[18F]fluciclovine vs. [18F]fluorocholine Positron Emission Tomography/Computed Tomography: A Head-to-Head Comparison for Early Detection of Biochemical Recurrence in Prostate Cancer Patients

Nowadays, there is still no consensus on the most accurate PET radiopharmaceutical to early detect prostate cancer (PCa) relapse. A tailored radiotracer choice based on a specific patient’s profile could ensure prompt disease detection and an improvement in patients management. We aimed to compare the [18F]fluciclovine and [18F]fluorocholine PET/CT detection rate (DR) in PCa patients restaged for early biochemical recurrence (BCR), according to clinical and biochemical features. A cohort of 138 PCa patients with early BCR (mean age: 71 y, range: 50–87 y) were homogeneously randomized 1:1 to a [18F]fluciclovine or a [18F]fluorocholine PET/CT group. The respective PET/CT DR, according to per-patient and per-region analysis, and the impact of the biochemical, clinical, and histological parameters, were compared. The PSA cut-off values predictive of a positive scan were also calculated. Overall, the [18F]fluciclovine PET/CT DR was 64%, significantly higher than the [18F]fluorocholine PET/CT DR of 35% (p = 0.001). Similarly, in the per-region analysis, the [18F]fluciclovine PET/CT DR was 51% in the prostate region, significantly higher compared to 15% of [18F]fluorocholine (p < 0.0001). Furthermore, a statistically significant higher DR in per-patient and per-region (prostate/prostate bed) analysis was observed in the [18F]fluciclovine group for 0.5–1 ng/mL (p = 0.018, p = 0.049) and >1 ng/mL (p = 0.040, p < 0.0001) PSA values. A PSA of 0.45 ng/mL for [18F]fluciclovine and of 0.94 ng/mL for [18F]fluorocholine was identified as the optimal cut-off value in predicting a positive PET/CT scan. Our results demonstrated a better [18F]fluciclovine PET/CT DR compared to [18F]fluorocholine for restaging PCa patients in early BCR, particularly in the detection of locoregional recurrence. The significantly higher [18F]fluciclovine DR for low PSA values (PSA < 1 ng/mL) supports its use in this setting of patients.


Introduction
According to the most recent GLOBOCAN 2020 database, prostate cancer (PCa) remains one of the most common tumors and the fifth leading cause of cancer-related death among men, with a worldwide estimation of 1.4 million diagnoses in 2020 and a prevalence of 59% (48-71%) [1].
Despite improvements in radical treatment with curative intent in clinically localized PCa, the risk of recurrence still occurs in 20-50% after radical prostatectomy (RP) and in 30-40% after radiotherapy (RT) [2,3].
Written informed consent was obtained for all the enrolled patients. The present study was approved by the local Ethics Committee (Prot. n. 0012052|08/02/2022|AOUCPG23| COMET|P) and performed in accordance with the principles of the Declaration of Helsinki and with national regulations.

Imaging Protocol and Analysis
All PET/CT examinations were performed in accordance with the European Association of Nuclear Medicine (EANM) guidelines [14]. No physical exercise the day before the exam and fasting for at least 4 h before injection of [ 18 F]fluciclovine were required; additionally, patients were asked not to void 30-60 min before imaging. A dose of approximately 370 MBq of [ 18 F]fluciclovine and 3-4 MBq/kg body weight (range 240-390 MBq/kg) of [ 18 F]fluorocholine was intravenously administered. All scans were obtained using a hybrid PET/CT scanner (Discovery 710, GE, General Electrics, Milwaukee, WI, USA). After 5 min post-injection, early [ 18 F]fluorocholine PET/CT images of the pelvis were acquired targeting the prostate area, followed by a whole-body image acquisition after 60 min, from the skull base to the proximal third of the femurs. To perform the [ 18 F]fluciclovine PET/CT exam, immediately after the intravenous radiopharmaceutical administration, a low dose CT scan without contrast agents started, followed by a PET acquisition at 3-5 min post-injection, from the mid-thigh to the base of the skull (5-6 bed positions). A 3D acquisition mode PET scan was performed for the same longitudinal coverage (2.5 min per bed position). Co-registered CT parameters were as follows: pitch 0.98, gantry rotation speed of 0.5 s/rot, 120 kV, and modulated tube current of 140 mA. CT images were used both for image fusion and anatomical localization and for attenuation correction of emission data.

Image Analysis
Image analysis was carried out using a dedicated console (AW Server 4.7, General Electrics, Milwaukee, WI, USA). PET/CT scans were independently evaluated by two nuclear medicine physicians with at least 3 years of experience in [ 18 F]fluciclovine and [ 18 F]fluorocholine PET/CT reading and aware of clinical data. In the event of disagreement, a final consensus was reached.
Maximum intensity projection (MIP), PET, CT, and PET/CT fused images in different planes (axial, sagittal, and coronal) were visualized simultaneously to correctly interpret scans. Examinations were considered positive in the presence of focal areas of detectable increased tracer uptake, visually more intense than the background, not correlating with physiological tracer uptake and inflammatory articular processes, with or without any underlying lesion identified on the co-registered CT [15,16]. The semiquantitative evaluation by the maximum standardized uptake value (SUV) and, for [ 18 F]fluciclovine PET/CT image, also by the SUV ratio (SUVmax in the lesion/SUVmean in the surrounding background) was used to aid visual analysis. In particular, bone marrow uptake of vertebra L3 was used as a reference for lesions larger than a 1 cm longest dimension, and abdominal aortic blood pool for lesions smaller than a 1 cm longest dimension [14].

Data Analysis
[ 18 F]fluciclovine and [ 18 F]fluorocholine PET/CT-positive findings were reported as "detection rate", defined as the proportion of scans containing one or more areas considered positive for cancer since histological confirmation was not available or feasible. The respective PET/CT DR were compared, according to per-patient and per-region analysis (prostate/prostatectomy bed, lymph node, bone). Finally, focusing on positive PET scans, the number of lesions in per-patient and per-region analyses and the impact of PSA level and PSAdt were compared between the two radiopharmaceuticals groups.

Statistical Analysis
Quantitative variables were expressed as mean with standard deviation. Student ttests provided an estimate of the difference between [
Patients' characteristics are described in detail in Table 1.
Comparing the two radiopharmaceuticals groups, a statistically significant higher per-patient DR was observed in the [ 18 F]fluciclovine group for 0.5-1 ng/mL (p = 0.018) and >1 ng/mL (p = 0.040) PSA levels, as reported in Table 2. This result was confirmed in the per-region analysis, especially in the prostate/prostatectomy bed (PSA 0.5-1 ng/mL: +32%, p = 0.049; PSA >1 ng/mL: +57%, p < 0.0001).  as the optimal cut-off values in predicting a positive PET/CT scan ( Figure 2). For PSAdt, the comparison between the two radiopharmaceuticals groups showed a statistically significant higher overall DR in the [ 18 F]fluciclovine group for a slower PSAdt value (p = 0.001). All per-patient DR results are reported in Table 3.

Clinical and Histological Parameters
Comparing the two radiopharmaceuticals groups, a statistically significant higher DR was identified in the [ 18 F]fluciclovine one for GS <8, low EAU BCR Risk Group, and in the absence of an ongoing HT. Table 4 reported all clinical-histological analysis results.

Clinical Cases
In Figures 3-6, representative clinical cases of our sample are reported.
Tomography 2022, 8,9 Finally, the PSA value and PSAdt significantly impacted the [ 18 F]fluorocholine PET/CT lesions number, characterized by a positive linear correlation with the PSA value (p = 0.004) and PSAdt (p = 0.010). Although not statistically significant, the same trend was observed in the [ 18 F]fluciclovine PET/CT group (p > 0.05).

Clinical Cases
In Figure 3, Figure 4, Figure 5, and Figure 6, representative clinical cases of our sample are reported.

Discussion
The cornerstone of BCR PCa patients management is the detection of disease relapse as early as possible for correctly selecting a local or systemic therapeutic strategy. For this purpose, the limited performance of conventional imaging has been overcome by PET/CT examination with a demonstrated superior DR for PSA level less than 2 ng/mL [17].
The most recent and updated 2022 European guidelines confirm the strategic role of PET/CT in this setting of patients. For PSA recurrence after primary treatment, radiolabeled PSMA PET/CT is recommended as the first-choice imaging method, when PSA is >0.2 ng/mL. Both radiolabeled choline and fluciclovine can be considered for possible identification of disease recurrence, in case of unavailability of PSMA, for PSA ≥1 ng/mL and if the imaging result will influence subsequent treatment decisions [5].
These recommendations are consequent to the known influence of PSA level and kinetic on choline DR and sensitivity, particularly dropping to suboptimal for low PSA values. Previous studies reported a radiolabeled choline PET/CT DR of 67-100% for PSA >5 ng/mL, which decreases to 5-24% when PSA is <1 ng/mL [18]. Despite this limitation, choline PET/CT demonstrated a pooled sensitivity and specificity of 86-89% and 89-93%, respectively, and a higher specificity in bone metastases detection compared to bone scan (98-100% vs. 75-100%) [8,19,20]. These data support choline use in this setting of patients, especially for PSA values over than 1 ng/mL [5,21]. Our results are consistent with the literature data, confirming the impact of PSA value and kinetic on the [ 18 F]fluorocholine PET/CT DR, which are 16% and 31% for PSA < 0.5 ng/mL and 0.5-1 ng/mL, respectively, and 21% for slower PSAdt (>12 months). The optimal PSA cut-off value of 0.94 ng/mL supports the recommendation of this radiotracer for PSA >1 ng/mL.
If the choice of [ 18 F]fluorocholine as a PET radiopharmaceutical is well-established, the use of [ 18 F]fluciclovine in clinical practice needs to be further investigated. Namely, our study aimed to compare the two [ 18 F]-labeled radiotracers DR, in order to guide the radiopharmaceutical choice based on a specific biochemical and clinical-histological patients' profile.
After FDA approval in May 2016, [ 18 F]fluciclovine became widely commercially available and it gained a growing scientific interest for its promising role in detecting local recurrent lesions. However, the literature data concerning its diagnostic performance in BCR patients with low PSA values are still limited and conflicting. In the largest cohort study (596 patients), Bach-Gansmo et al. reported a patient-level DR of 67.7%, with a lower DR of 41.4% for PSA values ≤0.79 ng/mL [22]. A similar pooled DR (65%) was observed by Dreyfuss et al. in a 328 PCa patient cohort but preserving an optimal DR (58%) also among 26 patients with PSA < 0.2 ng/mL [23]. Analogously, Marcus et al. found an overall DR of 58% in a population including 64 PCa patients with very low PSA value (≤0.3 ng/mL) [24]. These data were also recently confirmed in the retrospective study by Filippi et al., recording a promising performance for low PSA values (DR of 66.7% for PSA <0.57 ng/mL and of 71.4% for PSA 0.57-0.99 ng/mL). In addition, the authors identified 1.1 ng/mL as the optimal cut-off value in predicting a positive PET/CT scan [25]. The minimal PSA threshold to improve the [ 18 F]fluciclovine PET/CT DR in BCR PCa patients was already investigated by Armstrong et al., who suggested a PSA cut-off of 2.10 ng/mL [26]. Conversely, Wang et al. recommended performing [ 18 F]fluciclovine PET/CT in patients with PSA levels ranging from 0.3 to 1 ng/mL [27]. This range includes the PSA value of 0.45 ng/mL, identified as the optimal cut-off value in predicting a positive PET scan in a previous study of our group and confirmed in the current analysis [9]. Despite the different PSA thresholds proposed, most authors agree in considering [ 18 F]fluciclovine PET/CT as a reliable method also for PSA lower than the recommended EAU guideline value of 1 ng/mL.
In addition, the good diagnostic performance of the [ 18 F]fluciclovine PET/CT for a low PSA value (DR 43% for PSA < 0.5 ng/mL; 61% for PSA 0.5-1 ng/mL), highlighted by the literature data and confirmed in our study, should lead us to re-evaluate the current clinical recommendations for a more precise and specific application of PET prostate-radiotracers. Hence, to guide a personalized choice of the most adequate radiopharmaceutical, it is of utmost importance to compare the diagnostic performance of the [ 18 F]fluciclovine and [ 18 F]fluorocholine PET/CT, the latter representing the standard of reference for PCa imaging in the last decades.
To the best of our knowledge, the literature reported only a few head-to-head comparative studies between choline labeled with [ 11 C] and [ 18 F]fluciclovine PET/CT diagnostic performance in small BCR patient populations [28][29][30]. The authors reported a higher [ 18 F]fluciclovine DR of 20%-40% at patient-based and approximately 60% at lesion-based analysis compared to [ 11 C]choline, due to a negligible urinary excretion and a high tumorto-background ratio of the amino acid tracer [30].
However, despite the advantage of a lower rate of radioactivity in the bladder after [ 11 C]choline administration, [ 18 F]fluorocholine has a more widespread employment thanks to more favorable and lower positron energy (252 vs. 390 MeV) and positron range (0.66 vs. 1.27 mm), and a longer physical half-life (109.8 vs. 20.3 min), allowing its use in many PET centers without on-site cyclotron [28].
To Regarding the impact of different biochemical parameters on prostate-radiopharmaceuticals DR, the [ 18 F]fluciclovine PET/CT was superior both for low (0.5-1 ng/mL) and for high PSA levels (>1 ng/mL). Particularly, the results for the PSA range 0.5-1 ng/mL deserve consideration, since [ 18 F]fluciclovine could be very promising (DR +40%) also for PSA < 1 ng/mL. These data were confirmed in prostatic region analysis, both for low (0.5-1 ng/mL, +32%) and, especially, for high PSA levels (>1 ng/mL, +57%), further validating the greater performance in prostate/prostatectomy bed lesions detection.
Unlike The [ 18 F]fluciclovine PET/CT superiority was also evidenced for low GS (+36%) and for the low EAU BCR risk group (+44%). In these cases, the small amount of [ 18 F]fluorocholine uptake could be explained by a slow membrane metabolism, reflecting the weak proliferation of PCa cells, and by the different [ 18 F]fluciclovine mechanism of uptake. Therefore, the [ 18 F]fluciclovine PET/CT could prove to be very reliable imaging in low-risk patients, allowing accurate detection of the T and N parameters.
Taking into account that a precise and early detection of the recurrent site is imperative to correctly guide treatment strategies, the availability of an effective imaging modality in the case of low PSA values is crucial for optimal patient management [26].
Lastly, even if PSMA binding ligands have generally shown higher diagnostic performance when compared to other radiotracers in biochemical recurrence, the [ 18 F]fluciclovine PET/CT appeared a more accurate method in detecting localized recurrence close to the bladder, as PSMA urinary activity may mask avid lesions [33]. Furthermore, [ 18 F]fluciclovine could represent a promising alternative, especially for tumors characterized by PSMAexpression heterogeneity and for the non-negligible percentage of PSMA-negative patients [34].
Our study had some limitations: first, the unavailability of the histopathological confirmation of positive PET/CT, but consistent with clinical practice; second, the lack of comparative data between the [ 18 F]fluciclovine and [ 18 F]fluorocholine PET/CT about sensitivity, specificity, and their impact on patient management, because follow-up is still ongoing; third, the execution of the [ 18 F]fluciclovine or [ 18 F]fluorocholine PET/CT in our sample may have influenced the results of our study since the two radiopharmaceuticals have a different metabolism.

Conclusions
An accurate choice of the most adequate radiotracer for detecting PCa relapse should be tailored to patient profile, considering clinical, biochemical, and histological parameters.
Our comparative analysis demonstrated a better [ 18 F]fluciclovine PET/CT DR compared to [ 18 F]fluorocholine PET/CT for restaging PCa patients in early BCR, particularly in the detection of locoregional recurrence, also due to a negligible urinary excretion. The significantly higher [ 18 F]fluciclovine DR for low PSA values supports its use in clinical practice in this setting of patients (PSA < 1 ng/mL). This personalized approach allows for improved assessment of PCa recurrent disease, thanks to an earlier and more precise lesions detection, consequently optimizing patients management.
Funding: This research received no external funding.

Institutional Review Board Statement:
The study was conducted in accordance with the Declaration of Helsinki, and approved by the Ethics Committee of the Azienda Ospedaliera Universitaria Policlinico, Bari; Italy (protocol code: 0012052|08/02/2022|AOUCPG23|COMET|P; date of approval: 8 February 2022).