Detection Rate of

Accurate detection of prostate cancer lymph node metastases (LNM) through PET/CT before lymphadenectomy is crucial for successful therapy. PET/CT with choline derivatives used to be the standard tool for imaging metastases, whereas (68)Ga-PSMA (prostate-specific membrane antigen) PET/CT was introduced recently. Both PET techniques were investigated with respect to what extent the detection rate of LNM depends on the size of tumor deposits (TDs) within LNM. Methods: Documenting the switch from the use of (18)F-choline to (68)Ga-PSMA in 2014, we used 2 patient cohorts undergoing a template lymphadenectomy because of a PET/CT indicating LNM. Forty-four and 40 patients underwent PET/CT with (18)F-choline or (68)Ga-PSMA ligand, respectively. In total, 226 LNM (125 (18)F-choline, 101 (68)Ga-PSMA) originated from 73 salvage lymphadenectomies at biochemical recurrence and from 11 primary lymphadenectomies at radical prostatectomy. LNM eligible for direct correlation of PET/CT to histopathology were identified from lymphadenectomies conducted in small anatomic subregions, with 1 LNM (condition 1) or 1–2 LNM (condition 2). Longitudinal and short diameters of TD within LNM were determined by histopathology, allowing linking of the size of TD in LNM to the detection threshold of PET/CT. Diameters associated with a detection rate of 50% and 90% (d(50%), d(90%)) were calculated on the basis of logistic growth curve models fitted. Results: Gleason score, number of removed LNs, and subregions for lymphadenectomy per patient did not differ significantly between the (18)F-choline and (68)Ga-PSMA groups. The median prostate-specific antigen level at imaging and number of LNM per patient were significantly higher in the (18)F-choline group (3.4 ng/mL, n = 34) than in the (68)Ga-PSMA group (2.2 ng/mL, n = 28; both P < 0.05). Longitudinal and short diameters of TD in LNM to reach d(90%) were 11.2 and 7.4 mm, respectively, for (18)F-choline PET/CT and 6.3 and 4.9 mm, respectively, for (68)Ga-PSMA PET/CT. Corresponding diameters to reach d(50%) were 5.5 and 3.3 mm, respectively, for (18)F-choline PET/CT and 3.7 and 2.3 mm, respectively, for (68)Ga-PSMA PET/CT. Detection rates were significantly higher under (68)Ga-PSMA (P = 0.005 and 0.04 for longitudinal and short diameter). Conclusion: (68)Ga-PSMA PET/CT is superior to (18)F-choline PET/CT in the detection of LNM. Whether those results will lead to an improved patient outcome after (68)Ga-PSMA PET–guided therapy needs to be investigated by further studies.