3306 - Dosimetric Assessment of ¹77Lu-PSMA-617 and SBRT In Locally Advanced Prostate Cancer: Interim Dosimetry of the STARLiT Trial
Presenter(s)
Q. Li1, R. Kashani1, S. Ash2, J. E. Shoag3, J. Brown4, Y. Dewaraja5, S. Lichtman-Mikol1, W. C. Jackson6, R. T. Dess6, D. J. Gorovets7, H. Nagar7, Q. Li2, A. T. Price1, Y. Sun8, J. Garcia4, D. E. Spratt9, and A. Y. Jia1; 1Department of Radiation Oncology, University Hospitals Cleveland Medical Center/ Seidman Cancer Center, Cleveland, OH, 2Department of Radiology, University Hospitals Cleveland Medical Center, Cleveland, OH, 3Department of Urology, University Hospitals Seidman Cancer Center, Case Western Reserve University, Cleveland, OH, 4Department of Hematology and Oncology, University Hospitals Seidman Cancer Center, Case Western Reserve University, Cleveland, OH, 5Department of Radiology, University of Michigan, Ann Arbor, MI, 6Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, 7Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, 8Case Western Reserve University School of Medicine, Cleveland, OH, 9University Hospitals Seidman Cancer Center, Case Western Reserve University, Cleveland, OH
Purpose/Objective(s): STARLiT (NCT06574880) is an ongoing trial evaluating stereotactic body radiotherapy (SBRT) combined with ¹77Lu-PSMA-617 in patients with locally advanced prostate cancer without the concurrent use of hormone therapy. The dosimetric determinants of response in non-metastatic settings remain undefined, as well as the dosimetry in locally advanced disease to the predicted absorbed dose. We conducted a dosimetric analysis of the first 10 patients on STARLiT to determine absorbed dose in the prostate.
Materials/Methods:
The first 10 patients enrolled on STARLiT received two doses of ¹77Lu-PSMA-617 (~200 mCi per dose) with prostate SBRT (37.5–40 Gy in 5 fractions; 25 Gy to elective nodes; 35–40 Gy to gross nodes). ¹77Lu-PSMA-617 was delivered 6 weeks prior to SBRT, and the second cycle of ¹77Lu-PSMA-617 was 6 weeks post-SBRT. Quantitative SPECT imaging was performed for voxel-based dosimetry at 48 and 72 hours after Cycle #1 and 72 hours after Cycle #2. Maximum absorbed dose of ¹77Lu-PSMA-617 (Dmax) to the dominant intraprostatic lesion and involved lymph nodes were recorded. Mean absorbed doses to organs at risk, including parotid glands and kidneys, were calculated after Cycle #1 and Cycle #2.Results:
Of the 10 patients, 5 had NCCN very high-risk disease and 5 had cN1 disease. Baseline median PSA was 9.06 ng/mL (IQR 4.47-30.2), and PSMA SUVmax was 16.05 (IQR 10.0–61.1). Median administered activity was 203.1 mCi (IQR 195.0–205.9mCi) for Cycle #1 and 199.3 mCi (IQR 195.0–205.2mCi) for Cycle #2. The median prostate Dmax after Cycle #1 was 22.3 Gy (IQR 5.9–98.1Gy). Dmax strongly correlated with baseline PSMA PET/CT SUVmax (r = 0.90, 95% CI 0.61–0.98). The median Dmax from SBRT within the dominant intraprostatic lesion was 42.92 Gy (IQR 41.13–43.51), and median prostate Dmax from Cycle #2 was 7.76 Gy (IQR 4.29–11.78 Gy). The median prostate mean absorbed dose was 2.43 Gy (IQR) after Cycle #1, and 1.77Gy (IQR) after Cycle #2. Median absorbed dose to the lymph nodes was 3.88 Gy (IQR 1.73–67.0 Gy) after Cycle #1 and 0.87 Gy (0.32–1.60 Gy) after Cycle #2. The median organ mean absorbed dose to the parotid glands was 1.51 Gy (IQR 0.81–2.27 Gy) after Cycle #1 and 1.46 Gy (IQR 1.04–2.50 Gy) after Cycle #2. Corresponding kidney doses were 2.59 Gy (IQR 2.11–3.13 Gy) and 2.42 Gy (IQR 2.04–3.18 Gy), respectively.Conclusion:
In the first 10 patients enrolled on STARLiT, ¹77Lu-PSMA-617 delivered substantial additional absorbed dose to the dominant intraprostatic lesion beyond SBRT alone, with strong correlation with baseline PSMA PET SUVmax. Absorbed dose to PSMA-avid lymph nodes demonstrated marked interpatient variability. Enrollment continues and patients will be followed long-term to evaluate oncologic efficacy and safety.