Older regulations used to require a therapeutic dosage of radionuclides must be within 10%. Updated regulations for the NRC say that injected dosages must be within 20% of the prescription (or withing the specified range).
Note that this 20% requirement applies to both diagnostic and therapeutic dosages of nuclear material.
For further reading see:
https://www.nrc.gov/reading-rm/doc-collections/cfr/part035/part035-0063.html
https://www.nrc.gov/materials/miau/med-use-toolkit/faqs-part35.html