Pivotal Role of Dosimetry in High Activity 131i Mibg Therapy in Patients Affected by High Risk Relapsed Refractory Neuroblastoma: Towards a Deeper Metastatic Response Understanding

Abstract Background: Dosimetry in molecular radiotherapy for personalized treatment is assuming a central role in clinical management of aggressive/relapsed tumors. High-Risk relapsed/refractory metastatic Neuroblastoma (HRrrm-NBL) has a poor prognosis and high-activity 131I-mIBG therapy could represent a promising strategy.The aim of this study was to report the absorbed dose to whole-body (DWB), red marrow (DRM) and lesions (DLesion) correlated to clinical outcome.Methods: 14 patients affected by HRrrm-NBL were treated with high-activity 131I-mIBG therapy (two administrations separated by 15 days). The first administration was weight-based whereas the second one was dosimetry-based (achieving 4 Gy to whole-body). In all patients DWB and DRM has been assessed; 9/14 patients were selected for DLesion evaluation (for a total of 13 lesions). Treatment response was classified in Progression and Stable Disease (PD and SD), Partial and Complete Response (PR and CR) and correlated to the value of DLesion value.Results: The cumulative DWB, DRM and DLesion ranged from [1.49; 4.45], [0.99; 2.56] and [44.17; 585.75] Gy. After treatment 3, 2, 4 and 5 patients showed CR, PR, SD and PD respectively showing a correlation between the clinical outcome and DLesion with a threshold at 80 Gy.Conclusions: Our experience shows feasibility of high activity therapy of 131I MIBG in rrmHR-NBL children as two administration intensive strategy. Dosimetric approach allowed a tailored high dose treatment maximizing the benefits of radionuclide therapy for pediatric patients with a safety profile. The assesment of DLesion contributed to have a deeper understaning of metabolic treatment effects.