Since I-131 is selectively taken up by thyroid cancer cells and the range of beta particles emitted is 1-2 mm, there should not be any big concerns of overdosing OARs. https://ehs.stanford.edu/reference/i-131-radionuclide-fact-sheet http://www.docs.csg.ed.ac.uk/Safety/rpu/gn/GN009.pdf.