Development of a CT-less SPECT Acquisition Protocol for Kidney Dosimetry in ¹⁷⁷Lu-PSMA-617 Radioligand Therapy

Targeted radioligand therapy of metastatic castration-resistant prostate cancer (mCRPC) with ¹⁷⁷Lu-PSMA (RLT) requires sufficient dose monitoring of the kidneys. Currently, dosimetry using SPECT/CT-imaging is the most preferred method. However, SPECT/CT is a time-consuming procedure and comprises additional radiation exposure to the patient. Moreover, not every therapeutic nuclear medicine facility has access to SPECT/CT. Therefore, the aim of this study was to develop a new procedure of kidney dosimetry without the use of SPECT/CT and evaluate this method in a large cohort of patients with mCRPC undergoing RLT.

A dedicated torso phantom with kidneys filled with a solution of ¹⁷⁷Lu-PSMA was used for quantitative calibration of a SPECT-camera. The calculated sensitivity was adapted according to the individual attenuation of the patient in four directions from the kidney surface to the body surface (ventral, dorsal, left and right) obtained from a previously performed CT. A total of 196 patients undergoing 926 cycles of ¹⁷⁷Lu-PSMA therapy were retrospectively analyzed. Abdominal SPECT was performed 24, 48 and 72 h after administration of ¹⁷⁷Lu-PSMA including scatter and dead-time correction in every patient. Kidney dose was calculated using an individual attenuation-based procedure and compared to values from international literature.

Volumes of interest of the kidneys were drawn in the three sequential SPECT-images to calculate intra-renal effective half-life. Absolute quantification of activity in the kidneys was accomplished obtaining a patient individual sensitivity based on the individual attenuation in the patient. Kidney dose was then calculated applying a bi-exponential time activity curve in Microsoft EXCEL. Mean kidney dose per administered activity was 0.54 (± 0.26) Gy/GBq.

With the presented procedure a reliable kidney dosimetry is possible without the use of SPECT/CT. Facilities without SPECT/CT are therefore able to perform an adequate kidney dosimetry without additional radiation exposure for the patient.