Parathyroid imaging with thallium-201, activity in parathyroid and other tissue removed surgically

Parathyroid imaging by the T1-201 thallous chloride and Tc-99m pertechnetate subtraction technique is moderately successful, but its sensitivity is limited by the difficulty in correctly localizing small parathyroid lesions, particularly those of less than 0.5 g. In a series of 35 patients operated upon for primary or secondary hyperparathyroidism, only one parathyroid lesion out of ten less than 0.5 g was correctly localized, while twenty-nine out of thirty-five over 0.5 g were correctly localized. In order to quantify further the limitations of the test, percentage uptake of Tl-201 in parathyroid lesions has been estimated. Doses of 3.7 MBq (100 μCi) T1-201 were injected intravenously in 7 patients when the thyroid was exposed at operation before dissection or compromise of the blood supply occurred. Subsequently weighed and histologically confirmed samples of parathyroid, thyroid and skeletal muscle were counted against a standard in a well counter. Samples had been removed at times from 5 to 180 minutes after the injection of T1-201. Parathyroid activity varied from 0.011-0.033%/g (mean 0.018%/g), thyroid from 0.004-0.014%/g (mean 0.009%/g) and skeletal muscle from 0.001-0.004%/g. Activity in the tissues sampled tended to decrease with time after injection. The findings suggest that a parathyroid uptake of some 0.01% of the imaging dose (74 MBq, 2 mCi) defines approximately the lower limit for correct localization by current methods using T1-201