[Interstitial pneumonia with primary pulmonary atypical carcinoid tumor and multiple brain metastases: a case report].

Pulmonary carcinoid (PC) is a relatively rare neuroendocrine tumor, accounting for approximately 1%-2% of all lung tumors. According to histopathological characteristics, it can be categorized into low-grade typical carcinoid (TC) and intermediate-grade atypical carcinoid (AC), with AC being less common, representing about 10% of all pulmonary carcinoids. The clinical symptoms and imaging manifestations of AC lack specificity, with solitary pulmonary nodules as the major radiological feature. Cases of interstitial lung disease (ILD) combined with AC are extremely rare. Clinicians have limited awareness of this comorbidity, and AC nodules occurring against a background of ILD are easily confused with fibrotic or inflammatory lesions, leading to a high rate of clinical misdiagnosis and missed diagnosis. Herein, we report a case from our institution: the patient was found to have bilateral pulmonary interstitial changes and nodules on routine chest CT, and was initially diagnosed with connective tissue disease-associated interstitial lung disease (CTD-ILD) based on pathological and ancillary examinations. Several years later, the patient presented with dizziness related to brain metastases and was finally diagnosed with AC. By analyzing the entire diagnosis and treatment process of this case, we aim to improve clinicians' ability to recognize such rare cases and provide clinical guidance for their early diagnosis and appropriate treatment.