Microscopic dose distribution from 125I in the toxic thyroid gland and its relation to therapy

A pilot study (Greig, Smith, Gillespie, Thomson and McGirr, 1969) has shown that thyrotoxicosis can be rapidly controlled using doses of 125I in the range 15–50 mCi. The rationale of this form of radioiodine therapy is that 125I will irradiate the parts of the thyroid cells completing synthesis of hormone to a significantly greater extent than it will irradiate follicular cell nuclei. The potential advantage of 125I over 131I for the long term treatment of thyrotoxicosis may be rapid control of the disease without a high risk of post-irradiation hypothyroidism. In this paper known aspects of the histology of the normal and thyrotoxic thyroid glands relevant to dosimetry will be described, the energies and abundances of the photon and electron emissions from 125I will be determined, and dose distributions around 125I sources of various shapes will be calculated. The implications of the relationships in the toxic gland between functional histology and dose distribution will be discussed in the light of the proposed use of 125I in the treatment of thyrotoxicosis. In the normal thyroid, follicles are spherical with diameters in the range 50 μm to 300 μm; the average being approximately 150–200 μm. Colloid in the centre of the follicles occupies at least 50 per cent of the total gland volume, and possibly as much as 75 per cent, depending on age, race and diet.