Prognostic value of baseline evaluation of FDG and NaF PET/CT in castrate-resistant prostate cancer (CRPC): Early results from a prospective phase 1 trial

Objectives To evaluate baseline (BL) FDG and NaF PET/CT for predicting response to cabozantinib and abiraterone therapy (Tx) in CRPC.

Methods 9 men with CRPC had FDG and NaF PET/CT at BL and after 8wk on Tx. Standard imaging (bone scan and CT) was done at BL and every 8wk on Tx. FDG-avid disease extent was classified as widespread (FDG++), oligometastatic ie. ≤2 sites (FDG+) and non-FDG-avid (FDG-); SUVmax of the most intense site (MAX) and average SUVmax of up to the 5 most intense sites (AV) were assessed; Tx response using PCWG guidelines and days on trial (DOT) were recorded.

Results Of 9 men, 2 were classified as FDG++; both subsequently had progressive disease (PD) despite Tx (DOT 84 and 77d) and are now deceased or in hospice. 2 men were classified as FDG+; 1 had PD (DOT=231d), and 1 is without PD (still on trial after 540d). MAX and AV in the 2 FDG++ men were higher than in the 2 FDG+ men. Of 5 FDG- men, 4 had stable or improved disease on Tx (still on trial or stopped for non-oncologic reason; DOT = 225-407d), and only 1 had PD but with a longer DOT (282d) than in 3 men with FDG-avid disease. Despite small data size, Kaplan-Meier analysis showed a highly significant difference (p=0.012) among the 3 groups of men (FDG++, FDG+, FDG-) (Fig. 1). All 4 men with FDG-avid BL disease had FDG-avid disease at 8wk. All 9 men had NaF-avid disease (widespread in 8) at BL and at 8wk. Neither disease extent nor intensity (MAX or AV) on NaF PET was predictive of Tx response or prognosis. Standard imaging did not appear to have prognostic information.

Conclusions Based on our small sample size, BL FDG PET/CT may predict response to cabozantinib and abiraterone Tx for CRPC and may be superior to NaF PET/CT and standard imaging. Also, FDG has potential to stratify men with CRPC into 3 groups (widespread, oligometastaic and non-FDG-avid disease) to tailor Tx. Further evaluation is warranted.

Research Support RSNA, PCF and BWH