Red marrow toxicity evaluation in paediatric 131I-NaI molecular radiotherapy for differentiated thyroid cancer

Abstract 131 I-NaI molecular radiotherapy for differentiated thyroid cancer (DTC) has been shown to be safe for the red marrow (RM) in adults. In paediatric patients, the mean absorbed dose to the RM ( D RM ) is generally much lower than the accepted 2 Gy limit, although no clinical data-based studies have been available to date. This study was designed to address this gap. Sixty-six paediatric patients, mean age 13.2 years (4.3–17.9), were enrolled. The mean administered activity was 3.4 GBq (1.1–10.6). D RM was estimated through emission rate measurements in blood samples and whole-body acquisitions, accounting for auto-irradiation and remainder of body contribution, respectively. Haematological data included platelet (PLT), white blood cell (WBC), and red blood cell (RBC) counts, collected before and after treatment. Wilcoxon–Mann–Whitney test (WMW) was performed to assess count variation; the Pearson’s correlation coefficient (PCC) was used to assess the correlation between count difference and D RM . The D RM -to-activity mean ratio was 0.09 mGy/MBq (0.02–0.19), corresponding to a mean D RM of 0.29 Gy (0.07–0.92). WMW revealed significant pre- to post-treatment changes in WBC, RBC, and PLT ( p < 0.05), but no case showed all three parameters simultaneously below normal values. PCC values were 0.007, 0.164, and 0.095 for WBC, RBC, and PLT, respectively, confirming no significant correlation between D RM and blood count variations. All treatments delivered D RM values well below 2 Gy. Although modest changes in blood counts occurred, no clinically relevant RM toxicity was observed. Overall, 131 I-NaI molecular radiotherapy for paediatric DTC appears safe.