Imaging tumor phenotype: 1 plus 1 is more than 2

Exploring Tumor Heterogeneity Using PET Imaging: The Big Picture

Personalized medicine represents a major goal in oncology. It has its underpinning in the identification of biomarkers with diagnostic, prognostic, or predictive values. Nowadays, the concept of biomarker no longer necessarily corresponds to biological characteristics measured ex vivo but includes complex physiological characteristics acquired by different technologies. Positron-emission-tomography (PET) imaging is an integral part of this approach by enabling the fine characterization of tumor heterogeneity in vivo in a non-invasive way. It can effectively be assessed by exploring the heterogeneous distribution and uptake of a tracer such as 18F-fluoro-deoxyglucose (FDG) or by using multiple radiopharmaceuticals, each providing different information. These two approaches represent two avenues of development for the research of new biomarkers in oncology. In this article, we review the existing evidence that the measurement of tumor heterogeneity with PET imaging provide essential information in clinical practice for treatment decision-making strategy, to better select patients with poor prognosis for more intensive therapy or those eligible for targeted therapy.

Estrogen Receptor Binding (18F-FES PET) and Glycolytic Activity (18F-FDG PET) Predict Progression-Free Survival on Endocrine Therapy in Patients with ER+ Breast Cancer

PURPOSE

18F-fluoroestradiol (FES) PET scans measure regional estrogen binding, and 18F-fluorodeoxyglucose (FDG) PET measures tumor glycolytic activity. We examined quantitative and qualitative imaging biomarkers of progression-free survival (PFS) in breast cancer patients receiving endocrine therapy.

EXPERIMENTAL DESIGN

Ninety patients with breast cancer from an estrogen receptor-positive (ER+), HER2- primary tumor underwent FES PET and FDG PET scans prior to endocrine therapy (63% aromatase inhibitor, 22% aromatase inhibitor and fulvestrant, 15% other). Eighty-four had evaluable data for PFS prediction.

RESULTS

Recursive partitioning with 5-fold internal cross-validation used both FES PET and FDG PET measures to classify patients into three distinct response groups. FDG PET identified 24 patients (29%) with low FDG uptake, suggesting indolent tumors. These patients had a median PFS of 26.1 months (95% confidence interval, 11.2-49.7). Of patients with more FDG-avid tumors, 50 (59%) had high average FES uptake, and 10 (12%) had low average FES uptake. These groups had median PFS of 7.9 (5.6-11.8) and 3.3 months (1.4-not evaluable), respectively. Patient and tumor features did not replace or improve the PET measures' prediction of PFS. Prespecified endocrine resistance classifiers identified in smaller cohorts did not individually predict PFS.

CONCLUSIONS

A wide range of therapy regimens are available for treatment of ER+ metastatic breast cancer, but no guidelines are established for sequencing these therapies. FDG PET and FES PET may help guide the timing of endocrine therapy and selection of targeted and/or cytotoxic chemotherapy. A multicenter trial is ongoing for external validation. Clin Cancer Res; 23(2); 407-15. ©2016 AACR.

Oestrogen-related tumour phenotype: positron emission tomography characterisation with ¹⁸F-FDG and ¹⁸F-FES

This article outlines the role of 16α-[(18)F]fluoro-17β-oestradiol ((18)F-FES) positron emission tomography (PET) combined with 2-[(18)F]fluoro-2-deoxy-D-glucose ((18)F-FDG) in patients with oestrogen-related tumours for evaluating tumour phenotype. (18)F-FES-PET combined with (18)F-FDG is helpful in characterising the distinct phenotypic features of oestrogen-related tumours; that is, inter- and intrapatient tumour heterogeneity, which indicates its great potential as a determinant of individualised treatment and a prognostic predictor for patients with oestrogen-related tumours.

Between-patient and within-patient (site-to-site) variability in estrogen receptor binding, measured in vivo by 18F-fluoroestradiol PET

Heterogeneity of estrogen receptor (ER) expression may be an important predictor of breast cancer therapeutic response. (18)F-fluoroestradiol PET produces in vivo quantitative measurements of regional estrogen binding in breast cancer tumors. We describe within-patient (site-to-site) and between-patient heterogeneity of lesions in patients scheduled to receive endocrine therapy.

METHODS

In 91 patients with a prior ER-positive biopsy, 505 lesions were analyzed for both (18)F-fluoroestradiol and (18)F-FDG uptake and the (18)F-fluoroestradiol/(18)F-FDG uptake ratio. Standardized uptake values (SUVs) were recorded for up to 16 lesions per patient, of 1.5 cm or more and visible on (18)F-FDG PET or conventional staging. Linear mixed-effects regression models examined associations between PET parameters and patient or lesion characteristics and estimated variance components. A reader study of SUV measurements for 9 scans further examined sources of within-patient variability.

RESULTS

Average (18)F-fluoroestradiol uptake and (18)F-fluoroestradiol/(18)F-FDG ratio varied greatly across these patients, despite a history of ER-positive disease: about 37% had low or absent (18)F-fluoroestradiol uptake even with marked (18)F-FDG uptake. (18)F-fluoroestradiol SUV and (18)F-fluoroestradiol/(18)F-FDG ratio measurements within patients with multiple lesions were clustered around the patient's average value in most cases. Summarizing these findings, the intraclass correlation coefficient (proportion of total variation that is between-patient) was 0.60 (95% confidence interval, 0.50-0.69) for (18)F-fluoroestradiol SUV and 0.65 (95% confidence interval, 0.56-0.73) for the (18)F-fluoroestradiol/(18)F-FDG ratio. Some within-patient variation in PET measures (22%-44%) was attributable to interobserver variability as measured by the reader study. A subset of patients had mixed uptake, with widely disparate (18)F-fluoroestradiol SUV or (18)F-fluoroestradiol/(18)F-FDG ratio for lesions in the same scan.

CONCLUSION

(18)F-fluoroestradiol uptake and the (18)F-fluoroestradiol/(18)F-FDG ratio varied greatly between patients but were usually consistent across lesions in the same scan. The average (18)F-fluoroestradiol SUV and (18)F-fluoroestradiol/(18)F-FDG ratio for a limited sample of lesions appear to provide a reasonable summary of synchronous ER expression for most patients. However, imaging the entire disease burden remains important to identify the subset of patients with mixed uptake, who may be at a critical point in their disease evolution.

Fluoroestradiol positron emission tomography reveals differences in pharmacodynamics of aromatase inhibitors, tamoxifen, and fulvestrant in patients with metastatic breast cancer

PURPOSE

To determine, by molecular imaging, how in vivo pharmacodynamics of estrogen-estrogen receptor (ER) binding differ between types of standard endocrine therapy.

EXPERIMENTAL DESIGN

The ER has been a highly successful target for breast cancer treatment. ER-directed treatments include lowering ligand concentration by using aromatase inhibitors (AI) and blocking the receptor with agents like tamoxifen (TAM) or fulvestrant (FUL). We measured regional estrogen-ER binding by using positron emission tomography with (18)F-fluoroestradiol (FES PET) prior to and during treatment with AI, TAM, or FUL in a series of 30 metastatic breast cancer patients. FES PET measured in vivo estrogen binding at all tumor sites in heavily pretreated women with metastatic bone soft tissue-dominant breast cancer. In patients with uterus (n = 16) changes in uterine FES uptake were also measured.

RESULTS

As expected, tumor FES uptake declined more markedly on ER blockers (TAM and FUL, average 54% decline) compared with a less than 15% average decline on estrogen-depleting AIs (P < 0.001). The rate of complete tumor blockade [FES standardized uptake value (SUV) ≤1.5] following TAM (5/5 patients) was greater than the blockade rate following FUL (4/11; 2-sided mid P = 0.019). Percent FES SUV change in the uterus showed a strong association with tumoral change (ρ = 0.63, P = 0.01).

CONCLUSIONS

FES PET can assess the in vivo pharmacodynamics of ER-targeted agents and may give insight into the activity of established therapeutic agents. Imaging revealed significant differences between agents, including differences in the efficacy of blockade by different ER antagonists in current clinical use.

Functional oestrogen receptor α imaging in endometrial carcinoma using 16α-[¹⁸F]fluoro-17β-oestradiol PET

PURPOSE

To investigate the correlation between uptake of 16α-[(18)F]fluoro-17β-oestradiol (FES) and expression of oestrogen receptors as well as other related immunohistochemistry markers, positron emission tomography (PET) was performed in patients with endometrial carcinoma before surgery.

METHODS

Nineteen patients with endometrioid adenocarcinoma underwent preoperative PET studies with FES and 2-[(18)F]fluoro-2-deoxy-D: -glucose (FDG). Standardized uptake values (SUVs) for each tracer and the regional FDG to FES SUV ratio were calculated using images after coregistration. PET values were compared with postoperative stage, differentiation grade and immunohistochemical scores including oestrogen receptor subtypes (ERα, ERβ), progesterone receptor B (PR-B), Ki-67 and glucose transporter 1 (GLUT1).

RESULTS

FES uptake showed a significantly positive correlation with expression of ERα. The FDG to FES ratio showed a significantly negative correlation with expression of ERα and PR-B. The FES uptake and FDG to FES ratio did not correlate with expression of ERβ, Ki-67 or GLUT1. FDG uptake was not correlated with any of the immunohistochemical scores. The PR-B score was strongly correlated with the ERα score. Well-differentiated carcinoma (grade 1) showed a significantly higher FES uptake and significantly lower FDG to FES ratio than moderately or poorly differentiated carcinoma (grade 2-3). None of the PET parameters were significantly different between advanced-stage carcinoma (≥ stage IB) and early-stage carcinoma (IA) based on the Féderation International de Gynécologie et d'Obstétrique (FIGO) staging classification. Differentiation grade was the most closely correlated parameter to FES uptake and FDG to FES ratio by multivariate analyses.

CONCLUSION

FES PET combined with FDG would be useful for non-invasive evaluation of ERα distribution, as well as ERα function, which reflects differentiation grade in endometrial carcinoma.