Post therapy SPECT/CT uptake correlates with baseline [¹⁸F]FDG-PET/CT parameters in relapsed non-Hodgkin lymphoma patients treated with [¹⁷⁷Lu]Lu-lilotomab satetraxetan.

Introduction: [¹⁷⁷Lu]Lu-lilotomab satetraxetan, a novel radioimmunoconjugate (RIC) is currently under investigation in a phase 1/2 study for relapsed non-Hodgkin lymphoma. Patients undergo [¹⁸F]FDG-PET/CT at baseline and SPECT/CT-scans after RIC. In the present work we explore the correlations between PET-based total metabolic tumour volume (TMTV) and total lesion glycolysis (TTLG), and SPECT-based total radioimmunoconjugate tumour volume (TRTV) and total radioimmunoconjugate lesion uptake (TRLU) parameters.

Methods: A total of 15 patients treated with a single administration of [¹⁷⁷Lu]Lu-lilotomab satetraxetan were included in this sub-study. Baseline [¹⁸F]FDG-PET/CT were conducted and MTV and TLG-values were calculated as recommended by the EANM procedure guidelines. A SUV-max-threshold of 41 % was chosen. Only lesions above the liver PERCIST-threshold were included. Post-therapy SPECT/CT scans were acquired 96 hours post-treatment and used to determine the TRTV and TRLU. For the SPECT-derived parameters, a semi-automatic approach was applied. An initial manual segmentation was performed by a nuclear medicine specialist to exclude normal tissue uptake. Then, a thresholding with a 26 % cut-off based on the voxel with the highest uptake in the initial segmentation was carried out. This threshold was chosen after a visual optimization. The TRLU was defined as the total activity in the TRTV.

Results: The median TMTV was 131.0 ml (range: 32.3 - 585.4 ml), whereas the median TRTV was 204.0 ml (range: 38.5 - 531.2 ml). In the majority of cases the TRTV was greater than the TMTV (11/15 patients). Slight differences are to be expected given the different modalities and methodology; such as the cut-off values and the exclusion of lymphoma manifestations below the liver PERCIST-criteria for TMTV. The median TTLG was 742.9 (range 110.4 - 3884.4) and the median TRLU was 7.2 MBq with a minimum value of 1.1 MBq and a maximum value of 44.6 MBq. There was a significant correlation between the TRTV and TMTV (spearman rank test, r = 0.78, p = 0.001) and between TRLU and TTLG (spearman rank test, r = 0.57, p = 0.025).

Conclusions: Total uptake of [¹⁷⁷Lu]Lu-lilotomab satetraxetan could be measured by performing SPECT/CT-scans post-treatment in patients with non-Hodgkin lymphoma. Significant correlations were found between baseline [¹⁸F]FDG-PET/CT parameters and post-therapy SPECT parameters. We predict that baseline[¹⁸F]FDG-PET/CT may be of importance to reveal prospective information about radioimmunoconjugate uptake in patients treated with radioimmunotherapy.