Predictive Value of PSA Bounce Phenomenon after Stereotactic Body Radiation Therapy in a Large Population of Prostate Cancer Patients Treated By 1.5 T MR Guided Adaptive Technique

Introduction: Prostate-specific antigen bounce (PSAb), characterized by a transient elevation in PSA levels followed by a spontaneous decline, is a well-documented phenomenon in prostate cancer (PC) patients undergoing radiotherapy. While PSAb can cause diagnostic uncertainty, mimicking biochemical recurrence, its etiology and clinical implications remain poorly understood. This study aims to analyze the incidence, characteristics, and prognostic significance of PSAb in a cohort of PC patients treated with stereotactic body radiotherapy (SBRT) using a 1.5 T MR-Linac platform. Methods: A total of 305 patients with low-to-intermediate risk PC (stage T1-T2, Gleason score ≤ 4 + 3) were treated with SBRT in five fractions (35–36.25 Gy) using an adaptive workflow guided by pretreatment MRI and real-time cine-MRI monitoring. PSAb was defined as a temporary increase in PSA levels ≥ 0.2 ng/mL above the nadir, followed by a return to or below the nadir. Statistical analyses evaluated correlations between PSAb and clinical/dosimetric variables, including tumor volume, treatment scheduling, and toxicity. The Chi-square test was used for categorical variables, while Spearman′s correlation and linear regression assessed continuous variables. Results: PSAb occurred in 25% of patients, with a median time to bounce of 11 months posttreatment. The incidence of PSAb within the first 6 months was 86.4%. The median time to post-bounce nadir (PSAn) was 25 months, with a mean PSAn of 0.90 ng/mL (range: 0.05–5.02 ng/mL). Biochemical relapse was observed in only 2,95% of patients. No significant associations were found between PSAb and tumor volume metrics or treatment scheduling. Toxicity analysis revealed predominantly mild-to-moderate acute events, including cystitis (28.2%) and proctitis (4.6%), with minimal late toxicity (cystitis: 9.2%; proctitis: 2.9%; sexual impotence: 4.3%). Univariate analysis demonstrates a significant correlation between PSAb and improved biochemical relapse-free survival (p = 0.016). Conclusions: PSAb is a common and benign phenomenon in PC patients treated with SBRT, associated with favorable oncological outcomes. The absence of significant correlations with tumor volume or treatment scheduling suggests that PSAb may be influenced by individual biological factors rather than dosimetric or procedural variables. These findings underscore the importance of distinguishing PSAb from true biochemical recurrence.