Early Prostate-Specific Antigen Kinetics for Low- and Intermediate-Risk Prostate Cancer Treated With Definitive Radiation Therapy.

Abstract Purpose This study used a patient-specific model to characterize and compare ideal prostate-specific antigen (PSA) kinetics for low- and intermediate-risk prostate cancer after definitive radiation treatment with conventionally fractionated, hypofractionated, stereotactic body radiation therapy, or brachytherapy, both high-dose and low-dose rate. Methods and Materials This retrospective analysis includes low- and intermediate-risk patients with prostate cancer treated between 1998 and 2018 at an National Cancer Institute–designated comprehensive cancer center. Demographics and treatment characteristics were prospectively collected. Patients had at least 2 PSA measurements within 24 months of treatment and were free from biochemical recurrence. The incidence of, time to, and risk factors for PSA nadir (nPSA) and bounce (bPSA) were analyzed at 24 months after radiation therapy. Ideal PSA kinetics were characterized for each modality and compared. Results Of 1042 patients, 45% had low-risk cancer, 37% favorable intermediate risk, and 19% unfavorable intermediate risk. nPSAs were higher for ablative modalities, both as absolute nPSA and relative to initial PSA. Median time to nPSA ranged from 14.8 to 17.1 months. Over 50% treated with nonablative therapy (conventionally fractionated, hypofractionated, and low-dose rate) reached an nPSA threshold of ≤0.5 ng/mL compared with 23% of stereotactic body radiation therapy and 33% of high-dose rate cohorts. The incidence of bPSA was 13.3% and not affected by treatment modality, Gleason score, or prostate volume. PSA decay rate was faster for ablative therapies in the 6- to 24-month period. Conclusions Analysis of PSA within 24 months after radiation therapy revealed ablative therapies are associated with a latent PSA response and higher nPSA. Multivariable logistics modeling revealed younger age, initial PSA above the median, presence of bPSA, and ablative therapy as predictors for not achieving nPSA ≤0.5 ng/mL. PSA decay rate appears to be faster in ablative therapies after a latent period. Understanding the different PSA kinetic profiles is necessary to assess treatment response and survey for disease recurrence.