Imaging Analysis

Eggshell calcification of thyroid nodule not always benign

Stephanie L. Lee
Sung Hye Kong

A 73-year-old man had been diagnosed with colorectal cancer at an academic medical center. An F18-fluorodeoxyglucose PET-CT scan was performed for tumor staging, and a small hypermetabolic nodule was noted in the right thyroid lobe, as shown in Figure 1A.

The patient had no prior history of thyroid disease or head and neck radiation. He did not have a family history of thyroid disease or thyroid cancer. He had no symptoms or signs of thyroid dysfunction or local obstructive symptoms.

A thyroid ultrasound demonstrated a 1-cm solitary, hypoechoic left thyroid nodule with discontinuous surface macrocalcification (Figure 1B). The report indicated that no abnormal nodes or masses in the neck were observed. An ultrasound-guided thyroid biopsy of the nodule showed a papillary thyroid carcinoma.

The patient and providers decided to treat the colorectal cancer immediately. After chemoradiation therapy, transanal resection and adjuvant chemotherapy, there was no evidence of persistent colorectal cancer.

Figure 1. Initial images. (A) F18-FDG PET-CT scan: Axial fusion image showing a discrete 1-cm hypermetabolic mass (red arrow; SUV 22.9) in the right thyroid gland. (B) Thyroid ultrasound scan: Transverse image of the right thyroid lobe showing a 1-cm nodule with eggshell calcification with a focal areas of discontinuous calcification (yellow arrow). Abbreviations: TR: trachea. CA: carotid artery.

Images courtesy of Stephanie L. Lee, MD, PhD, ECNU, reprinted with permission.

Figure 2. Images after anaplastic transformation. (A) F18-FDG PET-CT scan: Axial fusion image showing a much larger hypermetabolic mass (red arrow; SUV 29) in the right thyroid gland. (B) Thyroid ultrasound scan: Sagittal image of the right thyroid lobe showing a 4.8-cm hypoechoic mass (green arrows) with infiltrative margins (blue arrows) extending out from the unchanged circle of eggshell calcification. An adjacent spongiform nodule (yellow arrow) had not changed in size from the prior exam (not shown). Abbreviation: TR: trachea.

After 37 months, the patient was referred to our center for a growing neck mass. The thyroid ultrasound showed the ring of calcification had not changed from the prior exam, but a 4.8-cm very hypoechoic mass with indistinct margins surrounded the calcification was observed (Figure 2B). A fine-needle aspiration biopsy was suspicious for anaplastic thyroid carcinoma. A staging F18-fluorodeoxyglucose (FDG) PET-CT scan confirmed the hypermetabolic mass (standardized uptake value [SUV], 29) had grown since the last exam, but without evidence of local or distant metastatic spread of tumor.

A total thyroidectomy and central node dissection was performed and confirmed the diagnosis of a 5.1-cm anaplastic thyroid carcinoma with extrathyroidal extension. The patient refused adjuvant external radiation. Within 4 months, he had vocal cord paralysis with a locally advanced tumor recurrence and lung metastases. He was treated with local neck radiation therapy and chemotherapy — dabrafenib (Tafinlar, Novartis) and trametinib (Mekinist, Novartis) — but died because of airway obstruction 7 months after the diagnosis of anaplastic thyroid carcinoma.

This is an uncommon scenario of an anaplastic transformation of a micropapillary thyroid cancer. Surface calcification of a thyroid nodule has previously been considered evidence of slow growth with a benign pathology. According to Kim and colleagues, among 93 histologically proven thyroid nodules with eggshell calcication, 63.4% were malignant, and 36.6% were benign.

A study of thyroid nodules with eggshell calcifications showed nodules with a disruption or intermittent thickening of the calcified rim or a halo of soft tissue outside the calcification rim were highly significant for thyroid cancer. The molecular events associated with the anaplastic transformation from a well-differentiated papillary thyroid cancer to an anaplastic thyroid cancer are incompletely understood, although the mutational events are thought to involve multiple tumor suppressor genes.

Disclosure: Kong and Lee report no relevant financial disclosures.