J Korean Endocr Soc.  2007 Dec;22(6):460-464. 10.3803/jkes.2007.22.6.460.

Mediastinal Uptake Misinterpreted as Metastasis in Papillary Thyroid Cancer

Affiliations
  • 1Department of Internal Medicine, Seoul National University College of Medicine, Korea.
  • 2Department of Pathology, Seoul National University College of Medicine, Korea.
  • 3Department of Thoracic Surgery, Seoul National University College of Medicine, Korea.
  • 4Department of Nuclear Medicine, Seoul National University College of Medicine, Korea.

Abstract

Radioactive iodine (RAI) therapy is used for the removal of remnant thyroid tissue or metastatic thyroid cancer cells in differentiated thyroid cancer. The main mechanism of the therapy is destruction of cells by radioactive iodine that penetrates the cells though the action of the sodium-iodide symporter (NIS). We experienced a case of a 26-year-old woman with mediastinal uptake as detected on a radioiodine scan, who was previously diagnosed with papillary thyroid cancer. For diagnostic tests including chest computed tomography (CT) and a radioiodine scan, the stimulated thyroglobulin level did not show a definite cause of the mediastinal uptake. During regular follow-up, the thymus became triangular with clear margins. The patient had neither specific symptoms nor physical findings related to the presence of a thymic mass. A subsequent CT scan showed an irregular margin of the thymus, suggestive of thymic metastasis. The patient underwent a mediastinectomy. The removed specimen was composed of normal thymic tissue. Moreover, we demonstrated the presence of human NIS by immunohistochemical analysis. After thymectomy, the mediastinal uptake was markedly decreased as compared to the previous scan. This case suggests that a clinician should be suspicious for the functional uptake of thymus when metastasis is unlikely in a clinical situation.

Keyword

iodine-131; sodium-iodide symporter (NIS); thymus; thyroid papillary carcinoma

MeSH Terms

Adult
Diagnostic Tests, Routine
Female
Follow-Up Studies
Humans
Iodine
Ion Transport
Neoplasm Metastasis*
Thorax
Thymectomy
Thymus Gland
Thyroglobulin
Thyroid Gland*
Thyroid Neoplasms*
Tomography, X-Ray Computed
Iodine
Thyroglobulin

Figure

  • Fig. 1 Radioiodine scans and stimulated thyroglobulin levels. (A) for 2 months after thyroidectomy (post-radioiodine treatment scan). (B) for 14 months after thyroidectomy (post-radioiodine treatment scan). (C) for 36 months after thyroidectomy (post-radioiodine treatment scan). (D) for 8 months after thymectomy (follow-up diagnostic scan): The mediastinal uptake was decreased after thymectomy. TSH, thyroid stimulating hormone; Tg, thyroglobulin; TgAb, antithyroglobulin antibody.

  • Fig. 2 Chest computed tomography (CT). (A) for postoperative 3 months (B) for postoperative 14 months: The thymus looks like an arrow head and it enlarged than previous CT (A). (C) for postoperative 3 years: The size and shape of thymic lesion was changed. The irregular nodular shape and enhancement in the lesion seems to be a metastatic lesion.

  • Fig. 3 Immunohistochemistry for hNIS protein in thymic tissue. Only a few thymic macrophages were stained by anti-hNIS (H&E, ×40).

  • Fig. 4 Immunohistochemical detection for hNIS protein in thyroid tissue. hNIS protein was detected in the basolateral membrane of the tissues of chronic thyroiditis around the papillary cancer (H&E, ×40).

  • Fig. 5 Immunohistochemistry for hNIS protein in tissue section of papillary thyroid carcinoma. Most of tumor cells reveal loss of hNIS expression except for focal membranous and cytoplasmic staining in a few tumor cells (H&E, ×100).


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