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

The activity of radioiodine (I-131) 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 I-131 activity. Retrospective analysis of 57 patients with differentiated thyroid cancer (DTC) after thyreidectomy and adjuvant I-131 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 I-131 uptake <0.1% in the thyroid bed in follow-up examination). Dosimetry during adjuvant I-131 therapy makes it possible to diversify the therapeutic activities of I-131 in order to obtain a uniform value of D.

Automated summary

Dosimetry based on Marinelli's formula is essential for determining the radioiodine activity in adjuvant therapy for thyroid cancer. The retrospective analysis showed that adjusting therapeutic activities of I-131 based on dosimetry measurements can lead to successful outcomes for patients.