Metastasis of lung adenocarcinoma to follicular thyroid carcinoma combined with papillary thyroid carcinoma in the contralateral lobe: A case report.

Cancer-to-cancer metastasis is uncommon. The present study describes a case in which lung adenocarcinoma (LC) metastasized to follicular thyroid carcinoma (FTC). A 46-year-old Chinese woman exhibited a solid carcinoma (~60×40 mm) that nearly replaced the entire thyroid tissue of the left lobe, along with papillary thyroid carcinoma (PTC) in the right lobe. The initial clinical manifestation appeared as a thyroid neoplasm. Histological examination revealed two admixed components within the neoplasm of the left lobe: FTC and an adenocarcinoma component. The mutational profile of LC was analyzed using next-generation sequencing, revealing an epidermal growth factor receptor gene mutation at exon 21 (L858R) in the LC tissue. Differentiating between two tumor populations can be challenging when donor tumor cells closely resemble primary neoplasms of the recipient organ. Beyond detailed morphological evaluation, ancillary approaches, such as immunohistochemistry and molecular testing, are essential to detect tumor-specific markers. Moreover, the degree of cell differentiation is an important determinant in diagnosing cancer-to-cancer metastasis. The present study discusses differences in tumor cell morphology and their diagnostic relevance in relation to previous reports, clinical history, histopathology and molecular pathology. The observations may contribute to improving diagnostic precision in cancer-to-cancer metastasis.