Dosimetric approach to 131I ablation therapy for the differentiated thyroid cancer.

Objective: The aim of this study is to compare the activity amounts used in the standard activity approach with the activity amounts calculated with the dosimetric method for the ablation of post-operative differentiated thyroid cancer residual tissue.

Subjects and methods: Seventeen patients (mean age=47.5±8.4 years) were included in the study. Time-activity curves were created by measuring iodine-131 (¹³¹I) uptake values of residual tissue at 4^th, 24^th, 48^th and 96^th hours following oral administration of ¹³¹I. In a dosimetric approach, activity amounts exposing 300Gy to residual tissue were calculated using medicalinternal radiation dose (MIRD) formulation.

Results: Dosimetric calculation could not be made in 3 patients because there was not enough radioactive iodine uptake (RAIU) at the neck to calculate the radioactive iodine treatment (RAIT) dose. The lowest and highest activity amounts determined by dosimetric calculations were 259MBq and 10860MBq, respectively. Dose amounts for the proposed treatment were decreased in 8 and increased in 6 patients compared to the standard activity approach. Cumulative activity (CA) and total cumulative activity (TCA) levels were found to be high in 1 patient who could not achieve adequate ablation.

Conclusion: We recommend the dosimetric approach for ablation of residual postoperative thyroid tissue to find patients with high levels of CA and TCA levels to consider higher activity doses compared to risk stratification assessment.