Physical and radiobiological bases of the use of125I in the management of thyrotoxicosis |
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Authors: | A R Reddy |
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Institution: | (1) Institute of Nuclear Medicine and Allied Sciences, Probyn Road, Delhi, India |
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Abstract: | Increasing interest is recently shown in the use of125I as an alternative isotope to131I for the management of thyrotoxicosis based on the postulate that there is a relative sparing of the reproductive integrity of the thyroid follicular cell and a consequent possibility of smaller incidence of hypothyroidism after therapy. A review of the dosimetric, radiobiological and clinical aspects of the use of125I are presented here.For the same activity though125I gives smaller mean glanddose than131I, the dose computations at microscopic level have revealed that there is a preferential irradiation of the apical membrane compared to the nucleus of the follicular cell. The ratio of the dose to the apical membrane and that to the nucleus increases with the decrease of the percentage colloid mass of the gland. Radiobiological significance of this non-uniform dose distribution across a follicular cell, with125I, is brought out using rat thyroid as the biological system. For the same mean gland dose the follicular cell survival is larger with125I than with131I. 24 h radioiodine uptake is reduced in case of125I treatment whereas it is not affected with125I. Pilot clinical trials using125I for the management of thyrotoxicosis are underway in some centres. Preliminary results from centres using doses 3–4 times that of the conventional131I dose are not very different from those with conventional131I therapy. Centres that used doses same as or less than the conventional131I doses, reported smaller incidence of hypothyroidism.Alexander von Humboldt Fellow in Klinikum Steglitz, Berlin |
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