Diagnostic Accuracy of 4D-CT for Parathyroid Adenomas and Hyperplasia

Discussion

4D-CT demonstrated high accuracy for the determination of both lesion side and quadrant location for single-gland disease. Our series suggests that 4D-CT agrees with the pathology findings in most of the cases and is a reliable choice for parathyroid adenoma detection. Importantly, 4D-CT accurately lateralized single-gland adenomas in 93% of cases, allowing the surgeon to perform a unilateral neck dissection with associated decreased morbidity and length of hospital stay. The ability to accurately lateralize the side of disease is a commonly used end point in parathyroid imaging because it enables a directed surgical approach. By comparison, sonography has a cited sensitivity for lateralization ranging from 57% to 86%.^5,11

4D-CT appears to hold potential for detection of multigland disease. Although this technique demonstrated suboptimal sensitivity of 44% in patients with multigland disease, it was 100% specific for ruling out multigland disease, as no cases that were thought to have multigland disease by 4D-CT had single-gland adenomas at pathology. These results compare favorably to those reported for sonography and sestamibi techniques. A review by Sugg et al,¹² including 23 patients with multigland disease, found that the combination of sonography and sestamibi imaging correctly predicted multigland disease in 30% of patients, incorrectly called single-gland disease in 30%, did not identify abnormal parathyroid glands in 30%, and yielded discordant results in 10%. In our cohort, 2 of the 3 false-negatives for parathyroid lesion detection were patients with multigland disease. Multigland lesions are particularly challenging to detect prospectively, likely related to the small lesion size. We would further posit, as other authors have suggested, that in the case of a negative 4D-CT examination, there is presumed multigland disease. Importantly, some authors suggest a nonselective operative approach in cases of negative or discordant imaging studies due to the increased likelihood of multigland pathology.^2,13

Unique angiographic and perfusion characteristics of parathyroid adenomas were described by Doppman and others in the 1970s, and used to identify abnormal hypervascular glands with preoperative digital subtraction angiography.^9,14 A characteristic “blush” has been described following the injection of intra-arterial contrast, indicating the presence of an abnormal gland that is hypervascular compared with normal thyroid and parathyroid tissue. Similar principles are applied to contrast-enhanced CT, where hyperenhancement, location, and size are important discriminators in detecting parathyroid adenomas.⁴ 4D-CT appears to further enhance detection, with the advantages of noncontrast imaging to distinguish the gland from surrounding thyroid tissue, early postcontrast imaging to assess for a hypervascular gland, and delayed postcontrast imaging to detect regions of altered enhancement and abnormal contrast retention.

It is important to be cognizant of the radiation dose delivered with CT, particularly multiphase CT. We use dose reduction techniques with 4D-CT, as described by Welling et al,⁸ including automatic tube current modulation and limiting the superior field of view to the mandibular teeth. Despite these techniques, 4D-CT delivers a significant radiation dose. Calculated CTDI_vol measurements range from 19.8 mGy (at 150mA) to 39.7 mGy (at 300 mA) per phase of imaging. These parameters are comparable with recently reported CTDI_vol values for neck CT ranging from 23.0 to 34.7 mGy.¹⁵ We have chosen not to exclude the noncontrast examination, as we think it provides important diagnostic information, particularly in its ability to show attenuation differences in lesions contiguous with the thyroid gland. The risk-benefit discussion must weigh the cost, morbidity, and risk of formal neck exploration with the exposure from 4D-CT. The examination will likely only require performance on a single occasion.

There are several limitations to our study. First, this study is a retrospective review and was not designed to make a direct prospective comparison of the accuracy of sonography, sestamibi, and 4D-CT imaging for the detection of parathyroid pathology. Second, there is a selection bias for cases of ectopic and multigland disease because these are more difficult to diagnose on traditional imaging modalities and thus more likely to progress to 4D-CT imaging. Our patient population may be skewed by this factor, with a relatively higher proportion of these subtypes of adenomas. Third, although the technique of 4D-CT at our institution has been performed with high accuracy, variability in diagnostic accuracy may occur across institutions, secondary to interpreter experience and study technique.

Nonetheless, our series demonstrates that 4D-CT is a valuable tool for appropriate operative planning. 4D-CT also shows a higher sensitivity and accuracy than commonly cited for sonography or sestamibi imaging. We believe the accuracy of this technique is particularly impressive when considering the patients referred for 4D-CT frequently had equivocal or indeterminate findings on prior sonography and sestamibi-SPECT studies. The multiplanar reconstruction capabilities provide beneficial spatial information for optimal operative planning and intervention.