Fast SPECT acquisitions for single time-point dosimetry in 177Lu-PSMA for metastatic castration-resistant prostate cancer (mCRPC) patients

Abstract 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 aimed 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 an MEGP collimator. Fast SPECT acquisition was quantitatively optimised 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.