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.

Abstract

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 ⁶⁸Ga-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. <b>Methods:</b> Documenting the switch from the use of ¹⁸F-choline to ⁶⁸Ga-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 ¹⁸F-choline or ⁶⁸Ga-PSMA ligand, respectively. In total, 226 LNM (125 ¹⁸F-choline, 101 ⁶⁸Ga-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. <b>Results:</b> Gleason score, number of removed LNs, and subregions for lymphadenectomy per patient did not differ significantly between the ¹⁸F-choline and ⁶⁸Ga-PSMA groups. The median prostate-specific antigen level at imaging and number of LNM per patient were significantly higher in the ¹⁸F-choline group (3.4 ng/mL, <i>n</i> = 34) than in the ⁶⁸Ga-PSMA group (2.2 ng/mL, <i>n</i> = 28; both <i>P</i> < 0.05). Longitudinal and short diameters of TD in LNM to reach d_90% were 11.2 and 7.4 mm, respectively, for ¹⁸F-choline PET/CT and 6.3 and 4.9 mm, respectively, for ⁶⁸Ga-PSMA PET/CT. Corresponding diameters to reach d_50% were 5.5 and 3.3 mm, respectively, for ¹⁸F-choline PET/CT and 3.7 and 2.3 mm, respectively, for ⁶⁸Ga-PSMA PET/CT. Detection rates were significantly higher under ⁶⁸Ga-PSMA (<i>P</i> = 0.005 and 0.04 for longitudinal and short diameter). <b>Conclusion:</b> ⁶⁸Ga-PSMA PET/CT is superior to ¹⁸F-choline PET/CT in the detection of LNM. Whether those results will lead to an improved patient outcome after ⁶⁸Ga-PSMA PET-guided therapy needs to be investigated by further studies.