Safe ¹³¹I treatment for children with advanced DTC

The radioiodine (¹³¹I) activities that are as high as safely administrable (AHASA) for the treatment of children with advanced differentiated thyroid cancer (DTC) are twice as high as those previously estimated, Verburg et al. report.

Advanced DTC in children and adolescents is treated by surgery and ¹³¹I therapy, but the AHASA of ¹³¹I therapy in children with this disease is not known.

...instead of giving a child 50 MBq/kg for initial ¹³¹I therapy or 100 MBq/kg for further therapy, I can now safely use the double of each...

“We wanted to know what we could theoretically give these children without inducing dangerous radiation exposure levels for the bone marrow. As we had data from extensive post-therapy measurements in Belarusian children with DTC after the nuclear reactor accident at Chernobyl treated at our center from 1995 to 2007, we thought we would try and derive such safety data,” explains study researcher Frederik A. Verburg of the University of Würzburg.

Data on the ¹³¹I treatment on 180 individuals of <20 years of age with advanced DTC were analyzed, which included 133 courses of ¹³¹I thyroid remnant ablation, performed with a median ¹³¹I activity of 51.8 MBq per kg of body weight, and 250 courses of subsequent ¹³¹I therapy, performed with a median ¹³¹I activity of 98.0 MBq/kg. Levothyroxine therapy had been stopped 4 weeks before ¹³¹I therapy.

The investigators used twice-daily measurements of ¹³¹I still present in each child or adolescent after ¹³¹I administration to calculate ¹³¹I whole-body retention, and these data were used to calculate the radiation dose absorbed to the blood per unit of ¹³¹I activity administered during each treatment course. The researchers considered the upper safe limit for the blood radiation dose to be 2 Gy, which is the accepted limit for adults with DTC. However, they acknowledge that experimental data on this safe limit in children are lacking.

“The most significant findings were that for initial ¹³¹I therapy no child had an AHASA activity that was lower than 100 MBq/kg and for further therapy no child came under 200 MBq/kg; both these values are twice what we conservatively assumed to be safe in current practice. Furthermore, most children can have even the double of these doses for treatment, but it takes pretherapeutic dosimetry to determine which children that concerns. The implication for clinical practice is that instead of giving a child 50 MBq/kg for initial ¹³¹I therapy or 100 MBq/kg for further therapy, I can now safely use the double of each of these values without individual dosimetry in the knowledge that I will do no harm to the bone marrow,” concludes Verburg.