Sean Reynolds MD, Presenter: Nothing to Disclose

James Brian Allison MD, Abstract Co-Author: Nothing to Disclose

Marleen Moore MS, Abstract Co-Author: Nothing to Disclose

Christin E. Young, Abstract Co-Author: Nothing to Disclose

Janusz Karol Kikut MD, Abstract Co-Author: Nothing to Disclose

The effect on radioiodine uptake (RAIU) related to displacement of the thyroid off the thyroid probe axis will be demonstrated.

Advances in ultrasound and thyroid assays led to decline in utilization of scinitigraphy. RAIU is still used to calculate I131therapeutic dose for the hyperthyroid. A thyroid probe with NaI crystal and flat field collimator is commonly used. It produces flat isoresponse lines over a narrow angle. Without imaging, gland alignment within the field may be offset by several centimeters. Resulting artificially low RAIU leads to excess I131 treatment dosage and patient absorbed dose. A Picker thyroid phantom was filled with dissolved 3.87 MBq I123. Decrease in counts with incremental offsets of the phantom from the physical axis of the probe will be presented for 20, 27, 34, 41 cm distances from the crystal. Counts were sampled twice for 1 minute, averaged and decay corrected. For example, displacing the phantom from 1 to 4 cm offset from the physical axis resulted in count loss; 23% at 20 cm, 8% at 27 cm, 4% at 34 cm, 3% at 41 cm.

When performing RAIU without thyroid scan inferred gland size or location, a larger distance (30-40 cm) from the crystal should be utilized to prevent underestimation of RAIU which may cause I-131 overtreatment with unnecessary absorbed dose to the patient.

Reynolds, S, Allison, J, Moore, M, Young, C, Kikut, J, RAIU Underestimation due to off Axis Thyroid Positioning.  Radiological Society of North America 2013 Scientific Assembly and Annual Meeting, December 1 - December 6, 2013 ,Chicago IL. http://archive.rsna.org/2013/13044274.html