Fast SPECT Acquisitions for Single Time-Point Dosimetry in 177Lu-PSMA for Metastatic Castration-Resistant Prostate Cancer (mCRPC) Patients

Background: Implementing multiple time point dosimetry in clinical settings faces limitations due to the imaging burden, increased workload, and additional costs. Single time-point (STP) dosimetry has emerged as a critical technique for calculating radiation doses efficiently. It has been suggested that 48 hours post-administration is preferred as the optimal imaging time for determining the absorbed doses in the kidneys and tumours for metastatic castration-resistant prostate cancer (mcrpc) patients treated with ¹⁷⁷ Lu-PSMA. This study aims to evaluate a phantom study for fast SPECT acquisition and its clinical integration using STP dosimetry in ¹⁷⁷ Lu-PSMA mCRPC patients. Methods: Phantom and patient acquisitions were conducted using a dual-head SPECT/CT GE Discovery 870 DR equipped with a MEGP collimator. Fast SPECT acquisition was quantitatively optimized and calibrated at 5 seconds per frame over 60 frames per head, totalling 7 minutes per bed, using the Jaszczak phantom with known ¹⁷⁷ Lu activity. Images were quantitatively reconstructed using OSEM iterative reconstruction (16 iterations, 9 subsets, 128x128 matrix size) with Hybrid Recon 3.0. Whole-body SPECT/CT scans of eight treatment cycles (five patients), administered 5.6 – 8.1 GBq (mean of 6.8 GBq) of ¹⁷⁷ Lu-PSMA I&T, were imaged at approximately 48 hours post-administration. STP voxel-based dosimetry was performed to calculate the absorbed doses in the kidneys and tumours using the Hanscheid method in Hermes Dosimetry Software. Results: The mean absorbed doses in the kidneys were 2.04 ± 0.37 Gy, confirming that the total absorbed dose delivered to the kidneys remained within the acceptable limit of 23 Gy. The tumours (n=25) showed absorbed doses ranging from 0.98 – 16.05 Gy. Notably, different tumour sites exhibited significant variation in absorbed doses, with most of tumours receiving higher doses compared to the kidneys. Conclusions: STP dosimetry for ¹⁷⁷ Lu-PSMA, coupled with fast SPECT acquisition, demonstrates significant potential for clinical application. The successful transition from phantom studies to clinical settings marks a pivotal development towards more efficient and less time-consuming dosimetric practices in nuclear medicine, potentially improving patient care by reducing the imaging and procedural burden.