Inappropriate probe positioning during radioactive iodine uptake measurements cause inaccurate 131I quantification in patients with Graves' disease.

Background: To evaluate the effects of probe positioning on radioactive iodine uptake (RAIU) measurements and 131I quantification in Graves disease (GD) patients. Materials and Methods: We performed a cross-sectional study in GD patients between May 9 and June 27, 2013. 24 hr-RAIU was measured. Measurement A was the reference measurement and was obtained with the probe parallel to the bottom of the thyroid cartilage. Measurements B, C, D, and E were obtained by moving the probe 2.5 cm to the left, 5 cm to the left, 2.5 cm vertically upward, and 5 cm vertically upward, respectively. The 131I doses were calculated. Non-reference measurements were compared with measurement A using the paired t-test or Wilcoxon signed-rank test. Results: A total of 63 GD patients (17 men, 40.3±13.6 years old) were enrolled. Among them, 57 patients had 24 hr-RAIU measurement and 37 patients chose the 131I treatment. Significant changes were observed in 24 hr-RAIU and 131I dose between measurement A (24 hr-RAIU 60.40±12.67% and median 131I dose 8.17 mCi) and measurement E (24 hr-RAIU 48.67±11.74% and median 131I dose 9.50 mCi) (P<0.05). In one patient, the 24 hr-RAIU was 20.3% lower in measurement E compared with that in measurement A, resulting in a 30 mCi increase in the calculated 131I dose. Conclusions: The vertically upward movement by a distance of a radius of the collimator could affect the 24 hr-RAIU measurement and thus cause inaccurate calculation of the 131I dose. [ABSTRACT FROM AUTHOR]