In this study we evaluated the lesion detection efficacy of 18F-DCFPyL PET/CT in 248 patients with BCR. Suspected metastases were identified in the majority of PET examinations and even at PSA values <0.5 ng/ml, lesions were observed in 59% of patients. These results show clear improvement over the results with conventional imaging modalities, as well as choline PET/CT [16,17,18]. Compared to a recent meta-analysis on 68Ga-PSMA PET/CT , our study demonstrates equivalent results in patients with PSA values >2.0 ng/ml (i.e. 95% lesion detection with both tracers). Potentially improved detection is observed compared to 68Ga-PSMA in all PSA strata <2.0 ng/ml (most notably, 59% detection with 18F-DCFPyL when PSA <0.5 ng/ml vs. 45% with 68Ga-PSMA; Table 2) .
Recently revised European Association of Urology guidelines recommend performing PSMA PET/CT at BCR after RP if the PSA level is >0.2 ng/ml and if the results will influence subsequent treatment decisions . PSMA PET/CT is recommended in case of BCR after EBRT in patients fit for curative salvage treatment. The guidelines also recommend offering a PSMA PET to men with a persistent PSA >0.2 ng/ml after prostatectomy to exclude metastatic disease. Those recommendations are in line with our findings that even at the lowest PSA values, sites of PCa recurrence could be detected with 18F-DCFPyL PET/CT in the majority of patients.
Further, 18F-DCFPyL PET/CT revealed lesions outside the prostatic fossa even at very low PSA levels (e.g. 39% with PSA <0.5 ng/ml) (Figs. 2, 3). These findings are clinically relevant, because patients with BCR after RP are candidates for local salvage radiation treatment to the prostatic fossa, and such treatment is likely to be ineffective in the presence of (lymph node) metastases. Indeed, Emmett et al. recently demonstrated that 68Ga-PSMA PET/CT may be effective for stratification of patients into groups with a high probability of response to salvage radiotherapy (negative PET or detection of local recurrence only) versus lower probability of response (detected lesions outside the prostatic fossa). In that study (n = 164), PSMA PET findings predicted salvage therapy outcomes better than established predictors, most notably current PSA values .
To our knowledge, there is only one prior study on 18F-DCFPyL PET/CT for patients with BCR, analysing n = 62 18F-DCFPyL PET/CT scans in direct comparison with 68Ga-PSMA PET/CT . Dietlein et al.  found higher sensitivity with 18F-DCFPyL in patients with PSA values between 0.5–3.5 ng/ml (88% with 18F-DCFPyL versus 66% with 68Ga-PSMA, p = 0.042). Compared to Dietlein et al., we observed a higher detection efficacy, especially at PSA values <0.5 ng/ml (59% versus 13%, respectively). This difference might be due to the image interpretation protocol applied by Dietlein et al., which required corresponding findings on CT for all PET-positive lesions. Based on current standardised reporting systems, such a requirement is no longer applied for PSMA PET interpretation at our institutions [21, 22].
Recently, another 18F-labelled PSMA tracer was introduced: 18F-PSMA-1007 . 18F-PSMA-1007 is only minimally excreted via the urinary tract, which potentially improves visualisation of tumour deposits adjacent to the urinary bladder (mainly local recurrence). Our results with 18F-DCFPyL PET/CT appear comparable to the outcomes of 251 18F-PSMA-1007 PET/CT scans analysed by Giesel et al. (e.g. 62% positive scans with 18F-PSMA-1007 when PSA <0.5 ng/ml ) (Table 2). Conversely, Rahbar et al. report notably more positive 18F-PSMA-1007 PET/CT scans compared to both Giesel et al. and the present study (86% positive scans when PSA <0.5 ng/ml; 100% positive scans when PSA >1.0 ng/ml ). Rahbar et al. included only 100 patients, however, and may have included more advanced PCa cases. For example, 38% of RP patients analysed by Rahbar et al. had already received salvage radiotherapy (compared to 17% in our cohort), and 27% of patients received ADT around the time of examination (compared to 8% in our cohort). In general, precise comparison of the results with different PSMA tracers (including 68Ga-PSMA) is difficult. Most studies are retrospective, and many factors other than the applied radiotracers may have influenced the diagnostic results (e.g. included patient population, PET scanner technique, image reconstruction methods, experience of the PET readers). In particular for cohorts including patients treated with EBRT, such as our cohort, somewhat higher detection rates are generally found in comparison with populations treated with RP only.
In terms of detection of local recurrence, no evident benefit of 18F-PSMA-1007 over 18F-DCFPyL is seen. Giesel et al. included only patients with BCR after RP and detected local recurrence in 25% of patients. Rahbar et al. noted local recurrence in 37% of all patients (treated with either EBRT or RP) [24, 25]. In our study, local recurrence was detected in 24% of all patients treated with RP and in 37% of the entire population.
Contrary to our finding that PSA at PET/CT is the only predictor of scan positivity, a recent study by Rauscher et al. found that concurrent ADT was also a predictor of a positive scan in multivariable analysis . It should be noted that in our cohort, only 20 patients were receiving ADT at the time of PET/CT, the majority of whom received ADT in combination with EBRT; Rauscher et al. included only patients after RP, all with a PSA <1.0 ng/ml. Overall, the role of ADT in scan outcomes remains incompletely understood, for ADT may upregulate PSMA expression initially , but reduce lesion detection efficacy upon longer exposure . Patients who received ADT after prostatectomy due to known metastatic disease without salvage options—in other words, in a palliative setting—were not included in this cohort. Although there is no definition as to where BCR ends in the course of prostate cancer progression, in our opinion, a cohort of patients with BCR should only include patients who are candidates for salvage options before PSMA PET/CT is performed.
Accurate identification of patients with oligometastatic disease is of current interest , although the definition of oligometastatic disease remains unclear (either ≤3 or ≤5 metastases). The clinical benefit of (oligo)metastases-directed treatment has not yet been established . Nevertheless, it seems evident that any success of metastases-directed strategies will depend on accurate imaging studies, as well as adequate timing of diagnostic procedures. From our data it can be observed that performing 18F-DCFPyL PET/CT soon after diagnosis of BCR (i.e. at lower PSA values) favours detection of oligometastases (Fig. 4). At higher PSA levels (>5.0 ng/ml), more patients are diagnosed with polymetastases (>5). Yet even in these patients, a substantial proportion had oligometastastic disease on 18F-DCFPyL PET/CT (Fig. 4).
An important limitation of the present study, and studies on PSMA PET for BCR in general, is the lack of histopathologic confirmation of PET results [8, 12, 30]. PSMA PET-detected lesions are often smaller than 1 cm, making biopsy procedures difficult and burdensome for patients. In our study, histologic confirmation of PET findings (CT-guided biopsy, lymph node dissection, prostate biopsy) had been performed in only 15 of our patients, although follow-up of patients from outside our PET centres was often lacking. In 12 of these patients (80%), the malignant nature of detected lesions was confirmed. Overall, these numbers are insufficient to assess the extent of false-positive outcomes (the specificity) in our study. High specificity of PSMA PET has been demonstrated in primary PCa patients, however, where imaging findings can be correlated to histopathology from lymph node dissection (96% specificity with 68Ga-PSMA ; 96–99% specificity with 18F-DCFPyL ). We were also unable to evaluate the sensitivity of 18F-DCFPyL on a lesion basis, since the true number of PCa metastases remains unknown. PET is inherently limited by its image resolution to the detection of small metastases (<2–4 mm). We should therefore assume that 18F-DCFPyL PET/CT still underestimates the extent of disease.
Clinically, it is important to realise that improved detection of metastases is only of benefit to patients if followed by appropriate, proven-effective therapeutic strategies. The identification of additional metastases may delay treatment of local recurrences, trigger metastasis-directed treatment, or result in initiation of systemic therapy. None of the clinical outcomes of these (PSMA PET-based) decisions have yet been sufficiently studied in clinical trials [3, 29, 32].