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

SIMPLE SUMMARY: The calculation of PSMA-positive tumor volume (PSMA-TV) of the whole body from PSMA PET scans for response evaluation 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 tumor lesions. Changes in the whole-body PSMA-TV of 65 patients were comparable to the changes in PSMA-TV after including only the ten largest lesions. Moreover, changes in PSMA-TV correlated well with changes in PSA levels, as did the changes in PSMA-TV with the reduced number of lesions. We conclude that a response assessment using PSMA-TV with a reduced number of lesions is feasible and could lead to a simplified process for evaluating PSMA PET/CT. ABSTRACT: (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.