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Visual and quantitative evaluation of [18F]FES and [18F]FDHT PET in patients with metastatic breast cancer: an interobserver variability study

Purpose Correct identification of tumour receptor status is important for treatment decisions in breast cancer. [18F]FES PET and [18F]FDHT PET allow non-invasive assessment of the oestrogen (ER) and androgen receptor (AR) status of individual lesions within a patient. Despite standardised analysis techniques, interobserver variability can significantly affect the interpretation of PET results and thus clinical applicability. The purpose of this study was to determine visual and quantitative interobserver variability of [18F]FES PET and [18F]FDHT PET interpretation in patients with metastatic breast cancer. Methods In this prospective, two-centre study, patients with ER-positive metastatic breast cancer underwent both [18F]FES and [18F]FDHT PET/CT. In total, 120 lesions were identified in 10 patients with either conventional imaging (bone scan or lesions > 1 cm on high-resolution CT, n = 69) or only with [18F]FES and [18F]FDHT PET (n = 51). All lesions were scored visually and quantitatively by two independent observers. A visually PET-positive lesion was defined as uptake above background. For quantification, we used standardised uptake values (SUV): SUVmax, SUVpeak and SUVmean. Results Visual analysis showed an absolute positive and negative interobserver agreement for [18F]FES PET of 84% and 83%, respectively (kappa = 0.67, 95% CI 0.48–0.87), and 49% and 74% for [18F]FDHT PET, respectively (kappa = 0.23, 95% CI − 0.04–0.49). Intraclass correlation coefficients (ICC) for quantification of SUVmax, SUVpeak and SUVmean were 0.98 (95% CI 0.96–0.98), 0.97 (95% CI 0.96–0.98) and 0.89 (95% CI 0.83–0.92) for [18F]FES, and 0.78 (95% CI 0.66–0.85), 0.76 (95% CI 0.63–0.84) and 0.75 (95% CI 0.62–0.84) for [18F]FDHT, respectively. Conclusion Visual and quantitative evaluation of [18F]FES PET showed high interobserver agreement. These results support the use of [18F]FES PET in clinical practice. In contrast, visual agreement for [18F]FDHT PET was relatively low due to low tumour-background ratios, but quantitative agreement was good. This underscores the relevance of quantitative analysis of [18F]FDHT PET in breast cancer. Trial registration ClinicalTrials.gov, NCT01988324. Registered 20 November 2013, https://clinicaltrials.gov/ct2/show/NCT01988324?term=FDHT+PET&draw=1&rank=2.


Introduction
Breast cancer is the most common malignancy in women in the Western world. The majority of breast tumours express the oestrogen receptor (ER), which is the main indicator of potential response to anti-oestrogen therapies [1,2]. Therefore, it is mandatory to determine ER expression in breast cancer. Recently, the androgen receptor (AR) emerged as a possible target for breast cancer therapy. The AR is present in 70-80% of patients with breast cancer, and AR antagonists are under investigation in clinical trials [3][4][5][6].
A tumour biopsy is the gold standard to determine receptor expression. However, this is an invasive procedure, is not always feasible in case of inaccessible tumour sites, and is subject to sampling errors [7]. The 16α-[ 18 F]fluoro-17β-oestradiol ([ 18 F]FES) and 16β-[ 18 F]fluoro-5α-dihydrotestosterone ([ 18 F]FDHT) PET/CT have been developed to non-invasively visualise, respectively, the ER and AR status in the tumour lesions within a patient. Previously, it has been shown that [ 18 F]FES and [ 18 F]FDHT uptake correlate well with ER and AR expression levels in representative breast cancer biopsies [8][9][10]. As a diagnostic tool, [ 18 F]FES PET leads to better diagnostic understanding in 88% and to a change of therapy in 48% of the patients presenting with a clinical dilemma [11]. To predict treatment effects, [ 18 F]FES PET can be used to assess residual ER availability during treatment with, e.g. fulvestrant, a selective ER downregulator. Inadequate reduction of the [ 18 F]FES PET signal (< 75%) by fulvestrant treatment was associated with early progression [12]. Similarly, in patients with prostate cancer, [ 18 F]FDHT PET was used to determine the optimal dose of the AR blocker enzalutamide in a phase 1 trial [13]. Lastly, patients with ERpositive breast cancer and high [ 18 F]FDG uptake showed a worse progression free survival if [ 18 F]FES uptake was low in comparison to high [ 18 F]FES uptake (3 versus 8 months, respectively) [14].
For all these potential applications, reliable, observerindependent identification and quantification of [ 18

Patient population
This prospective two-centre interobserver variability study was part of a study investigating the correlation between [ 18 F]FES and [ 18 F]FDHT uptake and ER and AR expression in simultaneously biopsied metastases, of which the results have been published elsewhere [8]. Patients were recruited from September 2014 to August 2015 at the CCA-VUmc University Medical Center Amsterdam and the University Medical Center Groningen in the Netherlands.
Eligibility criteria included metastatic breast cancer and an ER-positive primary tumour, ≥ 1 extrahepatic tumour lesion, ECOG performance status of ≤ 2 and a postmenopausal status or use of LHRH-agonists. Patients were excluded if they had used ER or AR binding drugs during the 6 weeks before study entry, because these ligands compete with tracer binding.
All patients had to give written informed consent before study participation. The study was conducted in compliance with the ethical principles originating in or derived from the Declaration of Helsinki and in compliance with all International Conference on Harmonization Good Clinical Practice guidelines. The local medical ethics committee approved the study (NCT01988324).

Imaging protocols
[ 18 F]FES and [ 18 F]FDHT were produced as described previously [15,16]. On separate days, ≤ 14 days apart, 200 MBq (± 10%) of each tracer was injected. After 60 min (± 5 min), a low-dose CT was performed during tidal breathing for attenuation correction, followed by a whole-body PET scan (skull vertex to mid-thigh, 2 min per bed position). PET/CT scans were made using a Philips Gemini TF-64 PET/CT (Amsterdam) or Siemens 64 slice mCT PET/CT (Groningen). Acquisition and reconstruction protocols used on both scanners were according to the recommendations of the European Association of Nuclear Medicine (EARL) [17].
In addition, a high-resolution, contrast-enhanced CT chest-abdomen and bone scan was performed within 6 weeks of the PET scans for comparison.

Image analyses
Contrast enhanced CT scans were examined by experienced radiologists and bone scans by experienced nuclear medicine physicians, respectively, masked for the [ 18 F]FES and [ 18 F]FDHT PET results. Two independent observers from each centre (LM and CV), trained and supervised by two experienced nuclear medicine physicians, performed visual and quantitative analyses. The observers had knowledge of conventional imaging results (contrast enhanced CT and bone scans).
A visually PET-positive lesion was defined as focal uptake above local background incompatible with physiological uptake. Liver metastases were excluded from all analyses in this study because of high physiological [ 18 F]FES and [ 18 F]FDHT uptake in healthy liver tissue, making reliable identification of metastases difficult. In addition, if visual interpretation of uptake in a (potential) lesion was impossible, e.g. due to overlap with adjacent organs with high physiological tracer, the readers independently reported it as 'not evaluable' in the visual ratings, and these were excluded from further analyses. For each patient, the observers made a list that consisted of all lesions already detected on conventional imaging, followed by additional lesions discovered on [ 18 F]FES or [ 18 F]FDHT PET. An anatomical description of all the lesions was reported in order to match the results. In case a lesion was not reported by one of the two observers, it was scored as not visible for that observer. All visually PET-positive lesions were quantified, as well as PETnegative lesions that were identified on conventional imaging (i.e. lesions on bone scintigraphy and/or high resolution CT > 1 cm).
Each observer manually drew volumes of interest (VOI) on the tumour contours, using PET images for PET-positive lesions and low-dose CT images for PETnegative lesions (lesions only seen on bone scan or highresolution CT were visually matched on the low-dose CT). Lesions were separately analysed based on visibility on either PET or conventional imaging alone to investigate the influence of visibility on imaging techniques on interobserver agreement.
For every VOI, the standardised uptake values (SUV), i.e. the tracer uptake within a VOI normalised to the injected dose and body weight, were calculated using the software programs accurate (in-house build using IDL, observer 1) and syngo.via version VB10B, Siemens (observer 2). Both programs yielded identical results on test images. Three types of SUV were compared in this study: SUV max (voxel with highest SUV within the VOI), SUV peak (average SUV of a 1 cm 3 sphere containing the hottest voxels of the VOI) and SUV mean with isocontour 50% of SUV max (average SUV of all voxels with uptake ≥ 50% of SUV max ).
Based on previous studies, an SUV max [ 18 F]FES cut-off ≥ 1.5 was used to define ER-positivity (corresponding with a IHC cut-off of ≥ 1%) and an SUV max [ 18 F]FDHT cut-off ≥ 1.9 for AR positivity (corresponding with a IHC cut-off of ≥ 10%) [8,9].
For [ 18 F]FES and [ 18 F]FDHT, the SUV max tumourbackground ratio (TBR) was defined as the ratio of the SUV max of a tumour lesion and the SUV mean of healthy background tissue. To determine the SUV mean of healthy background tissue, a VOI was drawn on reference tissue in the unaffected contralateral site whenever available or in the unaffected surrounding tissue of the same origin [18].

Statistical analyses
For visual assessments, agreement was calculated with absolute and relative measures of interobserver agreement. Absolute agreement is the probability that if one observer would score a lesion as visible (positive agreement) or not visible (negative agreement) on the PET scan, the other observer would do the same. It is calculated by the following formulas: positive agreement = 2 × lesions visible to both observers/(2 × lesions visible to both observers + lesions only visible to observer 1 + lesions only visible to observer 2) and negative agreement = 2 × lesions not visible to both observers/(2 × lesions not visible to both observers + lesions only not visible to observer 1 + lesions only not visible to observer 2) [19]. In order to compare results with previous studies, also reliability (relative agreement) was calculated according to Cohen's kappa, and the results were interpreted as follows: kappa 0.01-0.20 as slight, 0.21-0.40 as fair, 0.41-0.60 as moderate, 0.61-0.80 as substantial and 0.81-1.00 as almost perfect interobserver agreement [20]. To account for potential within-person correlation in visual assessments, a chi-square test was performed to examine whether the percentage visual agreement differed per patient.
For quantitative assessments, parameters are presented as mean ± SD, and reliability was calculated with intraclass correlation coefficients (ICC) using a two-way random effect model with absolute agreement. For the interpretation of the ICCs, the following guideline was used: ≥ 0.90 as excellent, ≥ 0.75 as good, ≥ 0.50 as moderate and < 0.50 as poor [21].
Absolute agreement on quantitative assessments were analysed with Bland-Altman plots (differences between observers showed a normal distribution). For each lesion, it graphically shows the average SUV of observers 1 and 2 on the x-axes and on the y-axes the difference between observers for each lesion, expressed as percentage of the average SUV value. Percentage differences were used instead of absolute differences to achieve independence of magnitude of differences from magnitude of SUV values, and it facilitates comparisons between the SUV parameters SUV max , SUV mean and SUV peak , which may show large differences in absolute values.
To investigate the effect of TBRs on interobserver variability, differences between TBRs of [ 18 F]FES and [ 18 F]FDHT PETs were tested with Wilcoxon matched pairs signed rank tests. In addition, correlations between tracer uptake or tumour size and percentage interobserver differences were determined using the Spearman correlation coefficient (r). Finally, linear regression was performed to find the linear function between SUV max , SUV peak and SUV mean for [ 18 F]FES and [ 18 F]FDHT PET, and Cochran's Q and McNemar tests were used to analyse differences between visibility and quantitative uptake above or below cut-off for SUV max , SUV peak and SUV mean . P value < 0.05 was considered significant. Statistical analyses were generated using the SPSS software (version 22; IBM, SPSS statistics).

Patient characteristics
A total of 120 lesions were identified in 10 patients using the different imaging modalities (Table 1). Most lesions were skeletal (66%), followed by lymph node (25%) and visceral metastases (9%). The median number of lesions per patient was 9 (range 2-32).

Comparison of lesion detection on different imaging modalities
Of the 120 lesions in total (   In general, interobserver agreement was excellent for PET quantification (Fig. 3) 5) quantification was analysed separately with Bland Altman plots for all lesions visible on PET or lesions only visible on conventional imaging (hence, PET-negative lesions). For [ 18 F]FES PET, PET-positive lesions showed excellent quantitative interobserver agreement with mean differences < 2% and 95% limits of agreement (LOA 95% ) being narrower for SUV max (LOA 95% − 31.3 to 34.3%) and SUV peak (LOA 95% − 31.1 to 28.4%), compared to SUV mean (LOA 95% − 46.5 to 44.3%). More differences were shown for PET-negative lesions with mean interobserver differences < 14% and larger LOA 95% (within ± 75%), but note that absolute differences between observers were generally low due to a low SUV. Similarly, for [ 18 F]FDHT PET, interobserver agreement was better for PET-positive (mean interobserver differences < 7%, LOA 95% within ± 45 %) compared to PET-negative lesions (mean interobserver differences < 12%, LOA 95% within ± 76%). SUV max and SUV peak showed a better interobserver agreement in comparison to SUV mean for the quantification of lesions visible on [ 18 F]FES PET, while on [ 18 F]FDHT PET the different SUV parameters were comparable.
Higher levels of tracer accumulation in PET positive lesions were not associated with improved interobserver agreement (for [ 18 F]FES PET: Spearman r = 0.04, 0.26 and 0.14 for SUV max , SUV peak and SUV mean , respectively and for [ 18 F]FDHT PET: Spearman r = 0.00, r = 0.03 and r = − 0.17, respectively). In addition, there was no correlation between tumour size and interobserver agreement (for [ 18 F]FES PET: Spearman r = 0.10, r = 0.08 and r = 0.06, for SUV max , SUV peak and SUV mean , respectively and for [ 18   For diagnostic purposes, it is important to identify all receptor positive tumour lesions. Therefore, we compared visual and quantitative tracer uptake above/below cut-off levels ( Table 3). In 3% and 1% of the lesions scored visually positive on [ 18 F]FES PET by observer 1 and 2 respectively, SUV max was below the threshold of 1.5. For [ 18 F]FDHT PET, 14% of the visually positive lesions scored by observer 1 as well as observer 2 had an SUV max below the threshold of 1.9. There were no structural differences between observer 1 and 2. The discrepancies were mostly seen in lesions located in tissue with low background uptake such as skin and lung metastases (Supplementary table S1). Conversely, in 44% and 39% of the lesions scored visually negative on [ 18 F]FES PET by observer 1 and 2, respectively, SUV max was ≥ 1.5. Similarly, 31% and 52% of the visually negative lesions had an SUV max ≥ 1.9 on [ 18 F]FDHT PET, respectively. However, in most cases (60%), we observed overlap with organs having high physiological tracer accumulation such as the liver and bowel, followed by lesions that were determined to be visually positive at second glance (32%). After correction for these effects, ≤ 4% of the visually negative lesions had a SUV max above cut-off for both tracers.
Comparing the impact of the different SUV parameters on discrepancies between visual and quantitative assessments showed no significant differences with the only exception that SUV mean showed less visually negative lesions above cut-off on [ 18 FES]PET than SUV max or SUV peak for observer 1 (P = 0.008 and P = 0.001, respectively), but not for observer 2 (P = 0.125 and P = 0.063, respectively).

Discussion
Interobserver variability is an important step in the clinical application of diagnostic tools. Here, we showed that both visual and quantitative evaluation were highly reproducible between independent observers evaluating [ 18 F]FES PET at separate centres using different scanners and software. Visual positive and negative absolute agreement was > 80%, with a kappa of 0.67. Also, the interobserver reliability of quantitative metrics was excellent for SUV max and SUV peak (ICC of 0.98 and 0.97, respectively) and good for SUV mean (ICC of 0.89). In comparison, staging patients with breast cancer showed similar results for bone scintigraphy (kappa 0.62-0.78) and [ 18 F]FDG PET (kappa 0.65 and an ICC of 0.93 for the quantification of [ 18 F]FDG uptake) [22][23][24][25][26]. ]FDHT in the current study (2.0 for bone and 4.6 for lymph nodes) was also lower than in prostate cancer metastases (3.3 for bone and 5.7 for soft tissue metastases) with an SUV max three times higher in prostate cancer (7.1-9.1 versus 2.0 in the present breast cancer study) [27,28]. This suggests that higher AR expression likely results in better interobserver reliability.
Our study had some limitations. There were only a limited number of patients included in this study. However, receptor expression between lesions within a single patient can be heterogeneous [29], which was confirmed in the present study resulting in the coverage of a large range of data in 120 lesions [8]. In addition, we showed there was no within-patient correlation in visual assessments. A second limitation is a substantial number of 'not evaluable' lesions, due to overlap with adjacent organs with high physiological background. The decision for evaluability was left to each observer individually, which may have contributed to the low agreement (≤ 6%) on these 'not evaluable lesions'. For future studies, we recommend that all lesions with physiological background overlap from the liver, gallbladder, intestine, bladder and for [ 18 [8,9,30,31]. Some studies suggested an SUV max cut-off of 2.0 for [ 18 F]FES PET, taking into account the background [ 18 F]FES uptake in normal tissues which can exceed the cut-off of 1.5 [29][30][31]. Tissue specific cut-off values may indeed be more appropriate as there are responders to endocrine therapy with a tumour SUV max < 2.0. In the current study, up to 20% of the visually positive lesions had an SUV max < 2.0, while < 3% had an SUV max < 1.5 (Supplementary table S2).
For diagnostic purposes, simple visual assessment of [ 18 F]FES uptake may suffice to determine the receptor status of a tumour lesion (agreement was high between visual assessment and the applied SUV max cut-off value of 1.5 for ER-positivity). True discrepancies between visibility and corresponding uptake above or below cut-off were low (< 4%), making quantification of visually negative lesions not only cumbersome, but also unnecessary. Also, quantification of lesions without visual [ 18 F]FES uptake leads to higher interobserver variability due to differences in VOI definition. However, quantification remains a helpful tool for nuclear medicine physicians in 'equivocal [ 18 F]FES lesions'. In addition, quantification is useful to measure receptor availability over time for the evaluation of treatment effects. In contrast, quantification of [ 18 F]FDHT uptake is still required in future breast cancer studies, as we have shown relatively low visual agreement.
The role of [ 18 F]FES and [ 18 F]FDHT PET in addition to conventional imaging modalities needs to be defined further. It has to be taken into account that besides partial volume effects and constraints due to background tracer uptake limiting their detection, receptor expression can be heterogeneous and variable during the course of the disease [11,32]. In addition, treatment may induce changes in receptor expression, but also eradicated tumour cells can leave a visible lesion on conventional imaging (e.g. sclerotic bone lesions), in absence of viable tumour cells. In the current study with heavily pretreated patients, 42-46% and 26-29% of the lesions identified by conventional imaging were detected on [ 18 [33]. We are currently investigating [ 18 F]FES PET in staging patients with low grade, ER-positive locally advanced or recurrent breast cancer versus [ 18 F]FDG PET (NCT03726931), and in metastatic breast cancer versus addition to conventional diagnostics (NCT01957332). The non-invasive visualisation of receptor status in metastatic lesions with PET offers a number potential clinical advantages. For example, in case conventional diagnostics cannot establish a final diagnosis of suspected metastatic breast cancer lesions (e.g. as a result of inaccessible biopsy sites or repeated biopsy sampling errors). Also, PET imaging may help to determine the hormone receptor status of different tumour sites within a patient and guide treatment decisions, for instance, to decide on the origin of a metastatic lesion in case of multiple primary tumours or to determine whether receptor conversion occurred in metastases from a single primary tumour [11]. If validated, this may help with multimodality treatment strategies for heterogeneous tumour sites of breast cancer, such as endocrine therapy for [ 18 F]FES positive lesions combined with a local modality such as radiotherapy for concurrent [ 18 F]FES negative lesions [34].

Conclusion
In conclusion, our findings demonstrate that visual and quantitative evaluation of [ 18 F]FES PET has a high interobserver concordance and support the use in clinical practice. Although [ 18 F]FDHT PET showed relatively low visual agreement, presumably a result of the low AR expression and consequently low TBR in patients with breast cancer, there was good quantitative agreement between observers, acceptable for further [ 18 F]FDHT PET imaging studies in breast cancer.