Positron Emission Tomography in Prostate Cancer: Summary of Systematic Reviews and Meta-Analyses

Prostate cancer is a prevalent public health problem worldwide. Over the past decade, there has been tremendous research activity in the potential use of positron emission tomography with a number of radiotracers targeted to various biological aspects of this complex tumor. Systematic reviews and meta-analyses are important contributions to the relevant literature that summarize the evidence while reducing the effect of various sources of bias in the published data. The accumulation of relevant data in this clinical setting has recently provided the opportunity for systematic reviews. In this brief article, I summarize the published systematic reviews and meta-analyses of positron emission tomography in prostate cancer. Most robust evidence suggests a probable role for first-line use of positron emission tomography with radiolabeled choline in restating patients with biochemical relapse of prostate cancer with the diagnostic performance that seems to be positively associated with the serum prostate-specific antigen level and velocity. Future systematic reviews will be needed for other emerging radiotracers such as those based on the prostate-specific membrane antigen and gastrin-releasing peptide receptor.

Although there have been many narrative review articles on this evolving topic, formalized systematic reviews and meta-analyses have only recently appeared in the literature for a few radiotracers (Table 1). Systematic reviews and meta-analyses involve formulating a detailed search strategy and evaluating and statistically combining relevant evidence to produce a quantitative estimate of various performance parameters of interest (e.g., pooled sensitivity, specificity, etc.) (2). The goal of this brief article is to bring together a summary of the recently published systematic reviews and meta-analyses of PET in the imaging evaluation of prostate cancer.

Imaging Glucose Metabolism with FDG
FDG is the most common radiotracer in PET scintigraphy. The biological basis for increased FDG uptake in malignancy is related to the Warburg effect, one of the hallmarks of cancer (3). The only published systematic review and meta-analysis with FDG as the radiotracer in the clinical setting of prostate cancer is related to the prevalence and risk of malignancy of prostatic incidental uptake of FDG (4). In 6 studies that included 47 925 patients, the pooled prevalence of incidental prostatic FDG uptake was 1.8% (95% confidence interval [CI]: 1.3%-2.3%). The pooled risk of malignancy in patients with incidental prostatic FDG uptake verified by biopsy was 62% (95% CI: 54%-71%). The authors concluded that incidental peripheral prostatic FDG uptake might carry a significant probability of malignancy (Figure 1). of fatty acids from acetyl-CoA and malonyl-CoA through the upregulated action of fatty acid synthase in support of increased malignancy-related cell membrane biosynthesis (5,6). In a systematic review and meta-analysis of 11C-acetate PET/computed tomography (CT) in prostate cancer, 23 studies were included (7). For evaluating the primary tumor, pooled sensitivity and specificity were 75.1% (95% CI: 69.8%-79.8%) and 75.8% (95% CI: 72.4%-78.9%), respectively. For detecting recurrence, pooled sensitivity was 64% (59%-69%), and pooled specificity was 93% (95% CI: 83%-98%). Moreover, sensitivity for detecting recurrence was higher in postsurgical versus postradiotherapy patients and in patients with serum prostate-specific antigen (PSA) at a relapse of Ͼ1 ng/mL.
Most prostrate cancer studies have employed radiolabeled choline (8). Choline is an essential nutrient that was discovered by Adolph Strecker in 1862 (9). He noted that upon heating of lecithin from bile, a gelatinous substance is formed that he called choline. Lecithin was later characterized chemically as phosphatidylcholine, which was then found to incorporate choline (10). Dietary choline is absorbed in the intestine by choline transporters and then phosphorylated to phosphocholine (catalyzed by choline kinase) or oxidized to betaine in some cells.  Two other enzymes in sequence further catalyze phosphocholine to form phosphatidylcholine, which is then incorporated into the cellular membranes. Given that tumors grow rapidly and proliferate, there is a demand for increased cellular membrane synthesis, which forms the basis for the observed increased radiolabeled choline uptake in some tumors. Choline may also accumulate in some benign conditions. As such, choline uptake in tissues is not cancer-specific.
Choline may be labeled with 11C (half-life: 20 minutes) or 18F (half-life: 110 minutes). The physiologic biodistribution of 11C-choline and 18F-fluorocholine are similar (relatively high uptake in liver, pancreas, kidney, salivary glands, and variable uptake in intestine) except that there is low renal tracer excretion with 11C-choline; hence, there is less bladder urine activity accumulation. For both tracers, the critical organ receiving the highest dose is the kidney. The US Food and Drug Administration has approved only 11C-choline for local production and use at the Mayo Clinic in Rochester, Minnesota, for restaging prostate cancer (11).
Umbehr et al. provided a systematic review and metaanalysis of 11C-choline and 18F-fluorocholine PET for the initial staging of prostate cancer. They reported a pooled sensitivity and specificity on a per patient basis (10 studies, 637 patients) of 84% (95% CI: 68%-93%) and 79% (95% CI: 53%-93%), respectively (12). In restaging patients with biochemical recurrence, they reported a pooled sensitivity and specificity on a per patient basis (12 studies, 1 055 patients) of 85% (95% CI: 79%-89%) and 88% (95% CI: 73%-95%), respectively (7). A similar report by von Eyben et al. examined 47 articles and data from 3 167 patients with regard to the diagnostic utility of choline PET/CT in staging and restaging of prostate cancer (13). They found that there were statistically significant more positive results in the prostate bed of biochemically relapsed patients who had previously undergone external beam radiation therapy rather than those patients who had radical prostatectomy as the initial treatment strategy. Moreover, choline PET/CT led to a change in treatment in 381 (41%) of 938 patients, leading to complete PSA response in 101 of 404 (25%) patients.

Imaging Amino Acid Metabolism with Anti-18F-FACBC
Anti-18F-FACBC is a synthetic nonmetabolized analogue of the amino acid leucine that accumulates in prostate cancer cells through the upregulated alanine, serine, cysteine (ASC) transport system (17). More recently, it has also been observed that androgens augment the expression of the ASC transporter system, thereby increasing the uptake of anti-18F-FACBC in prostate cancer (18).
Ren et al. reported on a systematic review and meta-analysis of 6 studies comprising 251 patients who were suspected of having recurring prostate carcinoma and who underwent anti-18F-FACBC PET/CT. The authors reported pooled sensitivity and specificity of 87% and 66% on a per patient-based analysis, respectively, in detecting prostate carcinoma recurrence (19) (Figure 3).

Comparison of PET Radiotracers in Prostate Cancer
The relatively large array of potential PET radiotracers and paucity of comparative studies in specific clinical situations in prostate cancer lead to both challenges and opportunities. It may be that a particular radiotracer is best suited for a particular phase of the disease while another may be most appropriate for another phase along the natural clinical history of prostate cancer. Our group at the University of Southern California recently reported on a comprehensive extracting and reanalysis of the PET detection data for FDG, 11C-acetate, 11C-or 18Fcholine, and anti-18F-FACBC (20). We found that FDG exhibited the lowest detection rate for any suspected disease. In addition, 11C-acetate tended to be advantageous over radiolabeled choline in detecting local recurrence and lymph node lesions, although the difference was not statistically significant. Anti-18F-FACBC had the greater likelihood of detecting local recurrence compared with radiolabeled choline, although again this difference was not statistically significant.

SUMMARY
Overall, the most robust evidence is for the probable first-line use of choline PET/CT in restating prostate cancer patients who have biochemically relapsed. More evidence and experience will be needed for the other candidate PET radiotracers (e.g., those targeted to prostate-specific membrane antigens, the gastrinreleasing peptide receptor, and others) in the imaging evaluation of prostate cancer. Other pertinent issues such as availability, access, regulatory, and reimbursement hurdles will also need to be resolved.
Disclosure: The author declares no conflicts of interest. Parts of this article appeared in the Society of Nuclear Medicine and Molecular Imaging PET Center of Excellence Newsletter Vol.12, Issue 1, 2015.

ACKNOWLEDGMENTS
This work was supported by the National Institutes of Health, National Cancer Institute grants R01CA111613 and P30CA014089. Abbreviations: PET, positron emission tomography; PSA, prostate-specific antigen.
Systematic Review of PET in Prostate Cancer