[Replacement of neck phantom with a stand for irradiation source support in thyroid radioiodine uptake test]

To assess the possibility of replacing neck phantom with a stand to support the irradiation source in thyroid radioiodine uptake test.

The counts per minute (cpm) of 9 radioiodine samples of step-wise increasing irradiation doses were measured when different placements of the irradiation source, namely via neck wax phantom, a supporting stand or direct fixation, were adopted, with the uniform distance between the standard source and the crystal surface of the detector being 26 cm. In clinical trial, the values of radioactive iodine uptake (RAIU) of 30 patients with hyperthyroidism were obtained adopting 3 and 24 h separately after oral radioactive iodine administration both neck phantom method and stand-supported standard source measurement, and statistical analysis of the results were performed.

Statistical analysis revealed significant differences between the cpm measured using the 3 methods (F=59.339, P=0.000), and that measured by neck wax phantom was 16% higher than those obtained from measurements using a stand (P=0.000) or direct fixation (P=0.000). Despite that no significant difference was noted between the latter 2 methods (P=0.324), the 3 approaches were closely correlated with each other. The results of RAIU obtained by stand-supported source placement were significantly higher than those from neck phantom approach (t=26.033, P=0.000), showing close correlation between them (r=0.996, P=0.000) that could be expressed as RAIUnp=0.863xRAIUst.

The results of RAIU from the 2 approaches of source placement methods are significantly different but highly correlated, and the 2 methods are not equal but mutual replacement is possible when some technical treatment of the results is made.