engleski

I-131 beta S-factors for various geometry models in dosimetry of thyroid remnants

The effectiveness of radioiodine ablation therapy, after total thyriodectomy in patients with well differentiated thyroid cancer, could be optimised using quantitative dosimetry. For this purpose the accurate knowledge of the geometric features of the remnant tissue is important, since the S-factor (mean absorbed dose per unit cumulated activity) depends on remnant mass and beta self-absorbed fraction. Using Monte Carlo (MC) simulation, beta absorbed fraction, &#934; &#946; , was calculated for I-131 within following geometrical models: sphere, cylinder, and ellipsoid. The complete beta spectrum of I-131 was used, as well as uniform beta activity throughout the object. According to the MIRD model, beta S-factors (mGy/MBq*s) were calculated for chordal lengths < 1cm and/or masses < 1g, where the escape of beta energy becomes relevant. For all geometrical models, &#934; &#946; is always smaller for the complete beta spectrum, then for just using mean 131I beta energy (0.182 MeV). This leads, for example, to 37 % overestimation of the S-factors for sphere of radius 1 mm, which drops to 11 % for sphere with r=3 mm. For thin elongated objects (cylinder with h=10 mm, r=1 mm, or spheroid with a=b=1 mm, c=5 mm) S-factors are Scyl= 7.60E-01 and Sspheroid= 1.11E+00, which are greater than the S-factor for sphere of the same maximal chordal length (diameter, d=10 mm), Ssphere= 5.47E-02 mGy/(MBq*s) . The values of S-factors presented could be helpful in assessing the potential dosimetric errors related to the inaccurate remnant mass determination due to the resolution loss of the imaging equipment.

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

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