Utility of positron emission tomography in biochemically recurrent prostate cancer: A comparison of carbon-11 acetate & 68Ga-prostate specific membrane antigen radiotracers
Carbon-11 acetate (C11 ac) and 68Ga-prostate specific membrane antigen (Ga PSMA) based PET/CT are often used to assess biochemical recurrence (BcR). We retrospectively analyzed 230 BcR patients who received PET/CT scans. 160 used C11 ac and 70 used Ga PSMA. Our objectives were to quantify how pre-scan PSA influenced the probability of a positive scan and determine the validity of radiotracer-based lesion characterization.
A database of BcR patients was queried for PET/CT scans and sufficient post-scan data. All scans were read by a board-certified radiologist and PET-specific findings suggestive of malignancy were characterized separately from non-PET findings. Site-specific radiotracer avidity was recorded. Pre-scan PSA range categories (i.e. .0 - .5 ng / mL) were set based on Ga PSMA and C11 ac subpopulation PSA equivalence as determined by the Mann-Whitney test (p = .49 - .97). These categories were subjected to Pearson's chi2 test regarding percent of scans read as positive to assess for significant variance between radiotracers. Positive scans were classified as true or false and a lesion-specific positive predictive value (PPV) was calculated for each radiotracer. Three methods were used to confirm a positive site: 1) histology, 2) non-confounded, post-targeted therapy PSA trend, and 3) non-PET imaging. False positives were confirmed by histology.
The rate of positives was greater in higher PSA categories for both radiotracers. At PSAs < 2 ng / mL, no significant difference was observed at any PSA range category or when considering PSAs < 2 as a group. At PSAs of 2 - 4, Ga PSMA begins to demonstrate a significantly (p = .041) higher rate of positives. For PSAs 2 - 20 (PSA rank sum p = .64), Ga PSMA was positive in 35/36 vs. 62/79 with C11 ac (p = .01). C11 ac PPVs were 93.9% and 88.4% for all cases and biopsy-confirmed cases, respectively. Ga PSMA PPVs were 97.4% and 92.9%.
C11 ac and Ga PSMA demonstrate high PPVs for prostatic malignancy at avid sites. Ga PSMA increases the rate of positive reads at PSA values greater than 2 and produces studies with a higher PPV relative to C11 ac.