Case Report: Do not diagnose lung cancer as pneumonia: continue to monitor a case of invasive mucinous adenocarcinoma as it progresses from small to large.

Background: Invasive mucinous adenocarcinoma (IMA) is a rare malignant tumor of the lung, characterized as a distinct subtype of lung adenocarcinoma, with unique histological features and clinical behavior. Because the diagnosis is often delayed due to its imaging characteristics resembling pneumonia, this neoplasm is associated with a poor prognosis. Case presentation: This case report describes a 69 years-old man who underwent an asymptomatic health examination, during which small ground-glass opacities were found in the periphery of both lungs. There was no significant change in the lesions during the subsequent 2 years follow-up. However, at the third year of follow-up, the lesions had significantly enlarged. A CT-guided puncture biopsy was performed, and the pathological results indicated IMA of the lung. Subsequently, the patient underwent surgical treatment, and the postoperative pathological findings were consistent with those of the biopsy. In this case, the patient believed that he had been following the doctor's orders for asymptomatic health checks and follow-up reexamination, yet his cancer diagnosis was still significantly delayed. Therefore, the patient demanded that the doctor take medical responsibility for the alleged negligence. After the doctor carefully described the imaging features of lung IMA, the patient ultimately decided to forgo pursuing medical responsibility and expressed satisfaction with the doctor's diagnosis. Conclusion: This case illustrates the evolving imaging signs of lung IMA. Medical professionals should avoid diagnosing lung cancer as pneumonia, with the aim of enhancing the accuracy of early diagnosis and assisting in clinical evaluation. Additionally, it serves as a reference for patients to better understand this disease.