The 20% rule: a simple, instantaneous radioactivity measurement defines cure and allows elimination of frozen sections and hormone assays during parathyroidectomy.

BACKGROUND: Although primary hyperparathyroidism is a physiologic disease, surgeons rely on anatomical characteristics (gross and histologic) to determine appropriate operative decisions. After the development of radioguided parathyroidectomy, we hypothesized that the amount of radioactivity contained within resected tissue would be the only information needed to establish the nature of the tissue and to determine a cure for the disease.
METHODS: A total of 1290 tissue specimens were obtained from 345 patients who had sporadic primary hyperparathyroidism. Ex-vivo radioactivity, in counts per second, was measured in parathyroid and other tissues within 3.5 hours of sestamibi injection. Background radioactivity was measured after tissue excision, and ratios were calculated.
RESULTS: Lymph nodes, normal parathyroids, and fat never contained more than 2.2% of background radioactivity, whereas thyroid and hyperplastic parathyroids contained 5.5% and 7.5%, respectively, and never more than 16%. In contrast, adenomas contained 59% +/- 9% of background radioactivity (P < .000001 vs all other tissues), with a range of 18% to 136%.
CONCLUSIONS: Radioactive ratios are an instantaneous measure of metabolic activity, thereby determining parathyroid function. Any excised tissue containing more than 20% of background radioactivity in a patient with a positive sestamibi scan result is a solitary parathyroid adenoma. This alleviates the need to identify other glands, obtain frozen sections, or measure serum parathyroid hormone levels intraoperatively.