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23 March 2022 - Case of the Month #513

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Thanks to Dr. Jose Mantilla, University of Washington, Seattle, Washington, USA for contributing this case and discussion and to Dr. Andrey Bychkov, Kameda Medical Center, Kamogawa City, Chiba, Japan for reviewing the discussion.

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Case of the Month #513

Clinical history:
A left hemithyroidectomy was performed in a 71 year old woman with a large thyroid nodule.

Gross and histopathology images:

What is your diagnosis?

Click here for diagnosis, test question and discussion:

Diagnosis: Metastatic melanoma involving oncocytic follicular adenoma (Hürthle cell adenoma)

Test question (answer at the end):
Which of the following is true regarding tumor-to-tumor metastasis in general?

A. Carcinomas of the breast are among the most common "donor" tumors
B. Melanoma is the most common "recipient" tumor
C. Most "recipient" tumors are benign
D. Sarcomas are the most common "donor" tumor
E. Their definition includes metastasis to lymph nodes affected by lymphoma



Tumor-to-tumor metastasis is a rare phenomenon defined as the involvement of a true neoplasm via a metastatic disease from a second neoplasm. This definition excludes direct tumor extension ("collision tumor"), tumor embolism and nodal metastasis to lymph nodes affected by leukemia / lymphoma (J Cutan Pathol 2020;47:1196, Int J Surg Pathol 2017;25:63).

Carcinomas of the lung and breast are the most commonly described "donor" neoplasms (Int J Clin Exp Pathol 2012;5:367). Approximately 60% of the "recipient" neoplasms are malignant and the most common type is renal cell carcinoma (J Cutan Pathol 2020;47:1196, Urology 1987;30:35).

Metastases to the thyroid (not limited to tumor-to-tumor metastasis) are rarely detected in the clinical setting. However, they may be seen in up to 22% of autopsies of patients with widespread metastatic malignancy (Ann Surg Oncol 2017;24:1533). The most commonly described histologic types include melanoma, breast carcinomas and renal cell carcinoma (Cancer 2002;95:1869, Am J Pathol 1931;7:187, Ann Surg Oncol 2017;24:1533, Histopathology 2021;78:508). Involvement of thyroid neoplasms, such as in this case, has been reported in a small minority of cases (< 100 reported cases at this time), the most common of which are follicular adenomas (Cancer 2002;95:1869, J Cutan Pathol 2020;47:1196).

Diagnosing tumor-to-tumor metastasis presenting as thyroid nodules may be challenging in the absence of an adequate clinical history of malignancy. Given that the initial evaluation of these lesions is routinely done using fine needle aspiration, a high level of suspicion is necessary to identify both kinds of neoplasm. Pertinent immunohistochemical work up, performed on the cell block material or resection specimen, can be helpful to definitively classify these lesions.

Test question answer:

A. Carcinomas of the breast are among the most common "donor" tumors

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