False negative scans in parathyroid scintigraphy – Factors involved and its implications in the era of minimally invasive parathyroidectomy

Abstract

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Objectives: Parathyroid imaging helps to localize the abnormal gland pre-operatively and plays a significant role to guide minimally invasive parathyroidectomy. This study aims to analyze false negative scans that can complicate surgical procedure and outcome.

Methods: 139 patients who had parathyroid scans and subsequent parathyroidectomy were analyzed retrospectively. Of these, 133 patients had primary hyperparathyroidism. Weights, pre-op PTH and calcium of the missed glands(FN cases) were compared to the correctly identified glands(TP cases) using regression analysis. Cure rate(response in calcium <10.6 and PTH <65 after surgery) and surgical approach were taken into account to understand how scintigraphy impacts the surgical management.

Results: The sensitivity, specificity, PPV and NPV for adenoma were 85%, 50%, 99% and 5% respectively. Mean gland weight of the FN cases(451mg,n= 24) on parathyroid scan was significantly lower (p=0.005) than TP cases (1283mg,n= 109). There was no significant difference in mean PTH (p=0.3)and calcium (p=0.5)values among FN and TP groups. There was a significant difference in occurrence of bilateral exploration (FN:17/24,70%; TP:16/109,14%,p=0.01) without any significant difference in the cure rates (FN:83%, TP:75%,p=0.3).

Conclusions: In our experience, gland weight is significantly associated with correct localization and a parathyroid scan might miss low weight lesions (n=24,18%). PTH and calcium cannot be used to select which patients should undergo pre-operative scintigraphic localization. The consequences of missing a lesion on parathyroid scan did not affect cure rate because of intra-op PTH monitoring. Rather, it affected the surgical technique and the rate at which minimally invasive procedures were performed.