1119 - Evaluating Molecular Response Using <sup>68</sup>Ga-PSMA-PET/CT in Prostate Cancer Patients with Pelvic Lymph Node Metastasis Undergoing Definitive Radiotherapy: Clinical Significance and Implications
Presenter(s)
H. C. Onal1,2, O. C. C. Guler2, N. Torun3, B. Demirhan4, A. Elmali1, P. Hürmüz5, M. Deek6, P. T. Tran7, M. Reyhan3, and V. Murthy8; 1Baskent University Faculty of Medicine, Department of Radiation Oncology, Ankara, Turkey, 2Baskent University Faculty of Medicine, Adana Dr Turgut Noyan Research and Treatment Center, Department of Radiation Oncology, Adana, Turkey, 3Baskent University Faculty of Medicine, Adana Dr Turgut Noyan Research and Treatment Center, Department of Nuclear Medicine, Adana, Turkey, 4Iskenderun Gelisim Hospital, Hatay, Turkey, 5Hacettepe University Faculty of Medicine, Department of Radiation Oncology, Ankara, Turkey, 6Department of Radiation Oncology, Rutgers Cancer Institute of New Jersey, New Brunswick, NJ, 7University of Maryland School of Medicine, Baltimore, MD, 8Tata Memorial Centre, Homi Bhabha National Institute, Mumbai, India
Purpose/Objective(s): This study aims to evaluate the prognostic significance of metabolic response assessed by Gallium-68-labeled prostate-specific membrane antigen positron emission tomography/computed tomography (68Ga-PSMA-PET/CT) in prostate cancer (PCa) patients with pelvic lymph node metastases (PLNM) undergoing definitive radiotherapy (RT) with androgen deprivation therapy (ADT).
Materials/Methods: This retrospective analysis included 107 PCa patients with PLNM treated between October 2015 and June 2023. All patients underwent two 68Ga-PSMA-PET/CT scans: a pre-treatment scan for initial staging and RT planning, and a post-treatment scan performed 3–6 months after RT completion. Inclusion criteria included histologically confirmed adenocarcinoma, availability of pre- and post-treatment 68Ga-PSMA-PET/CT scans, a minimum of 18 months of ADT, and at least 24 months of follow-up. The primary endpoint was distant metastasis-free survival (DMFS), with progression-free survival (PFS) and prostate cancer-specific survival (PCSS) as secondary endpoints. Kaplan-Meier survival estimates and Cox proportional hazards models were used to identify independent predictors of DMFS, PFS, and PCSS.
Results: The median follow-up was 60.4 months. Post-treatment 68Ga-PSMA-PET/CT, performed at a median of 4.1 months after RT, showed a reduction in SUVmax in 98.1% of primary tumors and 93.5% of lymph nodes. Complete metabolic response (CMR) was achieved in 43.9% of primary tumors and 65.4% of lymph nodes. Among the 44 patients (41%) who experienced disease progression following RT, the median time to progression was 24.4 months (IQR: 11.3–40.3 months). Distant metastasis (DM) was the most common recurrence pattern, occurring in 34 patients (31.8%). Patients achieving primary tumor CMR had significantly better 5-year outcomes, with DMFS at 81.6% compared to 61.4% (p = 0.006), PFS at 72.2% versus 41.0% (p = 0.01), and PCSS at 96.4% versus 84.7% (p = 0.04). Lymph node CMR was also associated with superior 5-year outcomes, with DMFS at 87.3% compared to 42.6% (p < 0.001), PFS at 72.6% versus 25.9% (p < 0.001), and PCSS at 97.5% versus 77.1% (p < 0.001). In multivariable analysis, longer ADT duration (=24 months) and lymph node CMR were identified as independent predictors of improved DMFS, PFS, and PCSS. A Gleason score (GS) greater than 7 was associated with poorer DMFS (p = 0.02) and showed a trend toward worse PFS (p = 0.07).
Conclusion: This study underscores the prognostic significance of metabolic response assessed by 68Ga-PSMA-PET/CT in PCa patients with PLNM. Lymph node CMR and extended ADT duration (=24 months) were independent predictors of improved survival outcomes. These findings support the integration of 68Ga-PSMA-PET/CT into treatment response assessment and highlight the need for prospective validation to optimize therapeutic strategies for high-risk PCa patients.