Prostate Adenocarcinoma Within a Thymoma: A Rare Case of Tumor-to-Tumor Metastasis

Tumor-to-tumor metastasis is defined as when metastasis from a primary tumor (donor) grows in a different primary neoplasm (recipient). Due to the structure of the thymus and the low incidence rate, thymic epithelial neoplasm has been rarely described in the literature as a recipient for metastases. In this report, a patient with advanced prostatic cancer and under control after chemo/hormone therapy was directed to our thoracic surgery unit for an anterior mediastinal mass detected during the staging workup for prostate disease. A limited uptake at fluorodeoxyglucose-positron emission tomography (FDG-PET) in the mediastinal lesion, while the surrounding tissue showed diffusely negative hypermetabolism, suggested a second primary thymic epithelial tumor with a possible carcinomatous differentiation. A thymectomy through a median sternotomy was carried out. Histopathological analysis after thymectomy revealed a type A thymoma with multiple elements of prostate adenocarcinoma within it. The foci of prostate adenocarcinoma were co-located in the context of the thymoma, revealing what is defined as a tumor-totumor metastasis. To our knowledge, this is the first report describing a thymoma as the recipient of metastases coming from a primary extrathoracic tumor without the involvement of other thoracic organs.