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 max 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 maxall , ΔSUV max10 , ΔSUV max5 , ΔPSMA-TV all , ΔPSMA-TV 10 , ΔPSMA-TV 5 , 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 max10 /ΔSUV max5 or ΔPSMA-TV 10 /ΔPSMA-TV 5 compared to ΔSUV maxall and ΔPSMA-TV all . For patients treated with taxanes, up to 6/21 (28.6%) showed clinically relevant deviations between ΔSUV maxall and ΔSUV max10 or ΔSUV max5 , but only up to 2/21 (9.5%) patients showed clinically relevant deviations between ΔPSMA-TV all and ΔPSMA-TV 10 or ΔPSMA-TV 5 . For patients treated with radioligand therapy (RLT), up to 5/28 (17.9%) showed clinically relevant deviations between ΔSUV maxall and ΔSUV max10 or ΔSUV max5 , but only 1/28 (3.6%) patients showed clinically relevant deviations between ΔPSMA-TV all and ΔPSMA-TV 10 or ΔPSMA-TV 5 . The highest correlations with ΔPSA were found for ΔPSMA-TV all (r ≥ 0.59, p ≤ 0.01), followed by ΔPSMA-TV 10 (r ≥ 0.57, p ≤ 0.01) and ΔPSMA-TV 5 (r ≥ 0.53, p ≤ 0.02) in all cohorts. ΔPSA only correlated with ΔSUV maxall (r = 0.60, p = 0.02) and with ΔSUV max10 (r = 0.53, p = 0.03) in the mHSPC cohort, as well as with ΔSUV maxall (r = 0.51, p = 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.