engleski

Possible Dosimetric Errors in Radioiodine Ablation of Thyroid Remnant Tissue

We present a method for calculation of patient specific beta S-factor, S_beta (mean absorbed doses per unit cumulated activity), for thyroid remnant tissue in radioiodine (I-131) ablation. S_beta calculation requires measurement of remnant mass, m, and calculation of beta self-absorbed fraction, PHI_beta. Remnant mass was determined by processing the patient post ablation neck pinhole scintigrams. Obtained remnant dimensions, in particular geometric model (sphere, cylinder, ellipsoid) were used for (volume) mass calculation, and as a geometrical input for Monte Carlo (MC) code written for calculation of PHI_beta. Patient specific S_beta is then: DELTA(PHI_beta/m) ; DELTA=3.04x10-6 kg cGy MBq-1 s- 1 (equilibrium dose-rate constant for I-131). For sphere model, PHI_beta is significantly <1 for dimensions (diameters) d< 6 mm, and the usual assumption: PHI_beta =1, leads to overestimation of S_beta by 10-37%. More important, if measured dimensions are used without a resolution loss recovery, mass is overestimated and S_beta is actually underestimated by 60% (for spheres with d<=1 cm). Therefore, accurate measurement of remnant mass is crucial, in order to avoid serious dosimetric errors when calculating absorbed dose to thyroid remnant tissue. Remnant shape is also important. For example, S_beta results, for sphere of diameter d=1 cm, and spheroid with same height (c=1 cm) but thickness of only a=b=2 mm, are quite different ; S_beta (sphere)=0.05 mGy/(MBq*s) ; S_beta (spheroid)=1.11 mGy/(MBq*s).

iodine ablation; dosimetry; Monte Carlo

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