Thyroid gland
Other thyroid carcinoma

Topic Completed: 17 February 2020

Minor changes: 24 September 2020

Copyright: 2003-2021,, Inc.

PubMed search: CASTLE thyroid

Xiaofeng Zhao, M.D., Ph.D.
Shuanzeng Wei, M.D., Ph.D.
Page views in 2020: 2,848
Page views in 2021 to date: 154
Cite this page: Zhao X, Wei S. CASTLE. website. Accessed January 19th, 2021.
Definition / general
Essential features
  • Intrathyroidal (ectopic) thymic carcinoma located at the lower pole of thyroid
  • Also called intrathyroidal epithelial thymoma, primary thyroid thymoma, lymphoepithelioma-like carcinoma of the thyroid, CD5 positive thyroid carcinoma
ICD coding
  • ICD-0: 8589/3 - carcinoma showing thymus-like element
  • Commonly involves the lower pole of the thyroid and surrounding soft tissue
Clinical features
  • Present with a slow growing neck mass with hard consistency and poor mobility
  • Some may present with hoarseness due to recurrent laryngeal nerve paralysis (Am J Clin Pathol 2007;127:230)
  • Clinical suspicion of thyroid mass followed by a fine needle aspiration which may not reach a definitive diagnosis or a resection in which the diagnosis is given based on histologic examination
Radiology description
  • Cold nodule on scintigraphy
  • Ultrasound: solid, heterogenous and hypoechoic mass
  • CT: well defined soft tissue density without calcification
  • MRI: isointensity on T1 weighted images and hyperintensity on T2 weighted images
  • Nodular masses located in the lower neck between the inferior pole of the thyroid and the upper mediastinum (Br J Radiol 2016;89:20150726)
Radiology images

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CT scan


Prognostic factors
  • Good prognosis, with 5 year and 10 year survival of 90% and 82%; poorer prognosis if nodal metastases or tumor extension to surrounding tissue (Am J Clin Pathol 2007;127:230)
  • Rare aggressive case can occur with brain, liver and lungs as common metastatic sites (Hum Pathol 1991;22:349)
Case reports
  • Total thyroidectomy with selected neck dissection and adjuvant intensity modulated radiotherapy
Gross description
  • Well defined, lobulated white-tan mass with hard texture, fibrous septa
Gross images

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Lobulated, solid and tan colored

Tumor and trachea

Microscopic (histologic) description
  • Lobular and expansive growth
  • Fibrous bands separating variably sized solid islands / nests of squamoid cells
  • Variable lymphocytes (mixed mature T and B cells versus immature T cells in thymoma) and plasma cell infiltration
  • Tumor cells either have ill defined cell border with vesicular nuclei and prominent nucleoli, or show squamous differentiation with distinct cell borders and eosinophilic cytoplasm
  • Low mitotic count
Microscopic (histologic) images

Contributed by Shuanzeng Wei, M.D., Ph.D.

Intrathyroid thymic carcinoma / CASTLE

Contributed by Andrey Bychkov, M.D., Ph.D.

Central necrosis

Tumor and thyroid tissue

Fibrous bands

Squamoid cells

Invasive growth

Cytology description
  • Nonspecific; features favoring include syncytial fragments of malignant cells with pleomorphic large nuclei, vesicular chromatin and prominent nucleoli in a background of lymphocytes (Acta Cytol 2016;60:421)
  • Resembles nasopharyngeal carcinoma (Diagn Cytopathol 1996;15:224)
Cytology images

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Sheets and clusters with keratin

Discohesive polygonal to ovoid cells

Positive stains
Negative stains
Electron microscopy description
Molecular / cytogenetics description
  • EGFR gene amplification and EGFR gene mutations were identified in some cases by FISH
  • No mutations in c-kit identified (Am J Clin Pathol 2015;143:223)
Sample pathology report
  • Thyroid, total thyroidectomy:
    • Intrathyroid thymic carcinoma, 1.2 cm, confined to the thyroid (see comment)
    • No lymphovascular invasion
    • Negative margins
    • Comment: Immunohistochemistry performed on block 1A with adequate controls show that the tumor cells are positive for p63, CD5, CD117 and negative for thyroglobulin and TTF1. The findings support the above diagnosis, which also is known as carcinoma showing thymus-like differentiation (CASTLE).
Differential diagnosis
Board review style question #1

    A 40 year old male presented with a mass in the left lower pole of the thyroid. The tumor is shown in the photomicrograph. The tumor is positive for p63, CD5 and CD117, and negative for thyroglobulin and TTF1. Which of the following is most likely the correct diagnosis?

  1. Squamous cell carcinoma
  2. Intrathyroid thymic carcinoma / carcinoma showing thymus-like differentiation (CASTLE)
  3. Medullary carcinoma
  4. Follicular carcinoma
  5. Papillary carcinoma
Board review style answer #1
B. Intrathyroid thymic carcinoma / carcinoma showing thymus-like differentiation (CASTLE)

Reference: Thyroid gland - CASTLE

Comment Here
Board review style question #2
    Which of the following is true about intrathyroid thymic carcinoma / carcinoma showing thymus-like differentiation (CASTLE)?

  1. Most commonly located in upper pole of thyroid
  2. PAX8 can be used to distinguish from thyroid carcinoma
  3. Most patients have a poor prognosis and die from this tumor
  4. Nests of squamoid cells with lymphocytes infiltrate is the key microscopic feature
  5. CD5 in CASTLE only highlights the lymphocytes
Board review style answer #2
D. Nests of squamoid cells with lymphocytes infiltrate is the key microscopic feature

Reference: Thyroid gland - CASTLE

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