Reduced Segmentation of Lesions Is Comparable to Whole-Body Segmentation for Response Assessment by PSMA PET/CT: Initial Experience with the Keyhole Approach.

(1) Background: Prostate-specific membrane antigen (PSMA) positron emission tomography (PET)-derived parameters, such as the commonly used standardized uptake value (SUV) and PSMA-positive tumor volume (PSMA-TV), have been proposed for response assessment in metastatic prostate cancer (PCa) patients. However, the calculation of whole-body PSMA-TV remains a time-consuming procedure. We hypothesized that it may be possible to quantify changes in PSMA-TV by considering only a limited number of representative lesions. (2) Methods: Sixty-five patients classified into different disease stages were assessed by PSMA PET/CT for staging and restaging after therapy. Whole-body PSMA-TV and whole-body SUV were calculated. We then repeated this calculation only including the five or ten hottest or largest lesions. The corresponding serum levels of prostate-specific antigen (PSA) were also determined. The derived delta between baseline and follow-up values provided the following parameters: ΔSUV, ΔSUV, ΔSUV, ΔPSMA-TV, ΔPSMA-TV, ΔPSMA-TV, ΔPSA. Finally, we compared the findings from our whole-body segmentation with the results from our keyhole approach (focusing on a limited number of lesions) and correlated all values with the biochemical response (ΔPSA). (3) Results: Among patients with metastatic hormone-sensitive PCa (mHSPC), none showed a relevant deviation for ΔSUV/ΔSUV or ΔPSMA-TV/ΔPSMA-TV compared to ΔSUV and ΔPSMA-TV. For patients treated with taxanes, up to 6/21 (28.6%) showed clinically relevant deviations between ΔSUV and ΔSUV or ΔSUV, but only up to 2/21 (9.5%) patients showed clinically relevant deviations between ΔPSMA-TV and ΔPSMA-TV or ΔPSMA-TV. For patients treated with radioligand therapy (RLT), up to 5/28 (17.9%) showed clinically relevant deviations between ΔSUV and ΔSUV or ΔSUV, but only 1/28 (3.6%) patients showed clinically relevant deviations between ΔPSMA-TV and ΔPSMA-TV or ΔPSMA-TV. The highest correlations with ΔPSA were found for ΔPSMA-TV (r ≥ 0.59, ≤ 0.01), followed by ΔPSMA-TV (r ≥ 0.57, ≤ 0.01) and ΔPSMA-TV (r ≥ 0.53, ≤ 0.02) in all cohorts. ΔPSA only correlated with ΔSUV (r = 0.60, = 0.02) and with ΔSUV (r = 0.53, = 0.03) in the mHSPC cohort, as well as with ΔSUV (r = 0.51, = 0.01) in the RLT cohort. (4) Conclusion: Response assessment using PSMA-TV with a reduced number of lesions is feasible, and may allow for a simplified evaluation process for PSMA PET/CT.