Underestimation of PET/CT Scan in Assessing Tumour Burden of Men With Nodal Recurrence From Prostate Cancer: Head-to-Head Comparison OF 68Ga-PSMA and 11C-Choline in a Large, Multi-Institutional Series of Extended Salvage Lymph Node Dissections

INTRODUCTION: The aim of the study was to compare 11C-Choline and 68Ga-PSMA in men undergoing SLND for nodal recurrent PCa. MATERIALS AND METHODS: The study included 641 patients who experienced PSA rise and nodal recurrence after radical prostatectomy and underwent SLND. Lymph node recurrence was documented by PET/CT scan using either 11C-Choline (n=407; 63%) or 68Ga-PSMA ligand (n=234; 37%). The outcome was underestimation of tumour burden (difference between number of positive nodes on final pathology and number of positive spots at PET/CT). Multivariable analysis tested the association between PET/CT tracer (11C-Choline vs. 68Ga-PSMA) and underestimation of tumour burden. RESULTS: Overall, the extent of underestimation of tumour burden was significantly higher in the 11C-Choline group compared to the 68Ga-PSMA (p<0.0001). This was confirmed on multivariable analysis (p=0.028). Repeating these analyses according to PSA, the underestimation of tumour burden was lower with 68Ga-PSMA only when the PSA was ≤1.5 ng/ml. Conversely, the underestimation of the two tracers became similar when PSA was >1.5 ng/ml. Furthermore, we evaluated the risk of underestimation by number of positive spots on PET/CT scan. The higher the number of positive spots the higher the underestimation of tumour burden regardless of the tracer used (p=0.2). CONCLUSIONS: PET/CT scan significantly underestimates the burden of PCa recurrence, regardless of the tracer used. 68Ga-PSMA was associated with a lower rate of underestimation in patients with a PSA below 1.5 ng/ml and a limited nodal tumour load. In all other men, there was no benefit from 68Ga-PSMA over 11C-Choline in assessing the extent of nodal recurrence.