoroestradiol PET Imaging Shows Return of ER Expression. Journal of the National Comprehensive Cancer Network. 2016;14(2):144-147. doi:10.6004/jnccn.2016.0017. PubMed
7. van Geel JJL, Moustaquim J, Boers J, et al. Intrapatient 16α-[18F]Fluoro-17β-Estradiol PET Heterogeneity as a Prognostic Factor for Endocrine Therapy Response and Survival in Patients with Estrogen Receptor-Positive Metastatic Breast Cancer. Journal of Nuclear Medicine. 2025;66(2):194-200. doi:10.2967/jnumed.124.268984. PubMed
8. Ulaner GA, Silverstein M, Nangia C, et al. ER-Targeted PET for Initial Staging and Suspected Recurrence in ER-Positive Breast Cancer. JAMA Network Open. 2024;7(7):e2423435. doi:10.1001/jamanetworkopen.2024.23435. PubMed
9. U.S. Nuclear Regulatory Commission. 10 CFR Part 35: Medical Use of Byproduct Material. ecfr.gov
10. International Commission on Radiological Protection. ICRP Publication 107: Nuclear Decay Data for Dosimetric Calculations. Annals of the ICRP. 2008;38(3). icrp.org