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Hinson AM, Lawson BR, Franco AT, Stack BC. Association of Parathyroid Gland Biopsy Excision Technique With Ex Vivo Radiation Counts During Radioguided Parathyroid Surgery. JAMA Otolaryngol Head Neck Surg. 2017;143(6):595–600. doi:10.1001/jamaoto.2016.4635
What proportion of a hyperfunctional parathyroid gland must be evaluated with the gamma probe to satisfy the Norman 20% rule?
In this case series parathyroid biopsies satisfied the 20% rule with an accuracy that varied significantly with the relative proportion of the parent gland represented: half or more (96.6%), a quarter to one half (87.0%), less than a quarter (63.6%).
Parathyroid biopsies accounting for less than half of a parathyroid gland are not sensitive enough to reliably rule out a hyperfunctional parathyroid gland. Manipulation and excision of half of 1 or more parathyroid glands for ex vivo counts may increase the patient’s risk for developing hypoparathyroidism.
Parathyroid biopsy represents a means for normal and hyperfunctional glands to be distinguished intraoperatively. However, no data exist to guide surgeons regarding how much of a parathyroid gland must be biopsied to satisfy the 20% rule.
To quantify the relative proportion of a hyperfunctional parathyroid gland that must be evaluated with the gamma probe to satisfy the 20% rule.
Design, Setting, and Participants
A retrospective review of surgical data for 24 consecutive patients (16 women, 18 men; mean [SD] age, 66.6  years; range, 51-83 years) who underwent surgery for primary hyperparathyroidism between May and October, 2015, in a tertieary academic medical center.
Main Outcomes and Measures
Extirpated parathyroid glands were sectioned into parallel or pie-shaped biopsies and evaluated ex vivo with a gamma probe to determine what percentage of a hyperfunctional gland must be sampled to meet the Norman 20% rule. The hypothesis was formulated during data collection.
In total, 253 ex vivo biopsy specimens were obtained from 33 surgically removed parathyroid glands. Parathyroid biopsies satisfied the 20% rule with an accuracy that depended on the relative proportion of the parent gland represented: half or more (96.6%; 95% CI, 91.7%-100.0%), a quarter to one-half (87.0%; 95% CI, 79.3%-94.7%), less than a quarter (63.6%; 95% CI, 54.5%-72.8%). When less than a quarter of the gland was removed, pie-shaped biopsies were more likely to satisfy the 20% rule compared with parallel biopsies of the same weight (78.4% vs 56.2%; absolute difference, 22.2%; 95% CI, 4.7%-39.7%).
Conclusions and Relevance
Unless half of a parathyroid gland is biopsied during radioguided parathyroidectomy, the 20% rule cannot reliably rule out the presence of a hyperfunctional parathyroid lesion. Pie-shaped biopsies originating from the center of the gland are associated with a lower rate of false-negative results compared with peripheral biopsies of similar size. Pie-shaped biopsies and biopsy of half or more of each nonexcised parathyroid gland for ex vivo counts may increase the risk of remnant devascularization and resultant hypoparathyroidism.
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