Early prediction of tumour response to PRRT. The sequential change of tumour-absorbed doses during treatment with 177Lu-octreotate.

Unlabelled: [177Lu-DOTA0,Tyr3]-octreotate (177Lu-octreotate) in peptide receptor radionuclide therapy (PRRT) offers direct intra-therapeutic dosimetry. The aim of this study was to compare tumour and non-tumour parameters and assess intra-individual variations.
Patients, Methods: Retrospective analysis of 53 consecutive PRRT treatment cycles (mean activity of 7.53 ± 0.46 GBq 177Lu-octreotate, intended four cycles at intervals of 10-14 weeks, standard nephroprotection) in 27 GEP NET patients. Extended planar dosimetry with serial whole-body imaging on selected, non-superimposed tumour and non-tumour regions; liver (LM), bone (BM), and other (OM) metastases. The per-cycle variation was compared with post-treatment response (CT/MRI three months post-treatment, modified SWOG criteria).
Results: Residence time in tumor lesions (133-147 h) exceeded that in kidneys (93 h). Tumour-to-kidney absorbed dose ratios ranged from 14 to 28 (LM, BM, OM). Intra-individual per-cycle dose variation was insignificant for kidneys, but significant for metastases (LM, BM, and OM; p < 0.05). The mean per-cycle decrease of tumour absorbed dose (ΔD/A0[%]) was linked to morphologic response after PRRT. A mean decrease of >20% was predictive of a partial or minor remission in all 11 evaluable patients, while absent significant dose reduction indicated stable or progressive disease in 4/5 patients. The dose decrease was unrelated to volume effects and also observed for BM.
Conclusion: Besides confirmation of a favourable tumour-to-kidney parameter relation for 177Lu-octreotate, stepwise intra-lesional comparison seems to imply a prognostic impact of tumor dosimetry: The early per-cycle change ΔD/A0 between treatment cycles may predict the outcome after PRRT. Larger studies are needed to confirm this finding.