The impact of different computational assumptions in 131 I dosimetry for hyperthyroidism therapy.

Abstract

The goal of the current study was to investigate the impact of different computational models on 131 I dosimetry prior to hyperthyroidism therapy. It was also aimed to highlight an accurate and cost-effective method for routine dosimetry of graves and toxic adenoma patients. A cohort of 45 patients was recruited in the current study with Graves (n=30) and Toxic Adenoma (n=15) diseases. The eligibility criterion was determined using the patients' blood test, 99m Tc- pertechnetate scintigraphy, and ultrasound scan. A properly calibrated thyroid probe equipped with sodium iodide crystal [NaI(Tl)] was used to obtain the uptake measurements at 2, 24, 48, 72, and 96h following administration of 0.27-0.73MBq 131 I tracer. The absorbed radiation dose of the thyroid gland/nodule was calculated by three different methods. The calculation models were based on the time-integrated activity (recommended by MIRD), effective half-life (recommended by EANM), and ellipsoidal-shape assumption. The mean effective half-life was 138±41h and 110±48h in Graves and Toxic Adenoma patients, respectively. The mean residence time was 125±5h in Graves patients, while it was 93±55h in Toxic Adenoma. The amount of 131 I activity required to deliver 200Gy to the thyroid gland in Graves patients was calculated as 436±381MBq, 426±370MBq, and 488±455MBq according to MIRD, EANM, and ellipsoidal-shape model, respectively. However, the activity required to impart 300Gy in the toxic nodules was computed as 622±332MBq by MIRD, 907±588MBq by EANM, and 1060±639MBq by the ellipsoidal-shape model. Overall, no significant difference was found between the MIRD and both of the EANM and ellipsoidal-shape models in the Graves patients (R2 =0.99, P>0.05). In contrast, less agreement (R2 =0.86) was shown between EANM and MIRD in Toxic Adenoma patients with no statistically significant difference (P>0.05), while the difference was significant (Pvalue <0.05) between the MIRD and the ellipsoidal-shape model with moderate association (R2 =0.66). It was deduced that the effective half-life-based model (EANM model) is a successful and affordable method for performing dosimetry in Graves patients. While, unit density sphere model sounds the most appropriate approach to be used in Toxic Adenoma dosimetry. However, using the ellipsoidal-shape assumption in the thyroid gland/or nodule dose calculation leads to redundantly larger activity administration.