Possibility of limiting the un-justified irradiation in ¹³¹I therapy of Graves' disease: A thyroid mass-reduction based method for the optimum activity calculation

Abstract 

Objective

In Graves' disease therapy, the amount of ¹³¹I is usually decided following two different modalities: the administration of a fixed activity or of an activity individually calculated based on a fixed value of target absorbed dose. Although the effectiveness of each of these approaches is good (about 80% of patients cured), the ALARA principle must be applied avoiding the un-justified radioactivity to the patient himself, the people living/working near him and the environment. In this paper a new approach to the ¹³¹I therapy in Graves' disease, based on the optimum value of the final thyroid mass, is presented.

Design

97 Graves' disease patients (29 males) were randomly assigned into three groups (GR1, GR2, GR3). In two of them (GR1, GR3) the radioiodine administering activity was calculated based on two fixed thyroid absorbed dose values (100Gy for GR1; 400Gy for GR3), in GR2 it was calculated based on the desired final optimum thyroid mass value m_f=0.24m₀/U₀

Results

The rate of cured patients are 48% (GR1), 97% (GR2) (z-test, p<0.001) and 97% (GR3). The average activity administered to GR2 (393±157MBq) is lower than that administrered to GR3 patients (524±201MBq) (p=0.007, two-tails unpaired t-test); the thyroid absorbed dose in GR2 (262±78Gy) is lower than in GR3 patients (407±23Gy) (p<0.001, two-tails unpaired t-test).

Conclusion

Our results demonstrate that the thyroid-mass based approach optimizes the treatment avoiding an un-justified excess or a not-effective too low activity without time and resources consuming.

Keywords: Radioimmunotherapy, Thyroid, ¹³¹I, Internal dosimetry, Graves' disease