Dosimetry during adjuvant (131)I therapy in patients with differentiated thyroid cancer-clinical implications

The activity of radioiodine ((131)I) used in adjuvant therapy for thyroid cancer ranges between 30 mCi (1.1 GBq) and 150 mCi (5.5 GBq). Dosimetry based on Marinelli's formula, taking into consideration the absorbed dose in the postoperative tumour bed (D) should systematise the determination of (131)I activity. Retrospective analysis of 57 patients with differentiated thyroid cancer (DTC) after thyreidectomy and adjuvant (131)I therapy with the fixed activity of 3.7 GBq. In order to calculate D from Marinelli's formula, the authors took into account, among other things, repeated dosimetry measurements (after 6, 24, and 72 h) made during scintigraphy and after administration of the therapeutic activity or radioiodine. In 75% of the patients, the values of D were > 300 Gy (i.e. above the value recommended by current guidelines). In just 16% of the patients, the obtained values fell between 250 and 300 Gy, whereas in 9% of the patients, the value of D was < 250 Gy. The therapy was successful for all the patients (stimulated Tg < 1 ng/ml and (131)I uptake < 0.1% in the thyroid bed in follow-up examination). Dosimetry during adjuvant (131)I therapy makes it possible to diversify the therapeutic activities of (131)I in order to obtain a uniform value of D.