Unexpected Presentation of Occult Papillary Thyroid Cancer;Retropharyngeal Cystic Lymph Node Metastasis Mimicking Abscess

Significance Statement

Initial retropharyngeal metastases from papillary thyroid cancer are very rare. Cystic changes are seen in almost half of metastases. This makes the differential diagnosis difficult, especially in the presence of a single metastatic lesion. The consideration of the possibility of metastases of thyroid cancer in the retropharyngeal region is very important both in the diagnosis and in the follow-up of the patient.

A 36-year-old male patient was admitted to our center with neck pain and dysphagia of increasing severity over a 1-week period. He had no known chronic diseases and no history of trauma. Physical examination revealed a mass protruding into the air column on the right posterior oropharyngeal wall. Palpation of the neck revealed no lymphadenopathy or enlargement/nodularity of the thyroid gland. Contrast-enhanced computed tomography (CT) of the patient revealed a 20 mm × 14 mm central hypodense lesion in the right retropharyngeal area at the level of the oropharynx (Figure 1). As the cystic area was peripherally contrasted, a retropharyngeal abscess was considered in the preliminary diagnosis. Fine needle aspiration was performed through a transoral approach. Histopathological examination of the aspiration was reported as non-specific cyst content. The patient subsequently underwent contrast-enhanced magnetic resonance (MR) imaging of the neck. MR images showed a right retropharyngeal mass measuring 25 mm × 16 mm, hyperintense on T1-weighted sequence and heterogeneously hypointense on T2-weighted images. These signal features were attributed to possible post-aspiration hemorrhage. The cystic lesion had peripheral and nodular contrast enhancement on post-contrast series (Figure 2). Diffusion-weighted images showed no significant restriction in these areas. There was also a 7 mm × 5 mm nodule in the left lobe of the thyroid with cystic openings, heterogeneous enhancement and no diffusion restriction. Punch biopsy was performed with preliminary diagnosis of necrotic lymphadenopathy due to nodular contrast in retropharyngeal cystic lesion. Papillary thyroid carcinoma was diagnosed on pathological examination of the tissue sample. The same result was obtained by fine needle aspiration of the thyroid nodule. No distant metastases were observed on subsequent Positron emission tomography (PET)-CT. Total thyroidectomy and trans-oral retropharyngeal LAP excision were performed. No recurrence was observed at 6 months follow up.

Figure 1.

Axial (A), sagittal (B) and coronal (C) plane contrast-enhanced neck CT images show a peripherally contrasted cystic lesion (arrows) in the retropharyngeal area at the level of the oropharynx.

Figure 2.

Coronal section T1-weighted image (A) shows hyperintense (*) and T2-weighted sagittal image (B) shows heterogeneous intensity and leveling of the retropharyngeal lesion (arrow). Subtraction T1-weighted postcontrast axial section (C) demonstrates nodular contrast enhancement (arrow).

Lymph node metastasis of papillary thyroid cancer is observed in 30%–80% of patients and it is the sole finding in 10%–20% of cases (occult). Metastases are frequently seen in the central and lateral compartment lymph nodes. Retropharyngeal metastasis, which has been shown as a few cases in the literature, is rare. It is generally accepted that metastasis to the retropharyngeal lymph nodes occurs due to collateralization of the lateral cervical lymph nodes. This explains why most of the cases are persistent or recurrent. It has also been shown that 20% of the thyroid upper pole lymphatics are directly related to the retropharyngeal lymph nodes as an anatomical variation. Our case fits this anatomical variation.^1-3

Approximately 40% of lymph node metastases of papillary thyroid cancer completely cavitates the lymph node. This complicates the differential diagnosis of benign cystic lesions on radiological imaging, especially in single lymph node metastasis. Among the cystic lesions seen in the retropharyngeal space, abscess, and more rarely bronchogenic cyst and ganglion cyst may be considered. To our knowledge, this is the first case in the literature of papillary thyroid cancer initially presenting with purely cystic retropharyngeal lymph node metastasis. Cystic lymph nodes are observed as a central hypodense lesion with thin or thick wall on CT. If present, calcification is hyperdense. On MR imaging, the signal may vary on T1 and T2 weighted images depending on the cystic content. In postcontrast series, wall/nodular contrast enhancement may be observed. In metastatic lymph nodes of papillary thyroid cancer, the presence of nodular contrasting areas and calcification may be helpful in the diagnosis. Fine needle aspiration may not give accurate results in the diagnosis of retropharyngeal metastatic lymph node as in our case. The treatment of retropharyngeal lymph node metastasis is transoral/transcervical or transmandibular surgical excision.^2-4 Postoperative follow-up of thyroid cancer patients is performed with ultrasound, which may cause possible retropharyngeal metastases to be missed. Periodic CT or MR imaging will be useful in the follow-up of patients.^3,5