Detection Rate of 18F-Choline PET/CT and 68Ga-PSMA-HBED-CC PET/CT for Prostate Cancer Lymph Node Metastases with Direct Link from PET to Histopathology: Dependence on the Size of Tumor Deposits in Lymph Nodes

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 Ga-68-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 F-18-choline to Ga-68-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 F-18-choline or Ga-68-PSMA ligand, respectively. In total, 226 LNM (125 F-18-choline, 101 Ga-68-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 F-18-choline and Ga-68-PSMA groups. The median prostate-specific antigen level at imaging and number of LNM per patient were significantly higher in the F-18-choline group (3.4 ng/mL, n = 34) than in the Ga-68-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 F-18-choline PET/CT and 6.3 and 4.9 mm, respectively, for Ga-68-PSMA PET/CT. Corresponding diameters to reach d(50%) were 5.5 and 3.3 mm, respectively, for F-18-choline PET/CT and 3.7 and 2.3 mm, respectively, for Ga-68-PSMA PET/CT. Detection rates were significantly higher under Ga-68-PSMA (P < 0.005 and 0.04 for longitudinal and short diameter). Conclusion: Ga-68-PSMA PET/CT is superior to F-18-choline PET/CT in the detection of LNM. Whether those results will lead to an improved patient outcome after Ga-68-PSMA PET-guided therapy needs to be investigated by further studies.