PSMA PET/CT Patterns of Recurrence After Mono-Brachytherapy in Men with Low and Intermediate Prostate Cancer and Subsequent Management

Abstract Purpose Brachytherapy as monotherapy is a recommended treatment option for men with low to intermediate risk prostate cancer, achieving biochemical progression-free survival rates >95% at 5 years. Local recurrence is difficult to identify due to sensitivity of conventional imaging and post-brachytherapy artefacts. Biochemical recurrence (BCR) in this setting is often considered as systemic recurrence, treated with androgen-deprivation therapy (ADT) as primary salvage therapy. This study investigated PSMA PET/CT for recurrence after low-dose rate (LDR) and high-dose rate (HDR) brachytherapy as monotherapy in men with low or intermediate prostate cancer, as well as subsequent management when recurrence occurred only within the prostate. Methods We performed a retrospective single-centre analysis for patients who were treated with brachytherapy as monotherapy for prostate cancer from May 2002 to May 2021 to identify men who underwent [68Ga]Ga-PSMA-11 or [18F]DCFPyL PET/CT (PSMA PET/CT) for BCR. We report the findings on PSMA PET/CT, quantitative parameters, as well as the subsequent management of the patients. Results Forty patients were identified who underwent PSMA PET/CT (26 LDR and 14 HDR) to investigate a rising PSA at a median (IQR) of 7 years (3.0-10.8) after initial therapy. Median (IQR) PSA at time of PSMA PET/CT was 6.6 ng/mL (3.9 - 15.5). On PSMA PET/CT, 20/40 (50%) men had prostate-only recurrence, 5/40 (12.5%) had nodal recurrence, 2/40 (5%) had bone-only metastases, 10/40 (25%) had multiple sites of recurrence and 3/40 (7.5%) had no visible sites of recurrence. Of the 20 patients with prostate-only recurrence, 8/20 (40%) had recurrence in a high-dose radiation zone, such as within the brachytherapy seeds, versus 7/20 (35%) in an under-covered zone, such as seminal vesicles or prostate base. On PSMA PET/CT, recurrence within the prostate had median (IQR) SUVmax 10.4 (5.1-15.7) and volume 2.9 mL (2.0-11.2). Subsequent management of these patients with local recurrence included surveillance followed by ADT (9/20, 45%), salvage SBRT (3/20, 15%), salvage brachytherapy (2/20, 10%), salvage radical prostatectomy (1/20, 5%) and not treated/lost to follow-up (5/20, 25%). For those with surveillance followed by ADT, the mean time before introduction of ADT was 4.1 years (range 1 to 8 years). Conclusion In the setting of BCR following mono-brachytherapy, PSMA PET/CT effectively identified local recurrence occurring within the prostate, including within high-dose radiation zones. It may be safe to delay the introduction of ADT, though more research is required.