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).
Tertiary care academic medical center.
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.
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.
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 (ρ = 0.579), in vivo to background (ρ = 0.770), basin to background (ρ = 0.525), and in vivo–basin to background (ρ = 0.788). Regression analysis indicated that the Norman ratio decreased over time at 11% per hour (P = .31).
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.