Comparison of day of treatment vs 1 day post-treatment Lu177 SPECT/CT images in patients treated with lutetium Lu 177 vipivotide tetraxetan.

Introduction: Ga-68 and F-18 PSMA agents provide excellent quality diagnostic PET/CT images of tumor foci when obtained at approximately 1 hour post tracer injection. It is unclear if modern SPEC/CT images obtained on the day of lutetium-177 (Lu-177) vipivotide tetraxetan therapy provide similar quality information or if longer delays post injection are necessary.

Methods: After IRB approval, we retrospectively explored in a clinical practice setting based on patient access, SPECT/CT images targeted at 0-4 hr post Lu-177 vipivotide tetraxetan (on the day of treatment) vs. targeted at least a day after injection (1 day post treatment) in patients with metastatic castrate resistant prostate cancer. SPECT/CT images (non contrast enhanced diagnostic CT) were acquired. This study evaluated image quality and lesion detection capability of the two imaging time points in clinical patients imaged at one of the two time points (unpaired studies). Two readers (VP and RW) qualitatively assessed image quality (scored 1-4), number of lesions, and quantitatively assessed (mean ± standard deviation (SD) of the tumor (T) to blood pool (BP) ratio and tumor-to-parotid gland (P) ratio) on the SPECT as well as baseline PET images.

Results: Forty-six patients underwent SPECT/CT either on the day of treatment (n=8; 17.4%) or a targeted one day post treatment (n=38; 82.6%; 1/38 was scanned 2 days p.i.). Both readers qualitatively assessed the early SPECT/CT images to be inferior in diagnostic quality to the delayed images. BP background was viewed as excessive on all day of treatment images rendering lesion detection difficult. T/BP at early point SPECT images were found to be lower (10.9 ± 7.1) as compared to late images (45.9±28.0). T/P at early and late time point SPECT images were found to be nearly identical; 3.0 ± 1.6 and 3.4 ± 2.3 respectively. On PET images, T/P and T/BP were 2.6 ± 1.7 and 23.0 ± 14.6, respectively.

Conclusions: Lu-177 vipivotide scans obtained within 4 hours of injection are qualitatively and quantitatively inferior in quality to those obtained a targeted 18-24 hours post therapeutic injection, specifically for lesions with lower uptake (mild to moderate) on PET. While patient convenience is greatest for the images obtained on the day of treatment, these results have moved our clinical practice to abandon day of treatment SPECT/CT imaging and target at least one day post Lu-177 vipivotide tetraxetan SPECT/CT imaging.