Incidental Detection of PSMA-Avid Thymic Squamous Cell Carcinoma on [68Ga]Ga-PSMA-11 PET/CT During Initial Prostate Cancer Staging

A man in his 70s with biopsy-confirmed prostate adenocarcinoma underwent gallium-68-labeled prostate-specific membrane antigen (PSMA)-11 ([68Ga]Ga-PSMA-11) positron emission tomography/computed tomography (PET/CT) for initial staging. In addition to intense PSMA uptake in the prostate (maximum standardized uptake value [SUVmax], 15.3), PET/CT incidentally revealed a moderately PSMA-avid anterior mediastinal mass (SUVmax, 6.1). Contrast-enhanced chest CT showed a 6.4 cm heterogeneously enhancing anterior mediastinal mass suspicious for an invasive thymic malignancy, with possible invasion of the adjacent lung and pericardium. Multiple left pleural nodules raised concern for pleural metastases. The patient underwent total thymectomy, lung wedge resection, pericardial excision, and pleural nodule biopsies. Histopathology confirmed moderately differentiated thymic squamous cell carcinoma with lung and pericardial invasion and pleural metastasis (pT2N0M1a, TNM stage IVA; Masaoka–Koga stage IVa). Tumor-cell positivity for p40, CD5, CD117 (c-kit), and GLUT1, together with the histopathological and anatomical findings, supported the diagnosis. The patient subsequently completed four cycles of cisplatin, doxorubicin, and cyclophosphamide chemotherapy, with stable disease in the response assessment, while the concurrent prostate cancer was managed with androgen deprivation therapy. This case illustrates the importance of CT morphology and tissue confirmation when an atypically located PSMA-avid lesion is encountered during prostate cancer staging.