Li, W. B., Friedland, W., Pomplun, E., Jacob, P., Paretzke, H. G., Lassmann, M. and Reiners, C. Track Structures and Dose Distributions from Decays of 131I and 125I in and around Water Spheres Simulating Micrometastases of Differentiated Thyroid Cancer. Radiat. Res. 156, 419–429 (2001).
The disintegration of the radionuclides 131I and 125I and the subsequent charged-particle tracks left behind in water (as a model substance for a biological cell) are simulated by the Monte Carlo track structure simulation code PARTRAC, using new inelastic electron scattering cross sections for condensed water. Every photon and electron emitted was followed in detail, event by event, down to 10 eV. From the spatial information on the track structures, absorbed dose distributions per 131I and 125I decay were calculated in and around water spheres simulating micrometastases as well as in the tissue surrounding such metastases. These radionuclides were assumed to be distributed uniformly inside spheres of different diameters (0.01, 0.03, 0.1, 0.3, 1.0 and 3.0 mm). The respective electron degradation spectra, the nearest-neighbor distance distributions between inelastic events, and the distance distributions for all activations for both iodine radionuclides were calculated. The absorbed fractions of the initial electron energies, absorbed doses and energy depositions, and single-event distributions, F1(ɛ), inside the six water spheres described above and in the surrounding tissue were also calculated. The absorbed doses per decay inside the six water spheres, i.e., the calculated S values (listed from 0.01 to 3.0 mm), were 6.8 × 10–4, 7.2 × 10–5, 5.5 × 10–6, 4.9 × 10–7, 3.1 × 10–8 and 1.8 × 10–9 Gy Bq–1 s–1 for 131I, and 3.4 × 10–3, 1.7 × 10–4, 5.1 × 10–6, 2.0 × 10–7, 5.6 × 10–9 and 2.2 × 10–10 Gy Bq–1 s–1 for 125I. It is concluded that, in the treatment of thyroid cancer, the geometrical track structure properties of 125I might be superior to those of 131I in micrometastases with diameters less than 0.1 mm; however, in this medical context, many other factors also have to be considered.