Table 1.

The ICRP classification and justification of radiolabeled doses to be administered to human volunteers participating in clinical trials ²⁰

ICRP risk category	Radioactive dose		Justified for	Drug developmental stage	Ethically allowed in children?
	µSv	µCi
I	100	0.1–1 but preferably lower	An increase of knowledge	At any stage in drug development	Yes
IIa	1,000	10–100	An increase of knowledge and health benefit	At the end of phase II in drug development, after radiological dosimetry using tissue distribution data from animals and demonstration of efficacy of a drug in humans	No

ICRP, International Commission on Radiological Protection.