Geometric models in dosimetry of thyroid remnant mass

Absorbed dose to thyroid remnant tissue after 131I ablation becomes mass/size-dependent. This is a direct consequence of the small remnant size and radiation escape starts to be relevant. The self-absorbed fraction becomes mass/size-dependent. We have used Monte Carlo simulations to investigate the influence of the thyroid remnant shape upon the absorbed fraction calculation.

Thyroid residue was modeled using spherical, cylindrical and elliptical shapes. Uniform beta activity distribution and unit density medium (water) within a remnant was assumed. For each of the geometrical models beta self-absorbed fraction (varphi(beta)) was calculated using Monte Carlo codes, while the mean absorbed dose per unit cumulated activity (S(beta)) was calculated using MIRD formalism.

For spherical objects varphi(mono) for mean beta energy (E = 0.182 MeV) of 131I is always greater than varphi(beta) calculated for the complete beta spectrum. For spheres having diameters 2-6 mm and assumption varphi(beta) = 1, S(beta) is over-estimated by 11-37%. For cylinder and prolate spheroid of the same length and thickness, S(beta) for cylinder is 30% smaller because of the greater mass. Similarly, elliptical cylinder and general ellipsoid of the same length and the same perpendicular dimensions (width and breadth), have similar varphi(beta), while S(beta) for elliptical cylinder is correspondingly smaller.

For accurate dosimetry of thyroid remnants having masses <1 g and chordal lengths <1 cm it is necessary to calculate varphi(beta) for the full beta spectrum, or S(beta) will be overestimated. The shape of the remnant may also be important since elongated non-spherical objects may also have varphi(beta) < 1.