Thyroid & parathyroid

Other uncommon thyroid carcinomas

Intrathyroidal thymic carcinoma



Last author update: 6 September 2023
Last staff update: 29 September 2023

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PubMed search: Intrathyroidal thymic carcinoma

Xiaofeng Zhao, M.D., Ph.D.
Shuanzeng (Sam) Wei, M.D., Ph.D.
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Cite this page: Zhao X, Wei S. Intrathyroidal thymic carcinoma. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/thyroidcastle.html. Accessed June 16th, 2024.
Definition / general
  • A malignant epithelial tumor with thymic differentiation occurring within the thyroid gland (WHO 5th edition)
  • Intrathyroidal (ectopic) thymic carcinoma (ITC)
  • CArcinoma Showing Thymus-Like differentiation (CASTLE)
  • Entity first described in 1985 (World J Surg 1985;9:128)
Essential features
  • Invasive intrathyroidal (ectopic) thymic carcinoma located at the lower pole of thyroid
Terminology
  • Acceptable: thymic carcinoma
  • Not recommended: CD5 positive thyroid carcinoma; lymphoepithelioma-like carcinoma of the thyroid; intrathyroidal epithelial thymoma; primary thyroid thymoma; thyroid carcinoma showing thymus-like differentiation; intrathyroidal carcinoma showing thymus-like elements (CASTLE)
ICD coding
  • ICD-O: 8589/3 - carcinoma showing thymus-like element
Epidemiology
Sites
  • Commonly involves the lower pole of thyroid lobes or attached to the thyroid (WHO 5th edition)
Etiology
Clinical features
  • Presents 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)
Diagnosis
  • Clinical suspicion of thyroid mass followed by a fine needle aspiration may not reach a definitive diagnosis; a diagnosis can be rendered based on histologic examination on resected specimen
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

Ultrasound

Prognostic factors
Case reports
Treatment
Gross description
  • Well defined, solid, lobulated pink-white mass with hard texture, fibrous septa; usually lacking calcifications or cysts on cut surfaces
Gross images

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Lobulated, solid and tan colored

Tumor and trachea

Microscopic (histologic) description
Microscopic (histologic) images

Contributed by Shuanzeng Wei, M.D., Ph.D. and Andrey Bychkov, M.D., Ph.D.
Tumor nests and fibrosis Tumor nests and fibrosis Tumor nests and fibrosis

Tumor nests and fibrosis

Central necrosis

Central necrosis


Tumor and thyroid tissue

Tumor and thyroid tissue

Fibrous bands

Fibrous bands

Squamoid cells

Squamoid cells

Invasive growth

Invasive growth

Cytology description
  • Nonspecific; features favoring ITC include syncytial or 3 dimensional tissue fragments of round or spindly tumor cells with 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
Electron microscopy description
Molecular / cytogenetics description
Sample pathology report
  • Thyroid, total thyroidectomy:
    • Intrathyroidal 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 and CD117 and negative for thyroglobulin and TTF1. The findings support the diagnosis above.
Differential diagnosis
Board review style question #1

A 40 year old man presented with a mass in the left lower pole of the thyroid. The tumor is shown in the photomicrograph above. 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. Follicular carcinoma
  2. Intrathyroidal thymic carcinoma
  3. Medullary carcinoma
  4. Papillary carcinoma
  5. Squamous cell carcinoma
Board review style answer #1
B. Intrathyroidal thymic carcinoma. Intrathyroidal thymic carcinoma often involves the lower pole of thyroid lobes with nests of squamoid epithelial cells, scattered lymphocytes and fibrous bands, along with typical immunohistochemistry phenotype. Answers A, C and D are incorrect because the morphology and immunoprofile are not consistent with this diagnosis. Answer E is incorrect because the immunoprofile shows thymic differentiation.

Comment Here

Reference: Intrathyroidal thymic carcinoma
Board review style question #2
Which of the following is true about intrathyroidal thymic carcinoma?

  1. CD5 in intrathyroidal thymic carcinoma only highlights the lymphocytes
  2. Most commonly located in upper pole of thyroid
  3. Most patients have a poor prognosis and die from this tumor
  4. Nests of squamoid cells with lymphocyte infiltrate is the key microscopic feature
  5. Polyclonal PAX8 can be used to distinguish from thyroid carcinoma
Board review style answer #2
D. Nests of squamoid cells with lymphocyte infiltrate is the key microscopic feature of intrathyroidal thymic carcinoma. Answer A is incorrect because CD5 also highlights the epithelial cells, which is a feature of thymic differentiation. Answer B is incorrect because intrathyroidal thymic carcinoma commonly involves the lower pole of thyroid lobes or attached to the thyroid. Answer C is incorrect because intrathyroidal thymic carcinoma has relatively good prognosis, with 5 year and 10 year survival of 90% and 82%. Answer E is incorrect because intrathyroidal thymic carcinoma arising from the intrathyroidal ectopic thymus also exhibits moderate to strong nuclear reactivity for polyclonal PAX8; thus, this cannot be used to distinguish from thyroid carcinoma. Monoclonal PAX8 was reported to be negative in intrathyroidal thymic carcinoma (Endocr J 2018;65:1171).

Comment Here

Reference: Intrathyroidal thymic carcinoma
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