Retropharyngeal ectopic parathyroid adenoma versus lymph node: Problem solving with CT neck angiogram.

A 66-year-old female underwent preoperative evaluation for primary hyperparathyroidism. Ultrasound (US) neck and technetium (Tc)-99m-sestamibi planar scintigraphy were negative, but single photon emission computed tomography/computed tomography (SPECT/CT) demonstrated a tracer-avid retropharyngeal nodule compatible with parathyroid adenoma (PTA). A retrospective review of CT neck angiogram (CTA) and neck magnetic resonance imaging (MRI) performed 4 months earlier for stroke evaluation revealed arterial phase hyperenhancing retropharyngeal tissue, which had been dismissed as a nonpathological lymph node. "Polar vessel sign" seen in two-thirds of PTA was also present on retrospective review of the CTA. The concordant findings between SPECT/CT and CTA were indicative of a solitary undescended ectopic PTA in the retropharyngeal space, an uncommon location. A successful surgical cure was achieved after minimally invasive parathyroidectomy. This case highlights the retropharyngeal space as an important ectopic site of PTA, limitation of US, and Tc-99m-sestamibi planar scintigraphy in identifying retropharyngeal PTA. We also discuss the role of CT and MRI and the challenge in differentiating retropharyngeal PTA from a lymph node.

Introduction

Surgical excision of parathyroid adenoma (PTA) is the standard of care for symptomatic primary hyperparathyroidism (PHPT) and for asymptomatic PHPT patients who satisfy the National institute of Health guidelines.[1] The goal of preoperative imaging in PHPT is to identify solitary PTA or the presence of multigland disease.[2] Localization of a solitary PTA on preoperative imaging facilitates minimally invasive parathyroidectomy (MIP).[2] When localization is negative, patient undergoes a standard four-gland bilateral neck exploration followed by total or subtotal parathyroidectomy. Intraoperative serial venous blood parathormone (PTH) assay is frequently used to assess adequacy of the excision.[3]

Most solitary PTA is perithyroidal in location. The prevalence of ectopic PTA is 6.3–16% in patients operated for PHPT.[4] Identifying an ectopic PTA on preoperative imaging is important to select the appropriate surgical approach and incision site, and to avoid potential surgical failure. Tc-99m-sestamibi single photon emission computed tomography/computed tomography (SPECT/CT) is widely used for localizing PTA including ectopic PTA. Ultrasound (US) is frequently used in combination particularly to evaluate coexisting thyroid disease. Contrast-enhanced 4D-CT or magnetic resonance imaging (MRI) with contrast is often used as problem-solving tools. In spite of the various imaging techniques, the challenge remains in differentiating PTA from its closest mimics, namely thyroid nodules and lymph nodes preoperatively.[3] Here, we report a case of undescended retropharyngeal ectopic PTA that was previously dismissed as a nonpathological lymph node on CT and MRI but later identified on Tc-99m-sestamibi SPECT/CT in an elderly woman with PHPT. She was treated successfully by MIP.

Case Report

A 66-year-old female with a recent clinical diagnosis of PHPT was referred for surgical evaluation. She complained of urinary urgency but was otherwise asymptomatic. Physical examination was significant for right-sided weakness from a cerebrovascular stroke sustained 4 months earlier. Her blood calcium was elevated at 11.8 mg/dl (normal range: 8.5–10.5 mg/dl), phosphorus was decreased at 2.0 mg/dl (normal range: 2.6–4.7 mg/dl), and PTH level was elevated at 196.9 pg/ml (normal range: 9–77 pg/ml) indicative of PHPT. Bone densitometry revealed osteoporosis in the left femoral neck. Patient consented for recommended parathyroidectomy. Preoperative imaging for localizing PTA was ordered. Combined neck US and dual phase Tc-99m-sestamibi scintigraphy with SPECT/CT are used as first-line imaging at our institution for evaluating PHPT. Neck US was negative for PTA or thyroid disease. Dual phase Tc-99m-sestamibi planar and SPECT/CT scintigraphy was obtained. Anterior planar images at 15 [Figure 1a] and 90 min [Figure 1b] were negative for PTA. Maximum intensity projections of the SPECT in anterior [Figure 2a] and lateral views [Figure 2b] showed focal intense tracer medial and posterior to left submandibular salivary gland, respectively. SPECT/CT fusion images [Figure 3] revealed retropharyngeal CT-isodense and tracer-avid soft tissue compatible with an ectopic undescended PTA in the clinical setting. However, a granulomatous or metastatic lymph node could have a similar appearance. We retrospectively reviewed the patient's neck CT angiogram (CTA) [Figure 4] and MRI [Figure 5] done 4 months earlier at the time of stroke evaluation. The imaging revealed an intensely enhancing 0.5 × 1.3 × 3.1 cm left-sided retropharyngeal oval-shaped discoid nodule superior to the upper margin of left thyroid lobe which previously had been dismissed as a nonpathological lymph node. CTA also showed a prominent vessel coursing toward the lower pole of the adenoma [Figure 4c], described in the literature as “polar vessel sign,” an imaging marker for PTA.[5] Our patient underwent minimally invasive parathyroidectomy with successful surgical cure on the basis of intraoperative PTH monitoring. A 1350 mg hypercellular parathyroid (normal 30-35 mg) was removed per pathology.

Figure 1

Dual-phase Tc-99m-sestamibi scintigraphy. Anterior planar images at (a) 15 and (b) 90 min were negative for PTA

Figure 2

Dual-phase Tc-99m-sestamibi SPECT/CT. Maximum intensity projection (a) anterior and (b) lateral images of the SPECT showed focal intense tracer uptake (arrows) medial to left submandibular salivary gland in anterior view and posterior to the gland in lateral view suspicious for ectopic PTA

Figure 3

(a-c) Axial, (d-f) coronal, (g-i) sagittal SPECT, noncontrast CT, and SPECT/CT fused images respectively showed retropharyngeal CT-isodense soft tissue with tracer uptake (arrows) suggestive of an ectopic undescended PTA

Figure 4

(a-c) Axial, sagittal, and coronal reformations of CT neck angiogram showed intensely enhancing 0.5 × 1.3 × 3.1 cm retropharyngeal PTA (yellow arrows), superior to left thyroid lobe (*), previously had been dismissed as a reactive lymph node. Retrospective review revealed a prominent vessel coursing toward the lower pole of the PTA (red arrow) described as “polar vessel sign” in literature

Figure 5

(a-c) Axial T2, sagittal short-TI Inversion Recovery (STIR), sagittal postcontrast MRI of the neck show the retropharyngeal PTA (arrows) as T2 hyperintense, enhancing tissue

Discussion

The retropharyngeal space is an important ectopic but uncommon location of PTA, even more so in patients considered for re-operation. In their series of sporadic PHPT, Gallagher et al. reported initial surgical failure of 17% for retropharyngeal glands compared to 3.1% for eutopic glands.[6] They found retropharyngeal glands to be four times more common at reoperation. These data highlight that the retropharyngeal space is a relative “blind spot” for PTA location.

While preference for imaging modality will vary with institution, Tc-99m-sestamibi SPECT/CT is a first-line preoperative imaging technique for PTA localization with a sensitivity of 90% and accuracy of 97.2%.[78] Although specificity of Tc-99m-sestamibi SPECT/CT is not widely quoted, false positives can occur due to uptake in thyroid nodules and lymph nodes which could be reactive, granulomatous, or metastatic in nature.[910111213] Reactive retropharyngeal lymph nodes are common in children below 5 years but isolated enlargement is unusual in adults and should raise suspicion for pathology. This pitfall is clearly demonstrated in our case. Also, a reactive cervical lymph node may demonstrate tracer retention on sestamibi scintigraphy as reported by Leslie et al.[12] Though such instances are very rare and constitute occasional case reports, they should be considered as important caveats for interpretation.[14] Multiphase contrast-enhanced 4D-CT or CTA can serve as a useful problem-solving tool as demonstrated in our case. On 4D-CT, PTA is typically hypodense on noncontrast images and intensely enhancing on arterial phase with rapid washout in venous or delayed phase.[29] Identification of “polar vessel sign” is useful to differentiate PTA from a reactive lymph node which also shows a fatty hilum on most occasions.[5] Though retrospectively identified in our case, the sign was helpful in confirming the scintigraphic diagnosis. The “polar vessel sign” is represented by an enlarged arterial branch or an early draining vein from the poles of the PTA and has been reported in two-thirds of solitary PTA, particularly with larger sized PTA.[5] The limitation of US in identifying retropharyngeal PTA is attributed to lack of acoustic window. The negative result on Tc-99m-sestamibi planar imaging is likely due to deeper location of the ectopic PTA, and the lower sensitivity of planar imaging has been documented. Also, given the fact that a significant percentage of PHPT is asymptomatic, a careful review of a patient's laboratory results and knowledge of the imaging appearance of PTA on 4D-CT and MRI can help identify asymptomatic PTA's whether the more common eutopic, or the uncommon ectopic retropharyngeal, as in our case.

Conclusion

Our case is one of the few surgically confirmed and cured patients in the literature of retropharyngeal PTA, an important ectopic location associated with relatively high initial surgical failure. Our case illustrates the utility of Tc-99m-Sestamibi SPECT/CT for accurately localizing ectopic retropharyngeal PTA, leading to cure by successful MIP. Our case also demonstrated the advantages and disadvantages of the various imaging modalities employed preoperatively, and the critical importance of image review with appropriate clinical information so that findings are not overlooked or misinterpreted.

Declaration of patient consent

The authors certify that appropriate patient consent was obtained.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.