Introduction

The availability of tracers other than 18f-fluorodeoxyglucose (FDG) suggests new opportunities for the diagnosis and management of prostate cancer (PCa). The use of different radiopharmaceuticals, such as radiolabeled choline, or radiolabeled ligands of prostate-specific membrane antigen (PSMA), has a significant impact in various clinical settings, from initial staging to the detection of a biochemical recurrence, enabling personalized treatment planning, and metastasis-directed therapy (MDT) [1, 2]. Such an approach relies on the diagnostic performance of the imaging modalities used to detect the real extent and location of metastases. Many studies on PCa patients have been conducted using PET/CT [3,4,5], but most clinical protocols consider magnetic resonance imaging (MRI) the principal imaging modality for staging and restaging of patients with PCa.

In recent years, the clinical availability of integrated PET/MRI scanners has made it possible to explore the practical potential of multimodal, combined metabolic-receptor, anatomical, and functional imaging. The present systematic review and meta-analysis summarize the diagnostic information obtained with PET/MRI in PCa patients.

Materials and methods

Search strategy and study selection

A literature search from 2013 up to 23rd March 2020 was conducted in the PubMed, Scopus, and Web of Science databases. The terms used were as follows: “choline” or “PSMA” AND “prostate cancer” or “prostate” AND “PET/MRI” or “PET MRI” or “PET-MRI” or “positron emission tomography/magnetic resonance imaging.” The search was carried out with and without the addition of filters, such as English language only, type of article (original article, research article), and subjects (humans only). Three reviewers (L.E., F.Z., and P.A.) conducted the literature search, and two other reviewers (G.C. and D.C.) independently selected the studies to consider, excluding duplicate papers. Any discrepancy was solved by a consensus. After combining all the records identified, the full texts were retrieved and further assessed by four of the reviewers (F.Z, P.A., G.C., and L.E.).

One reviewer (L.E.) ran a new search across the databases, checking the references of the studies already selected, to ensure their eligibility. Reviews, clinical reports, abstracts of meetings, and editorials were excluded. The qualitative analysis excluded reports that did not consider hybrid PET/MRI scanners or that enrolled a very low number of patients (< 5). Studies were eligible for inclusion in the meta-analysis if all the following requirements were met: (i) a sample size of more than ten patients; and (ii) the article included enough raw data to enable the completion of a 2 × 2 contingency table (or the authors made said data available on request).

Data extraction

General details were retrieved for each study considered, such as generic data (authors, journal name, year of publication, country, and study design), patients’ characteristics (number of patients and their mean or median age), disease phase (i.e., staging or restaging), type of treatment, mean or median PSA level at the time of PET, and radiotracer used for PET/MRI. A quality assessment on the studies was performed using the Quality Assessment of Diagnostic Accuracy Studies 2 (QUADAS-2) [6]. Data extraction and quality assessment were done independently by three reviewers (L.E., F.Z., G.C.), and differences were solved by discussion.

Statistical methods

The pooled detection rate of PET/MRI, with its sensitivities, specificities, and 95% confidence intervals (CIs), with both radiolabeled choline and radiolabeled PSMA, was calculated using random effects analysis. Heterogeneity was tested using the χ2 and the I2 tests. The χ2 test provided an estimate of the between-study variance, and the I2 test measured the proportion of inconsistency in individual studies that cannot be explained by chance. The values for heterogeneity (I2) of 25%, 50%, and 75% were considered low, moderate, and high, respectively [7]. Publication bias was assessed using Deeks’ funnel plot asymmetry test, and a P value above 0.05 suggested the absence of any publication bias. All statistical analyses were performed using the Meta-DiSc® version 1.4 (developed by the Clinical Biostatistics Unit at Ramón y Cajal Hospital, Madrid) and Comprehensive Meta-Analysis (CMA) software version 3.3.070 (Biostat, Englewood, NJ, USA).

Results

Qualitative results

In total, 50 studies were eligible for qualitative analysis (Fig. 1, Table 1), 20 of them were prospective, and 30 were retrospective. Overall, the analysis concerned 2059 patients who underwent hybrid PET/MRI. Disease staging was the most common reason for the test (n = 24 studies; totally, 940 patients), followed by restaging (n = 16; totally 844 patients), and both staging and restaging (n = 10; totally 275 patients). Radiolabeled PSMA was used in the majority of cases (n = 34 studies). In 25 studies, the main endpoint was the ability of PET/MRI to detect PCa, be it primary or recurrent disease. Comparisons were drawn between PET/CT and PET/MRI performed in the same populations in 7 reports.

Fig. 1
figure 1

The PRISMA method for study selection (*filters, journal article/humans/last 5 years/English language; **exclusion of reviews, no inclusion of PET/MRI in the title and exclusion of clinical case)

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Table 1 Characteristics of the selected studies
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Methodological quality

All 50 studies were assessed with the QUADAS-2 tool (Fig. 2). The risk of bias for patient selection was high in many papers [10, 15, 19,20,21, 23, 24, 31, 37, 40,41,42, 44, 49,50,51, 53, 55]. The flow and timing were also high in 17 studies [10, 15, 20,21,22,23,24, 26, 27, 31, 32, 35, 44, 45, 51,52,53]. The applicability of the studies was adequate in most cases, but unclear as regards the reference standard in 18 of them [15, 22,23,24, 27, 34,35,36,37, 41,42,43,44, 49, 50, 52, 53, 56].

Fig. 2
figure 2

QUADAS-2 findings on the qualitative assessment of the studies selected

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PET/MRI for initial staging

In the present review, 15 studies dealt with PET/MRI used only in the staging setting for the purpose of detecting primary disease [11, 12, 14, 18, 29, 31, 33, 35, 38, 40, 44,45,46,47, 54, 57].

Integrated PET/MRI proved to be of greater diagnostic value in locating PCa than either multiparametric (mp) MRI [11, 14, 18, 29, 35, 44, 45, 54, 58] or PET imaging alone [14, 18, 44]. 68Ga-PSMA-11 PET/MRI showed high lesion contrast and an excellent consistency in lesion detection [20]. Intense 18F-labelled PSMA uptake on PET and mpMRI changes correlated strongly with the dominant lesion in the prostate glands of men undergoing imaging before surgery [31]. These results are consistent with other studies where PET was used to identify PCa lesions. For instance, Park et al. [38] reported that PCa was detected by 68Ga-PSMA-11 PET in all of their 33 patients, whereas mpMRI with the PI-RADS (Prostate Imaging Reporting and Data System) pinpointed 4 or 5 lesions in 26 patients, but missed tumors in 3. Similarly, Ferrero et al. [47] found primary tumors PSMA-negative in only 3 of 60 patients, thus reaching a detection rate of 95%.

The assessment of extracapsular extension, tumor grade, and Gleason score plays an important part in treatment decisions, and in distinguishing aggressive from indolent disease. In one study, extracapsular spread of PCa was detected better with 68Ga-PSMA-11 PET/MRI than with mpMRI (69 vs. 46%) [54]. In another study, PET and PET/MRI produced a considerably lower proportion of equivocal results (i.e., PI-RADS 3) than mpMRI [35].

PET/MRI may have also an important role in detecting local and distant metastases. From a visual inspection of 60 patients’ imaging results, 68Ga-PSMA-11 PET/MRI revealed positive lymph nodes in 8 patients, with only one patient subsequently resulting false-positive. Most nodes were located in the pelvis, but distant nodes were found in the common iliac chain in 2 patients [35]. 68Ga-PSMA-11 PET/MRI provides valuable diagnostic information and improves patient selection for extended pelvic lymph node dissection by comparison with the currently-used clinical nomograms [38, 40, 47].

The rate of changes to patient management can express the impact of PET/MRI on the initial staging of PCa patients. Grubmuller et al. [46] reported that including PET/MRI in the initial workup of patients with PCa could alter the therapeutic strategy in at least 30% of cases.

PET/MRI in cases of biochemical disease recurrence

PET/MRI was used to seek biochemical recurrences of PCa in a total of 598 patients [8, 26, 32, 34, 39, 42, 51, 52, 56]. Taking the studies concerned together, the recurrent disease detection rate achieved with PET/MRI ranged between 54.5 [56] and 97% [8] (Table 2). In many cases, the authors also reported the detection rate by PSA category, which rose with antigen levels from low (< 0.2 ng/mL) to high (> 10 ng/mL). Hope et al. [26] reported a detection rate of 58–64% for PSA levels <0.5 ng/mL using 68Ga-PSMA-11 PET/MRI, while it was 100% for PSA > 2.0 ng/mL [26]. Grubmuller et al. [34] confirmed as much. A number of authors [26, 28, 34, 51] detected a change in patient management prompted by PET/MRI findings, in proportions of cases ranging from 53.2 to 74.6%. Based on the study by Kranzbuhler et al. [56], including PET/MRI in the diagnostic workup could prompt changes to radiotherapy planning for 39.4% of patients.

Table 2 Detection rates of PET/MRI in restaging
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PET/CT vs. PET/MRI

PET/MRI and PET/CT were compared in seven studies (Table 3; [10, 16, 20, 21, 25, 30, 59] encompassing 278 examinations, 225 of them using 68Ga-PSMA-11 (81%) and 53 with 11C-choline (19%).

Table 3 Detection rates for radiolabeled PSMA and Choline PET/CT vs. PET/MRI in Prostate Cancer
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The overall discrepancy in PET-positive findings between PET/CT and PET/MRI was very low, and agreement between the two methods was high, in the range of 71 to 95% [20, 30, 60]; this also was applied to the semiquantitative analyses [10, 30].

Five studies demonstrated that PET/MRI was superior to PET/CT in detecting PCa lesions, both in staging and restaging [16, 21, 25, 30, 59]. In particular, PET/MRI was more accurate than PET/CT in detecting local recurrences, thereby improving the detection rate for lower PSA levels. All authors [16, 21, 25, 30, 59] found the MRI component crucial in identifying local recurrences otherwise masked by the accumulation of the radiopharmaceuticals in the bladder, especially when 68Ga-PSMA-11 was used.

Regarding the assessment of lymph node involvement, PET/MRI achieved a slightly higher detection rate than PET/CT, probably due to a longer tracer accumulation time, as mentioned in the studies by Freitag et al. [16] and Lutje et al. [25].

As for identifying bone metastases, Eiber et al. [21] argue that PET/CT and PET/MRI are comparable for PSA levels < 2 ng/mL, and that PET/CT is more efficient for levels > 2 ng/mL. Freitag et al. [16] and Souvatzoglou et al. [10] claim instead that using multiple MRI sequences improves the detection of bone metastases, especially in cases of early bone marrow involvement.

PET/MRI demands a 79.7% (range, 72.6–86.2%) lower exposure to radiation than PET/CT [21, 59], but the acquisition time is much longer (60 vs. 20 min) [21]. This latter aspect is relative to the inclusion of a mpMRI of the prostate/prostatic fossa that improve significantly the resolution of prostate scan.

PET/MRI vs. mpMRI

Some papers compared the PCa detection rate or diagnostic performance of PET/MRI and mpMRI in terms of sensitivity and specificity (Table 1s; [13, 14, 18, 29,30,31,32,33, 35, 38, 45, 51, 52, 61]). PET/MRI achieved a higher primary tumor detection rate than mpMRI [14, 18, 45]. Judging from the data reported by de Perrot et al. [13] and Muehlematter et al. [41], PET/MRI was more sensitive than mpMRI in identifying primary tumor in the peripheral zone of prostate gland, and in revealing extracapsular extension and seminal vesicle infiltration. On the other hand, mpMRI provided more information about disease recurrence in the prostatic fossa [30, 51]. As for the detection of lymph node and distant metastases, PET/MRI was more sensitive than mpMRI, in both staging [38] and restaging [32, 51].

Radiolabeled PSMA vs. radiolabeled choline PET/MRI

The most papers included radiolabeled PSMA as radiopharmaceutical agent. The majority of them were focused on 68Ga-PSMA-11 (n = 32 studies), while 2 were based on 18F-PSMA [31, 33]. Radiolabeled choline PET/MRI was employed in the staging for 8/16 (50%) [11,12,13,14, 17, 29, 43, 53], while radiolabeled PSMA in 16/34 (47%) papers [16, 18, 27, 31, 35,36,37, 40, 44,45,46,47,48,49, 54]. Conversely, 5/16 (31%) [9, 21, 39, 42, 51] and 11/34 (32%) articles [8, 23, 25, 26, 28, 30, 32, 34, 52, 56] were focused on the resting phase for radiolabeled choline and PSMA, respectively.

For the identification of primary lesion, PSMA PET/MRI enriched a specificity of 88%, according to Hicks et al. [45], while choline PET/MRI registered a specificity equal to 76% [13]. Therefore, PSMA is more accurate in detecting primary PCa lesions, by reducing the rate of falsely positive findings. In restaging, PSMA PET/MRI showed a detection rate of 64% for PSA values < 0.5 ng/mL in 150 patients [26], therefore significantly higher than choline PET/MRI (detection rate of 12.5% in 58 patients for the same values of PSA) [51] (see Table 2).

However, no comparative data are now available about radiolabeled PSMA and choline PET/MRI in the same population, in each phase of disease (i.e., staging or restaging).

Other aspects explored

Six articles considered the image acquisition protocol [19, 23, 24, 37, 48, 53], four discussed technical aspects [15, 22, 41, 55], and eight focused on the interpretation of images obtained with PET/MRI [9, 12, 17, 27, 36, 43, 49, 50].

The best time per bed acquisition using PET/MRI for PCa is longer than 3 min [19, 23] because this can reduce the halo artifact in the bladder and kidney for 68Ga-PSMA-11 [24]. According to Heußer et al. [22], the halo artifact can also be reduced by lowering the maximum scatter fraction rate.

The choice of particular MRI sequences has an important influence on the detection of local and distant metastases, as suggested by Metser et al. [53].

The correlation between the apparent diffusion coefficient (ADC) and the standardized uptake value (SUV) is controversial. Wetter et al. [9] found an inverse correlation between ADC and SUV in bone metastases. Uslu-Besli et al. [49] and Tseng et al. [43] likewise reported an inverse correlation between the maximum SUV and the metabolic tumor volume, between uptake volume product and the ADC in primary tumor, respectively. Wetter et al. [12], on the other hand, found no correlation between ADC and SUV in primary cancer.

Quantitative results

A meta-analysis was performed on 23 studies (Fig. 1), 11 concerning the staging phase [16, 18, 27, 35, 38, 40, 44,45,46,47, 52], and 12 the restaging phase [8, 21, 23, 25, 26, 30, 32, 39, 42, 51, 56, 59]. Pooled sensitivities and specificities were obtained for the former (staging), and a pooled detection rate was computed for the latter (restaging).

Table 4 shows the pooled sensitivities and specificities for primary PCa and lymph node disease, showing a higher pooled sensitivity for primary lesions in the patient-based analysis (94.9% [95% CI 87.5–98.6]) than in the lesion-based analysis (61.5% [95% CI 40.6–79.8]). Vice versa, the pooled specificity was higher in the lesion-based analysis than in the patient-based analysis (90.9% [95% CI 80–97] vs. 62.5% [95% CI 43.7–78.9], respectively). For lymph node disease, the pooled sensitivity and specificity were similar in the two types of analysis. The heterogeneity between the studies ranged between 0 and 98.3%.

Table 4 Pooled sensitivity and specificity for staging
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At restaging, the pooled detection rate was 80.9% (95% CI 73.0–86.9%) (Table 5). The pooled detection rate was higher for studies using PET/MRI with radiolabeled PSMA than for those with radiolabeled choline (81.8 vs. 77.3%). The heterogeneity between the studies was high (> 80%). There was also evidence of publication bias, as illustrated by the funnel plot (Supplemental Figure 1).

Table 5 Pooled detection rate in restaging
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In the studies that compared PET/CT with PET/MRI in the same population, the pooled detection rates were 95.4% (95% CI 87.0–98.5) and 93.9% (95% CI 85.4–97.6), respectively; and, here again, the heterogeneity among the studies was > 80%.

Discussion and conclusions

The data emerging from the available literature suggest some considerations.

  1. 1).

    The ability of PET/MRI with radiolabeled PSMA to detect dominant lesions (pooled sensitivity for sextant-based analysis, 80%) may suggest a further search on prostate fusion biopsy of the suspected area. A recent paper by Westphalen et al. [62] reported a low positive predictive value (PPV) of PI-RADS for identifying primary PCa. After reviewing mpMRI images from 3449 patients for a total of 5082 lesions, the authors found a PPV of 5% for PI-RADS 2, 15% for PI-RADS 3, 39% for PI-RADS 4, and 72% for PI-RADS 5. Park et al. [38] found that PET/MRI with 68Ga-PSMA-11 had a higher PPV than mpMRI for bilateral tumors (70 vs. 18%, respectively). Two articles discussed about the role of PET/MRI for the diagnosis of PCa. Taneja et al. [37] and Jena et al. [44] showed that dual-phase simultaneous 68Ga-PSMA-11 PET/MRI is able to characterize prostate lesions, in 117 patients. In particular, Taneja et al. reported that malignant lesions have higher PSMA uptake than the benign ones, mainly in the delayed images (acquired after about 50 min form tracer injection) due to a possible role of receptor density and longer retention of PSMA in PCa over time. Moreover, Jena et al. [44] concluded that combining PET data, MRI data, PSA levels, and digital rectal examination resulted in a better characterization of prostatic lesions, with an AUC of 0.94 ± 0.29. However, in the setting of primary PCa, MRI-TRUS fusion biopsy using mpMRI will remain the standard for prostate cancer probably for longer time due to a very high-quality study [63]. Similar studies for PSMA PET/MRI-guided biopsy are needed to compete with mpMRI in order to elucidate the advantages in terms of diagnostic efficiency and costs.

    Although the detection of more lesions by use of PET/MRI in primary setting may not necessarily lead to better outcome in general, the identification of oligometastasic disease would be useful for guiding to an appropriate treatment management (extension of the radiation field, extension of lymph node adenectomy, etc.) therefore allowing a long-term prognosis of the patients.

  2. 2).

    Targeted therapies could be directed by PET/MRI with radiolabeled PSMA because of its ability both to detect the most aggressive lesion and to assess the extracapsular extension of disease. This latter information would be useful not only to guide to more precise surgical approach, but it can be useful for focal or less-invasive treatments.

  3. 3).

    PET/MRI with radiolabeled PSMA could be used for early disease recurrences (PSA levels < 0.5 ng/mL) because it can raise the detection rate to 65% and could also be helpful in guiding MDT. It seems that mpMRI can suffice for identifying PCa recurrences in the prostatic fossa. However, the added value of PET/MRI is its ability to detect also the lymph node involvement thus guiding to a specific salvage therapy, especially in case of radiotherapy. Furthermore, in case of a positivity only in the lymph node, a salvage lymph node dissection can be planned, by evaluating also the possible nerve or other neighboring structure involvement.

  4. 4).

    PSMA PET/MRI is more detectable than choline PET/MRI in staging and in restaging, although head to head comparative data are missing.

  5. 5).

    PSMA PET/MRI can prompt changes to the management of PCa patients in up to 75% of cases at restaging. It means that in population of 100 patients with a PCa, the inclusion of PET/MRI in the diagnostic algorithm has a deep effect on the management and therefore on the short- and long-term prognosis. However, more data are necessary for this latter indication, being the literature scarce.

This hybrid imaging modality has some limitations, however, such as the need for scatter correction and long acquisition times. The accurate description and interpretation of the results are also key challenges for radiologists/specialists in nuclear medicine and urologists alike.

In short, PET/MRI seems to have potential applications in the following: (1) the diagnosis of primary tumor; (2) facilitating biopsy targeting; (3) predicting or monitoring tumor aggressiveness (especially during active surveillance); (4) the early detection of recurrent PCa; and (5) guiding targeted therapies.