Collision tumor of thyroid: metastatic lung adenocarcinoma plus papillary thyroid carcinoma.

Collision tumors are rare entities wherein 2 separate, distinct histologic malignant neoplasms occur in adjacent anatomic proximity. The 2 malignancies may originate from the same organ or occur as metastases from other sites. We present a case of a collision tumor of metastatic lung adenocarcinoma and papillary thyroid carcinoma occurring in the thyroid gland of a 63-year-old woman who presented with a multinodular central neck mass. After excision and histopathologic examination, this tumor was identified as a collision tumor. This is the first report of this unique combination of tumors occurring in the thyroid gland. This case report emphasizes the role of detailed histopathologic analysis and the use of immunohistochemistry in better identifying rare thyroid neoplasms.