Molecular Characterization of an Endometrial Endometrioid Adenocarcinoma Metastatic to a Thyroid Hürthle Cell Adenoma Showing Cancerization of Follicles

Tumor-to-tumor metastasis is rare. Herein, we present a unique case of endometrial endometrioid adenocarcinoma metastatic to a thyroid Hürthle cell adenoma 9 years after initial diagnosis. On histologic examination of the thyroid, the malignant endometrioid glands and single cells (donor tumor) were dispersed within the Hürthle cell adenoma (recipient tumor). In several sections of the adenoma with still preserved microfollicular architecture, malignant endometrial adenocarcinoma cells were admixed within oncocytic adenomatous epithelium (so-called "cancerization of the follicles"). This unusual phenomenon, to our knowledge, is a novel finding in the thyroid gland. Immunohistochemistry, subsequently elicited clinical history, and morphologic comparison of the tumor in the thyroid to the primary endometrial tumor confirmed the origin of the donor tumor cells. Molecular analysis of both the metastatic and primary endometrial tumors demonstrated PIK3CA and PTEN mutations in both tumors, as is characteristic of well-differentiated endometrioid tumors of the endometrium. Amplification of chromosome 1q was detected in both sites; however, only the metastatic tumor showed loss of chromosomes 2, 9, and 22. The morphologic differential diagnosis of metastatic endometrioid adenocarcinoma in the thyroid includes columnar cell variant of papillary thyroid carcinoma (CCVPTC) arising in a preexisting adenoma, endocrine glandular atypia within an adenoma, and metastasis from other anatomic sites. Histomorphologic differences among these entities may be subtle; therefore, knowledge of and morphologic comparison with prior malignancies and immunohistochemistry can be helpful in rendering the correct diagnosis.