Thymic cyst with breast cancer metastasis

A 40-year-old woman presented with a cystic lesion in the anterior mediastinum. 12 x 8 cm in diameter, 4 years after mastectomy for breast cancer. Grossly, the lesion was cystic. resembling a multilocular thymic cyst. Histologically, the cyst was lined by tumor cells, forming papillae and ductlike structures. The cells were reactive with antibody to gross cystic disease fluid protein-15. Thymic tissue was present in the cyst wall, but not in the cyst lining. Breast cancer metastasis to the anterior mediastinum may present as a cystic lesion. It should be distinguished from non-neoplastic multilocular thymic cyst and from cystic thymoma.