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

Abstract

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-to-tumor 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.

Keywords: malignant thymoma; prostatic adenocarcinoma; thymic tumors; thymus; tumor-to-tumor metastasis.

Figure 1. Imaging of the mediastinal neoplasm at CT scan and at FDG-PET scan

The mediastinal mass of 81x73x54mm (red circle) at CT-scan (A). FDG-PET scan: a focal hypermetabolism with SUVmax 11.30 (white arrow) within negative surrounding tissue (B). Advanced prostatic adenocarcinoma with multiple metastases at the thoracic vertebrae, femur, sacrum and within the mediastinal lesion (black arrows) (C, D).

Figure 2. Pathologic specimen

Panoramic overview at 100x magnification, showing multinodular growing pattern and the clear area corresponding to the prostatic carcinoma metastasis (right, A). Classical type A thymoma (B). Prostate cancer metastasis (C ). Detail showing metastatic tissue (right) with the thymoma component (left), (D). Immunostain of the area showed in D with PSA and racemase that highlight prostatic tissue (E and F, respectively), and with p63 that highlight thymoma. B,C,D,E,F original magnification 200x.