Incidental Dual Time Point Pituitary 99m Sestamibi Uptake in Patients with Suspected Parathyroid Adenoma: An Observational Study

Objectives: Localization of parathyroid adenomas by Tc99m Sestamibi (MIBI) is a common preoperative study in patients with primary hyperparathyroidism. It is a common practice to image patients at two timepoints to characterize both uptake and washout of the thyroid and parathyroid glands. The practice at our institution is to augment planar images of the head and neck with a Single Photon Emission Computed Tomography (SPECT) on early images and a SPECT/Computed Tomography (CT) on delayed images. Commonly, the sella/pituitary is in the field of view on these images and often demonstrates very prominent uptake. As pituitary micro and macro adenomas are both relatively common and also characteristically associated with hyperparathyroidism in Multiple Endocrine Neoplasia Type 1 (MEN1), increased pituitary uptake may entail an increased risk of the presence of coincident pituitary disease. Focal MIBI uptake in the pituitary has been previously described in patients with pituitary adenomas. No widely accepted normal range or activity threshold exists of pituitary MIBI activity to guide providers in how best to interpret and quantify such activity when seen. We have conducted an interim assessment of an initial 50 patients to quantify pituitary MIBI uptake in patients without known pituitary disease undergoing dual timepoint parathyroid scintigraphy.

Methods: Our radiopharmacy database was queried to compile a list of all patients undergoing dual timepoint MIBI SPECT parathyroid scans from December 2014 through October 2019. Maximum and mean activity was measured of both the pituitary glands as well as that of the adjacent uninvolved brain parenchyma of each patient at each timepoint. Multiple nuclear medicine specialists assessed the visual prominence of the pituitary activity in each patient by consensus agreement. This qualitative assessment was correlated with the quantitative measurements to determine accuracy of visual characterization. Correlation was also made to calcium, phosphorus, and parathyroid hormone levels as well as to the presence of an identified parathyroid adenoma.

Results: This initial assessment reviewed the records and images of 77 consecutive patients imaged between December 2014 and June 2016. Twenty seven patients did not have complete visualization of the pituitary at both timepoints, and were excluded. 50 patients were evaluated, including 32 women and 18 men with a mean age of 59.2 (SD 12.6) years. Ratios were obtained of maximum pituitary activity compared to adjacent brain activity to establish a target-to-background measurement at each timepoint. The ratios measured 26.9 (SD 23.4) on early images and 25.6 (SD 26.6) on delayed images. Visual prominence of activity correlated with the ratio, with an accuracy of 0.72 (95% CI 0.575, 0.838) and a sensitivity of 0.63 and specificity of 0.83. There was no correlation to age, gender, lab results, or parathyroid imaging results. Three patients (6%) had been referred for sella MRI based on visual prominence on SEPCT images, none of which demonstrated a pituitary adenoma (one demonstrated a partially empty sella).

Conclusions: Pituitary MIBI uptake is regularly appreciated on SPECT/CT images of the head obtained during parathyroid localization studies, with a mean target to background ratio of over 25 on both early and delayed images. The significance of high pituitary MIBI uptake is unknown, but has been described in cases of pituitary adenomas. Visual assessment of this activity as prominent is only relatively accurate (0.72) in quantifying the activity. Further investigation is required to both determine the significance of patients with unusually high pituitary MIBI uptake as well as strategies to improve on the current visual assessment.