Role of Positron Emission Tomography/Computed Tomography (PET/CT) in Detecting Metachronous Primary Tumors: Diagnostic and Clinical Implications From a Single-Patient Case.

Metachronous primary tumors (MPTs) are defined as two or more independent primary tumors and require exclusion of metastatic disease. We report a rare case of metachronous quadruple malignancies in a 53-year-old male over a 10-year period. The patient was initially diagnosed with stage IIA colon adenocarcinoma (2014), followed by high-grade urothelial carcinoma of the left renal pelvis (2017) and non-invasive urothelial recurrence (2022). In 2023, fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) revealed a hypermetabolic left lower lobe lung mass with subcarinal nodal involvement (stage IIIB). Immunohistochemistry confirmed primary lung adenocarcinoma. The patient underwent concurrent chemoradiotherapy with a near-complete metabolic response, followed by successful lobectomy. In 2024, a fourth primary cutaneous squamous cell carcinoma was identified and excised. This case highlights the diagnostic challenge of distinguishing multiple primaries from metastatic disease and underscores the pivotal role of FDG PET/CT in staging, response assessment, and longitudinal management.