A Rare Case of Tumor-to-Tumor Metastasis of Thyroid Papillary Carcinoma within a Pulmonary Adenocarcinoma

Tumor-to-tumor metastasis (TTM) is a rare phenomenon first reported several decades ago in Fried's description of a bronchogenic carcinoma that metastasized to a meningioma.1 According to previous studies, renal cell carcinoma and meningioma are common recipients, whereas lung and breast cancer are common donors.2 Thus far, four cases of lung carcinoma harboring papillary thyroid carcinoma (PTC) metastasis have been reported in the English literature.3-6 Here we present another case of PTC that metastasized to pulmonary adenocarcinoma.Campbell et al. proposed the concept of TTM, which can be distinguished from collision tumor based on following criteria: 1) more than one primary tumor; 2) the recipient tumor may be a true benign or malignant neoplasm; 3) the metastatic neoplasm is a true metastasis with established growth within the host tumor, not the result of contiguous growth (collision tumor) or tumor emboli; 4) primary tumors spreading into the lymphatic system in the setting of general ized lymphoreticular malignancy are excluded.According to the 'seed and soil hypothesis'8 of cancer metastasis, interactions between cancer cells (seed) and specific organ microenvironments (soil) determine the outcome of metastasis.