A80-3-01 When History Rewrites the Diagnosis: Thoracic Splenosis Masquerading as Malignancy in an Elderly Ex-smoker

Rationale Despite advances in imaging and molecular testing, an attentive history remains the cornerstone of accurate diagnosis. Thoracic splenosis, the autotransplantation of splenic tissue into the thoracic cavity after trauma or splenectomy, is a rare condition that can closely imitate lung cancer decades later. This case highlights how recalling a remote injury prevented unnecessary intervention and revealed a benign physiologic mimic of malignancy. Case Presentation An 82-year-old man with severe emphysema and a 40-pack-year smoking history presented with progressive dyspnea and unintentional 20-lb weight loss. CT chest revealed a 2-cm pleural-based left lower-lobe mass concerning for malignancy. PET imaging showed mild hypermetabolism, and pulmonary function testing confirmed severe obstruction (FEV1 54% predicted; DLCO 59%). A CT-guided biopsy was initially planned; however, during multidisciplinary review, his chart revealed a remote abdominal gunshot wound from fifty years prior, complicated by splenectomy and pleurodesis—details previously overlooked. This historical context prompted reconsideration of the diagnosis, and nuclear scintigraphy using technetium-99m–labeled heat-damaged red blood cells was performed instead. The scan demonstrated splenic tissue within the thoracic cavity, confirming thoracic splenosis. The patient remained clinically stable, and six-month follow-up imaging showed no interval change. Discussion This case illustrates how historical insight can surpass even the most advanced diagnostics in guiding clinical judgment. In a patient whose age, symptoms, and imaging features strongly favored malignancy, one rediscovered detail—a remote splenectomy following abdominal trauma—completely redirected the diagnostic pathway. Given this history, a technetium-99m–labeled heat-damaged red blood cell nuclear scan was pursued, confirming ectopic splenic tissue and establishing the diagnosis of thoracic splenosis. This entity is frequently mistaken for primary or metastatic lung cancer, particularly in individuals with heavy smoking history or chronic lung disease. This case reinforces that every pulmonary lesion must be interpreted within the full clinical narrative, where even decades-old surgical history can redefine the diagnosis. Beyond influencing individual outcomes, such awareness reduces unnecessary invasive procedures, lowers healthcare costs, and sustains patient trust by preventing unwarranted alarm. Conclusion This case exemplifies how clinical context transforms outcomes. Awareness of prior trauma guided nuclear imaging that confirmed thoracic splenosis, replacing invasive intervention with conservative management. Even in an era of advanced diagnostics, history remains the foundation of precision medicine.