(UroToday.com) The 2024 Society of Nuclear Medicine & Molecular Imaging (SNMMI) annual meeting featured a session on prostate cancer, and a presentation by Dr. Mina Swiha discussing a comparison of post-therapy 4 and 24-hour 177Lu-PSMA SPECT/CT and pre-therapy PSMA PET/CT in assessment of disease in men with metastatic castrate-resistant prostate cancer.
177Lu-PSMA is an effective treatment for metastatic castrate-resistant prostate cancer and 177Lu-PSMA SPECT/CT 24 hours post-injection has shown potential as a response biomarker for 177Lu-PSMA therapy but is inconvenient. This study presented at the 2024 SNMMI annual meeting aimed to compare various imaging biomarkers between 4-hour 177Lu-PSMA SPECT/CT and 24-hour 177Lu-PSMA SPECT/CT in reference to 68Ga PSMA-11 PET/CT.
This prospective analysis enrolled 23 new patients diagnosed with metastatic castrate-resistant prostate cancer who were commencing 177Lu-PSMA-I&T therapy at St Vincent’s Hospital, Sydney, Australia between May and November 2023. Inclusion criteria included conducting a 68Ga-PSMA-11 PET/CT within three months of commencing 177Lu-PSMA therapy demonstrating suitable molecular features (SUVmax >= 15 and SUVmax >= 10 at all the measurable lesions not impacted by partial voluming (> 2 cm)), metastatic castration-resistant prostate cancer and ECOG <= 2. Post-therapy SPECT/CT was acquired at 4- and 24-hours after dose one (vertex to mid-thigh, 3 fields of view, 10s/frame, energy window 208 keV ± 10%, and scatter window 165 keV ± 6.5%). Reconstruction of SPECT projections was performed with an iterative ordered subset expectation maximization algorithm using four iterations and 10 subsets.
Baseline 68Ga-PSMA-11 PET/CT, 4-hour, and 24-hour 177Lu-PSMA SPECT/CT of 21 patients were analyzed both visually and semi-quantitatively. Of note, two patients were excluded due to technical difficulties in SPECT/CT acquisition. Visual analysis assessed disease distribution to determine the number, size, and organs harboring lesions between the studies. Quantitative analysis and a standardized semi-automated workflow to delineate regions of interest with a minimum SUVmax cut-off of 3 and lesion size of at least 0.5 mm. Output parameters included SUVmean, SUVmax, and total tumor volume.
All patients had metastatic castrate-resistant prostate cancer with prior androgen receptor signaling inhibition and 11 (52%) patients had prior docetaxel and 5 (24%) cabazitaxel. The median age was 78 years and median PSA 53.6 ng/mL. All patients received a dose of 8.5 GBq 177Lu-PSMA-I&T prior to their SPECT/CT imaging time points. The median time between 68Ga PSMA-PET/CT for screening and the first dose of 177Lu-PSMA-I&T was 34 days (IQR 24-37). 177Lu-PSMA SPECT/CT was acquired 4 hours (250 +/- 16 min) and 24 hours (1,447 +/- 55 min) after dose administration. On visual analysis, disease distribution was identical between the three studies despite visually higher background activity on the 4-hour 177Lu-PSMA SPECT/CT compared to 24-hour 177Lu-PSMA SPECT/CT and screening 68Ga PSMA-PET/CT:
Overall, 11 of 21 patients (52%) had missed lesions on 4-hour 177Lu-PSMA SPECT/CT compared to 24-hour 177Lu-PSMA SPECT/CT, with the median number of missed lesions of 1 (IQR 0-1). All lesions were small volume (< 2cm) and a mix of bone and lymph nodal sites. There were 14 of 21 patients (67%) that had missed lesions on both 4-hour and 24-hour 177Lu-SPECT/CT compared with 68Ga-PSMA PET/CT with the median number of missed lesions of 3 (IQR 0-5). In 13/14 patients (93%) the missed lesions were small and no large lesions were missed. There were 3 of 21 patients (14%) that had new lesions identified on post-therapy 177Lu-PSMA SPECT/CT that were not identified on the screening 68Ga-PSMA PET/CT, of which all lesions were small and in bone.
Semi-quantitative total tumor volume, SUVmax, and total body SUVmean were highly correlated between 4- and 24-hour 177Lu-PSMA SPECT/CT with Pearson’s coefficient of 0.995, 0.988, and 0.99, respectively (p < 0.001). Semi-quantitative total tumor volume on 4-hour and 24-hour 177Lu-PSMA SPECT/CT both correlated strongly to baseline 68Ga PSMA-PET/CT with Pearson’s coefficient of 0.964 and 0.948, respectively (p < 0.001). The median total tumor volume for 4- and 24-hour 177Lu-SPECT/CT and 68Ga-PSMA PET/CT was 517, 364, and 471mL, respectively with no statistically significant difference (p = 0.172). Likewise, median SUVmean was 8.03, 7.27, and 7.41 and SUVmax was 44, 47, and 36 (p = 0.386 and p = 0.867), respectively.
Dr. Swiha concluded his presentation discussing a comparison of post-therapy 4 and 24-hour 177Lu-PSMA SPECT/CT and pre-therapy PSMA PET/CT in assessment of disease in men with metastatic castrate-resistant prostate cancer with the following take-home messages:
- 177Lu-PSMA SPECT/CT at 4-hour is not inferior to 24-hour 177Lu-PSMA SPECT/CT in assessment of disease distribution, total tumor volume, SUVmax, and SUVmean
- Thus, it could represent a more convenient and cost-effective alternative to either 24-hour 177Lu-PSMA SPECT/CT and interim PSMA PET/CT in assessment of treatment response
Presented by: Mina Swiha, Fellow, Department of Nuclear Medicine, St. Vincent Hospital, Sydney, Australia
Written by: Zachary Klaassen, MD, MSc – Urologic Oncologist, Associate Professor of Urology, Georgia Cancer Center, Wellstar MCG Health, @zklaassen_md on Twitter during the 2024 Society of Nuclear Medicine & Molecular Imaging (SNMMI) Annual Meeting, Toronto, Ontario, Canada, Sat, June 8 – Tues, June 11, 2024.