engleski

New dosimetric patient-based modelling of thyroid remnant stunning and ablation

This work describes an effort to quantitatively investigate the effect of thyroid stunning and to derive a novel in-vivo dosimetric model as a useful guidance for clinical practice of radioiodine ablation in patients with well-differentiated thyroid cancer. This study involved 41 thyroid cancer patients following total thyroidectomy. The uptake of thyroid remnants was repeatedly measured by a conventional probe system and beta-gamma exposure rate meter during the first week after a median of 75 MBq and 1.9 GBq of I-131 had been given for diagnostic and therapeutic purposes, respectively, 4 and 5 weeks postthyroidectomy. The mass of thyroid remnants was determined from two orthogonal pinhole gamma camera images assuming an ellipsoidal shape. Thyroid remnant absorbed dose was calculated by applying a Medical Internal Radiation Dose (MIRD) formalism. The pre-ablation absorbed dose of diagnostic I-131 (D) was found to be a decisive cause of thyroid stunning (TS). A non-linear logarithmic model TS=-7.9+17.4xln(D), r=0.62, p<0.01 was established, revealing the onset of TS for D=2 Gy, 50% TS for D=28 Gy and complete TS for 492 Gy. The cut-off target maximal absorbed dose-rate and cumulative absorbed dose of ablative I-131 in thyroid remnant were set at 2 Gy/h and 150 Gy, respectively, so as to be on the safe side. Thyroid stunning has been proven to be a real phenomenon evident in vivo if thyroid remnant is internally irradiated by very few gray. If diagnostic administration of I-131 prior to radioiodine therapy is necessary, radiation absorbed dose delivered pre-therapy by diagnostic I-131 should be <2 Gy or the ablative activity of I-131 should be appropriately corrected for the effect of thyroid stunning in order to deliver an amount of radiation sufficient for a complete ablation. It appears that any amount of diagnostic I-131 activity used for pre-ablation whole-body scanning will induce thyroid stunning, even if therapeutic administration of I-131 occurs within the following 2-3 days.

thyroid cancer; dosimetry; thyroid stunning; thyroid remnant ablation

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