Prostate cancer with incidental thoracic splenosis: a diagnostic challenge unveiled by 18 F-PSMA PET/CT imaging

Thoracic splenosis is a rare condition caused by autotransplantation of splenic tissue into the thoracic cavity following thoracoabdominal trauma or surgery. Due to its radiological presentation, this condition often mimics malignancies, leading to potential misdiagnosis and unnecessary invasive procedures. A thorough clinical history, including prior splenic trauma or splenectomy, is therefore critical to raise suspicion. When imaging is inconclusive, non-invasive nuclear medicine techniques such as Tc-99 m heat-damaged erythrocyte scintigraphy represent the diagnostic gold standard, while biopsy should be reserved for cases with ongoing concern for malignancy. We report the case of a 61-year-old man with newly diagnosed high-risk prostate carcinoma with Gleason score 9 (5 + 4). Staging 18F-PSMA PET/CT demonstrated, in addition to prostatic and nodal uptake, intensely tracer-avid confluent pleural-based nodules in the left lower lobe. Their morphology was atypical for prostate metastases and raised suspicion of a primary malignancy. CT-guided biopsy showed nonspecific inflammatory tissue, which was not considered diagnostic. Given persistent concern for malignancy, robotic surgical exploration was performed, revealing multiple nodules consistent with thoracic splenosis, confirmed on histopathology. Previous reports have described splenosis mimicking metastases on 18F-PSMA PET/CT, usually confirmed non-invasively with spleen-specific scintigraphy. Our case is distinctive in that the coexistence of high-risk prostate cancer and non-diagnostic biopsy findings led to surgical resection for definitive diagnosis. This highlights both the diagnostic challenges of pleural PSMA-avid lesions and the risk of unnecessary invasive interventions when thoracic splenosis is not considered. Thoracic splenosis can closely mimic pleural malignancy on 18F-PSMA PET/CT, especially in patients with coexisting high-risk cancers. A detailed clinical history combined with non-invasive spleen-specific scintigraphy is crucial to avoid misinterpretation. Recognizing this pitfall in prostate cancer staging may prevent unnecessary invasive procedures and ensure appropriate patient management.