Bounce effect or local recurrence after low-dose-rate brachytherapy of the prostate? When prostate-specific membrane antigen positron emission tomography-computed tomography is false positive: a case report.

Abstract

Introduction: Low-dose-rate brachytherapy has been increasingly utilized as a minimally invasive treatment option in patients with low- and intermediate-risk prostate cancer. Following 1-2 years of treatment, a "bounce phenomenon" might occur in approximately 30% of patients undergoing low-dose-rate brachytherapy, characterized by a transient rise in prostate-specific antigen levels followed by a subsequent decrease. This phenomenon has been identified as a favorable prognostic factor. To date, only a few cases of a potential false-positive prostate-specific membrane antigen positron emission tomography-computed tomography related to the bounce phenomenon have been reported in literature. By presenting our clinical case, we aim to suggest refinements in the follow-up strategies and to assess the diagnostic value of prostate-specific membrane antigen positron emission tomography-computed tomography in managing cases with the bounce phenomenon.

Case presentation: A 66-year-old Caucasian (western European) male patient achieved a prostate-specific antigen nadir of 1.37 µg/l at 9 months after undergoing brachytherapy. At 21 months post-procedure, his prostate-specific antigen rose to 4.16 µg/l-following a period of stable and low prostate-specific antigen levels-prompting his general practitioner to refer him for prostate-specific membrane antigen positron emission tomography-computed tomography (298 MBq F-18-PSMA). Imaging revealed a prostate-specific membrane antigen-avid lesion within the prostate, suggesting a local recurrence, resulting in the offer of salvage therapy for the patient. However, a routine prostate-specific antigen screening before initiating salvage radiotherapy revealed a decrease to 3.75 µg/l with an additional reduction to 2.68 µg/l at 2 months later. The pattern of transient prostate-specific antigen elevation strongly suggested a bounce phenomenon rather than a recurrence, allowing any unnecessary treatment to be avoided. To date, prostate-specific antigen levels have been decreasing to as low as 0.48 µg/l, showing a satisfactory progress.

Conclusion: Our case illustrates a sporadically recognized false-positive prostate-specific membrane antigen positron emission tomography-computed tomography finding associated with a bounce phenomenon following low-dose-rate brachytherapy for prostate cancer. A single prostate-specific antigen test effectively ruled out the suspicion of local recurrence. While prostate-specific membrane antigen positron emission tomography-computed tomography is undoubtedly a valuable tool for detecting metastasis postoperatively, careful interpretation of local findings is essential owing to the potential for false positives. This consideration is vital when evaluating a patient with a rising prostate-specific antigen level after brachytherapy, to avoid premature initiation of salvage therapy.