Effect of a positron emitting source on thyroid probe data

Objectives: Thyroid uptake probes provide an accurate and reproducible method for determining thyroid uptake and other uptake measurements such as bioassays. Inaccuracies in thyroid measurements often are due to technical errors including inconsistent positioning of the thyroid probe and improper measurement of the neck phantom. Erroneous data from variations in background activity were recognized early in the use uptake probes as an uncommon source of error. With the advent of widespread imaging of positron emitting radiopharmaceuticals, background activity changes could be an under-recognized source of error. To evaluate the possible impact on uptake probe data by positron emitting radiopharmaceuticals, thyroid uptake and bioassay measurements were obtained while simulating a patient who has been administered a PET radiopharmaceutical resting in a PET uptake room and pausing in a hallway.

Methods: To simulate a patient in a PET uptake room, a cylindrical Ge-68 sealed source (CS-27, Siemens Healthcare) was placed on the patient stretcher in a shielded PET uptake room sharing a wall with the room containing a thyroid uptake probe. Repeated count measurements using the thyroid probe and bioassay settings were obtained with the thyroid probe in three positions simulating uptake measurements of a patient: pointed to the floor to simulate a thigh measurement for background, horizontal to the floor and parallel to the shared wall, and horizontal to the floor pointed towards the shared wall. Similar background measurements were made with the probe pointed to the floor and toward the shared wall without the Ge-68 source in the room. To simulate a patient who had been administered a positron emitting radiopharmaceutical pausing in a hallway sharing a wall with a thyroid uptake probe room, the source was placed on a cart and moved to a position along the wall. Measurements with the probe directed to the floor and the source wall were made with the source in position for 30 and 60 seconds using the thyroid uptake and bioassay settings. All measurements were started after greater than 10 half-lives had occurred for fluorine-18 and at least 48 hours after the last administration of a technetium-99m agent in the department. The variance of the data were assessed at each probe position. The effect of the variations in background activity on 24 hour uptake data from 30 consecutive patients was determined.

Results: The source in a PET uptake room resulted in higher background data than for the source in the hallway for 30 or 60 seconds (Figure 1). The probe pointed toward the source gave the highest counts for all source placement location ranging from 13 to 68 times greater than background counts with bioassay counts for the PET uptake room demonstrating the greatest difference from background. When compared to the thyroid uptake data from 30 patients, the counts from the source would erroneously account for 8.4% of the median thyroid counts at 24 hours (range 1.9% to 58.6%) and 82.9% of the median background measurement (range 59.7% to 94.1%). Bioassay count data were affected in a similar fashion.

Conclusions: Positron emitting radiopharmaceuticals can artifactually elevate counts during thyroid probe measurements in the experimental setting. When choosing a location for a thyroid probe, attention should be paid to the proximity to PET uptake rooms and the expected movement of patients undergoing PET imaging past the probe.