Pulmonary splenosis mimicking lung metastases in a breast cancer patient: A diagnostic trap

Pulmonary splenosis (PS) is a benign condition resulting from ectopic implantation of splenic tissue following splenic trauma or splenectomy. It can mimic metastatic disease, posing a significant diagnostic challenge, particularly in oncologic patients. We present a case of PS in a breast cancer patient initially misdiagnosed with lung metastases. A 43-year-old female patient with a history of splenectomy was diagnosed with grade III hormone receptor-positive (HR+), human epidermal growth factor receptor 2 negative (HER2-) invasive ductal carcinoma. Imaging suggested metastatic disease, with lung nodules and left subphrenic lesions. First-line treatment with letrozole and ribociclib was initiated. Subsequent imaging revealed stable disease and doubts about the metastatic nature of the lesions were raised. A history of prior splenic trauma and metabolic imaging consistent with splenosis led to a diagnosis of PS. The patient was reclassified as early-stage breast cancer, underwent right mastectomy with axillary lymph node dissection, and achieved a pathological complete response in the primary tumor. This case highlights the importance of considering PS in differential diagnoses for metastatic-like lesions, particularly in patients with a history of splenic trauma, to prevent misdiagnosis and unnecessary treatment. • Benign pulmonary lesions can mimic metastatic lesions and result in misdiagnosis. • It must be considered especially in cancer patients as it leads to mistreatment. • Pulmonary splenosis is a rare case in breast cancer patients. • This case underscores how pulmonary splenosis can lead to unnecessary treatments. • Full medical history is essential before treatment especially in cancer patients.