Objectives To reexamine the "Norman rule" (affected parathyroid gland would contain at least 20% radioactivity compared with background), report normative radiation data, offer alternative ratios, and explore the effect of lapsed time on minimally invasive parathyroidectomy (MIRP).
Design Retrospective analysis.
Setting Tertiary care academic medical center.
Patients A total of 116 consecutive patients who had a diagnosis of primary hyperparathyroidism, positive findings on sestamibi scan, and complete study data from 2000 to 2005 at a single institution.
Interventions Minimally invasive radio-guided parathyroidectomy (MIRP) for primary hyperparathyroidism.
Main Outcome Measures Ten-second gamma radiation counts at key procedural steps. Various ratios of measured radioactivity counts were studied.
Results A total of 116 patients who underwent MIRP had complete data; 91 patients waited 4 or more hours until surgery (78%), with some waiting 8 or more hours. Every patient had a successful surgery and was observed for 1 year thereafter. The Norman ratio of radiation counts (ex vivo to background) was compared with other radiation ratios using Spearman correlation; the comparisons included skin to background (
= 0.579), in vivo to background ( = 0.770), basin to background ( = 0.525), and in vivo–basin to background ( = 0.788). Regression analysis indicated that the Norman ratio decreased over time at 11% per hour (P = .31).
Conclusions Alternative ratios to the Norman ratio are reported. An ex vivo to background ratio greater than 20% as a rule of successful parathyroid adenoma excision was observed in all but 2 cases in our series.
