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—Dr Rudavsky argues that neither123I nor technetium Tc 99m are appropriate radionuclides for the imaging of substernal thyroid tissue. Although the γ-emissions of these isotopes are, indeed, less energetic than those of131I, we do not agree that they are incapable of penetrating the bones of the anterior thorax. In fact, the calculated attenuation of γ-photons by 2cm of bone are 0.26 for123I and 0.18 for131I; ie, there would be almost as many photons penetrating bone from123I (74%) as131I (82%). In addition to these theoretical considerations, we are unaware of any report of substernal thyroid tissue successfully imaged with131I after failure of visualization with123I.We do concur that Tc 99m is not as useful an agent for the assessment of substernal thyroid tissue, but principally because of its high thoracic background activity. The
Ladenson PW, Ridgway EC. Sequestered Substernal Goiter-Reply. Arch Intern Med. 1983;143(10):2025. doi:10.1001/archinte.1983.00350100209057