Left superior parathyroid adenoma. A-25 year-old African-American female presented with renal disease and prior removal of three parathyroid glands due to hyperparathyroidism. Her post-surgical serum calcium and PTH were persistently elevated. Parathyroid scintigraphy with Tc-99 m-MIBI was unsuccessful for parathyroid adenoma localization (Row (b)). The patient then underwent MRI (Row (a)—T1-weighted, STIR, T1-weighted with contrast left to right) which demonstrated a left superior parathyroid adenoma. This adenoma was also demonstrated on repeat scintigraphy (Row (c)) performed just 3 days prior to surgery. On reexamination, it is felt that this left parathyroid adenoma was in fact present on the first set of scintigraphic images (Row (b); note arrow) but was not well-seen due to poor patient positioning on that exam. This 2 × 3 cm parathyroid gland weighed 5 g, and the intraoperative PTH (iPTH) dropped from 4090 to 463 pg/mL
Ectopic parathyroid adenoma . This is a 55-year-old male with an incidental finding of elevated serum calcium and PTH and diagnosed with primary hyperparathyroidism. Ultrasound demonstrated a probable 3 mm × 3 mm left superior posterior parathyroid adenoma. Subsequent scintigraphy did not localize a parathyroid adenoma. MRI was then performed ((a)–T1-weighted axial, coronal, and sagittal images; (b)–T1-weighted post-gadolinium axial view) and successfully localized an ectopic anterior mediastinal parathyroid adenoma. Scintigraphy was performed again on the day of surgery with a larger field of view and demonstrated the ectopic gland (axial SPECT/CT image shown in (c)). The patient then underwent mediastinal parathyroidectomy (adenoma size was 13 mm × 19 mm). iPTH dropped from approximately 170–59 pg/mL
Causes of False Positive and False Negative Findings
There are a number of causes of false positive findings on MRI which include lymph nodes, thyroid nodules (adenomas, exophytic colloid cysts), enlarged cervical ganglia, and other neck masses such as sarcoid nodules and neurofibromasi (Fig. 15.3). In addition, a parathyroid adenoma may be discriminated from hyperplasia or parathyroid carcinoma based upon signal characteristics on T1- and T2-weighted imaging.
Thyroid adenoma . This is a 35-year-old euthyroid female who was diagnosed with primary hyperparathyroidism. An outside ultrasound showed a thyroid nodule in the right thyroid lobe, but an outside sestamibi scan reportedly did not localize a parathyroid adenoma. The patient then underwent MRI which demonstrated a relatively isointense mass on the right on T1-weighted imaging (Row (a), left image) which was hyper-intense on STIR (Row (a), middle image) and enhanced (Row (a), right image—post-contrast T1-weighted image). The patient then underwent a second confirmatory sestamibi scan at our institution, which did not demonstrate typical findings of a parathyroid adenoma, but was suspicious for a thyroid adenoma. Therefore, the patient underwent an I-123 scan which demonstrated a hot right thyroid lobe nodule with 4- and 24-h uptake values of 18 % and 41 % respectively. It was then felt that this represented a toxic thyroid adenoma. At surgery, a dominant hyperplastic right thyroid nodule and bilateral superior parathyroid adenomas were found (there was >50 % post-excision drop in iPTH for both of these parathyroid adenomas during surgery.). Of note, the parathyroid adenomas were not detected on scintigraphy, and the finding on MRI was false positive for parathyroid adenoma
Potential causes of false negative findings on MRI include concomitant thyroid disease, the presence of parathyroid gland hyperplasia, small parathyroid gland size (glands smaller than 5 mm in size may not be seen due to limited spatial resolution), or post-surgical anatomic distortion or atypical signal. Motion and other types of artifact may also cause false negative findings .
Positron Emission Tomography-Computed Tomography (PET-CT)
PET-CT is now a well-recognized and well-established imaging modality which provides functional imaging integrated with the anatomic imaging component of CT. The primary radiotracer utilized (and with reimbursement) is F-18-fluorodeoxyglucose (FDG) for the purposes of oncology, cardiac viability, and neurology cases. PET-CT is not widely used nor considered a first-line imaging modality for preoperative localization of a parathyroid adenoma or even for general evaluation of the parathyroid glands. There is limited data on the use of PET radiotracers for preoperative localization of parathyroid adenomas. We briefly discuss two of these radiotracers, FDG and C-11-methionine (MET). Like MRI, PET imaging with these radiotracers may have potential utility in cases where there has been failure to localize a parathyroid adenoma with other imaging modalities, for evaluation of possible ectopic glands, or when the patient cannot receive intravenous contrast [10, 11]. One of the larger studies evaluating MET-PET for preoperative parathyroid adenoma localization demonstrated high accuracy, a sensitivity of 91 %, and a positive predictive value of 93 %, similar to Tc-99 m-MIBI single photon emission tomography . A meta-analysis by Caldarella et al. also demonstrated MET-PET to be a sensitive and reliable tool for patients with suspected parathyroid adenoma . Regarding FDG, Neumann et al. found that FDG-PET had higher sensitivity for detecting a parathyroid adenoma compared to Tc-99 m-MIBI SPECT, which they felt might have been related to the size of the gland and the better spatial resolution of PET .
FDG is produced by a cyclotron and has a positron mode of decay with a half-life of approximately 110 min. Uptake is based upon glucose metabolism (FDG being a radiolabeled “sugar”) with increased accumulation in areas of tumor, inflammation, and infection. Such is the basis of preferentially increased FDG uptake in an abnormal parathyroid gland or adenoma .
MET is also produced by a cyclotron and has a positron mode of decay, though it has a half-life of only 20 min. Methionine is a naturally occurring amino acid, and its accumulation (predominately through an active transport mechanism) in the body occurs in cells with a high demand for this amino acid [14–16]. Thus, greater accumulation is seen with benign or malignant tumors (i.e., parathyroid adenomas) as well as with inflammation.
Advantages and Disadvantages
As mentioned, one advantage of PET-CT imaging for the detection of parathyroid adenomas is its higher spatial resolution, and thus, the potential for detection of smaller adenomas, as compared to Tc-99 m MIBI scintigraphy. PET-CT is also a whole-body imaging approach, and this may offer even greater advantage for detection of an ectopic parathyroid adenoma in the neck or chest. As with the use of an intraoperative gamma probe after Tc-99 m-MIBI administration, there is also interest and on-going development of hand-held beta-probe devices for PET to assist in localizing and confirming the of excision of F-18-FDG-avid tumors [17, 18]. In the future, this tool may become increasingly beneficial when utilizing PET radiotracers.