engleski

Geometric models in dosimetry of thyroid remnant mass

Aim: 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. Methods: 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 (&#966; &#946; ) was calculated using Monte Carlo codes, while the mean absorbed dose per unit cumulated activity (S&#946; ) was calculated using MIRD formalism. Results: For spherical objects &#966; mono for mean beta energy (E = 0.182 MeV) of 131I is always greater than &#966; &#946; calculated for the complete beta spectrum. For spheres having diameters 2&#8211; 6 mm and assumption &#966; &#946; =1, S&#946; is overestimated by 11&#8211; 37%. For cylinder and prolate spheroid of the same length and thickness, S&#946; 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 &#966; &#946; , while S&#946; for elliptical cylinder is correspondingly smaller. Conclusion: For accurate dosimetry of thyroid remnants having masses <1 g and chordal lengths <1 cm it is necessary to calculate &#966; &#946; for the full beta spectrum, or S&#946; will be overestimated. The shape of the remnant may also be important since elongated non-spherical objects may also have &#966; &#946; < 1.

Internal dosimetry; radioiodine ablation; absorbed fraction; Monte Carlo calculation

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