Reduced Segmentation of Lesions Is Comparable to Whole-Body Segmentation for Response Assessment by PSMA PET/CT:...

(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-TV₁₀, ΔPSMA-TV₅, 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-TV₁₀/ΔPSMA-TV₅ 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-TV₁₀ or ΔPSMA-TV₅. 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-TV₁₀ or ΔPSMA-TV₅. The highest correlations with ΔPSA were found for ΔPSMA-TV_all (r ≥ 0.59, p ≤ 0.01), followed by ΔPSMA-TV₁₀ (r ≥ 0.57, p ≤ 0.01) and ΔPSMA-TV₅ (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.