Evaluating Interim Progression: First Posttherapy 177Lu-PSMA SPECT/CT Relative to Baseline PSMA PET/CT.

Prostate Cancer Working Group 4 recommends using prostate-specific membrane antigen (PSMA) PET for response assessment of patients with androgen pathway receptor modulator-resistant prostate cancer treated with 177Lu-PSMA-617. For that purpose, the time interval between baseline PSMA PET and initiation of therapy is critical as interim disease progression may influence the performance of PET as a response assessment tool.

Methods

We visually assessed the frequency of interim disease progression seen on the first posttherapy 177Lu-PSMA-617 SPECT/CT in 87 patients with androgen pathway receptor modulator-resistant prostate cancer. The time interval between baseline PSMA PET and initiation of therapy, pretherapy prostate-specific antigen (PSA) measures, and treatment outcomes between patients with or without interim progression on SPECT/CT were also compared.

Results

For the 32 patients (36.8%) with interim progression, the PET to therapy interval (88 d) was significantly greater than those without progression (42 d, P < 0.001) despite adjusting for age, Gleason grade, and PSA doubling time. A time interval between baseline PSMA PET and initiation of therapy exceeding 30 d was seen in approximately 94% of patients with interim progression. PSA response and overall survival were not impacted by interim progression.

Conclusion

Baseline PSMA PET may not represent the true disease status if the first cycle of 177Lu-PSMA-617 occurs more than 30 d after baseline PET.