Abstract
Background: Radioactive iodine-131 (RAI-131) is used for the treatment and diagnosis of thyroid disorder. It is commonly used for internal radionuclide therapy of hyperthyroidism and differentiated thyroid cancer. The estimation of absorbed dose to thyroid remnants/tumors is the concern of nuclear physicians in RAI-131 therapy of carcinoma thyroid patients. The purpose of this study is to compare theoretically the estimated absorbed dose to thyroid remnants/tumors by dose commitment formula and medical internal radiation dose (MIRD) in RAI-131 therapy. The study is significant to recognize the limitations and advantages of the two techniques compared.
Methods: The absorbed doses to 21 different masses of thyroid remnants/tumors were estimated from MIRD and dose commitment formula; however, the residence time (τ), percentage uptake (%U), and initial administered activities (A0) were kept fixed. The previously published S-values of different masses of thyroid remnants/tumors were used in the study. The mean percentage difference and variation factor of absorbed doses between the two techniques were also measured.
Results: It was observed that the mean percentage difference between the two methods was 18.24% with a standard deviation of 19.25. However, the mean variation factor between the two methods was found to be 1.0687 (min: 0.766712 and max: 2.784375) with a standard deviation of 0.422885.
Conclusion: It is concluded that the methods are well matched only for some masses of thyroid remnants/tumors [3/21] but not for other masses [18/21]. It is also concluded that dose commitment formula can be used as an alternative to estimate the absorbed dose to single thyroid remnant/tumor as it is more useful than MIRD, because of some limitation of S-values in MIRD.

Key words: Absorbed dose, RAI-131 therapy, MIRD, dose commitment