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Although diffuse cysts in the lung can be found in many diseases, they are uncommon in metastatic lung adenocarcinoma. They are even more unusual after the administration of immune checkpoint inhibitors. A case of lung adenocarcinoma that developed diffuse cysts in the lungs during treatment with nivolumab is reported. The patient was a 60-year-old woman with postoperative recurrent lung adenocarcinoma in mediastinal lymph nodes and pleural dissemination. After first-line treatment with cisplatin, pemetrexed, and bevacizumab, computed tomography (CT) showed disease progression. Treatment was then switched to nivolumab. After 5 courses of nivolumab, CT showed multiple ground-glass nodules in her lungs. After 4 more courses of nivolumab, the ground-glass nodules increased in size, and cystic air spaces appeared in their centers. The patient did not have any symptoms. Laboratory tests showed no evidence of infection or nivolumab-induced pneumonitis. Sialyl Lewis X-i antigen increased, and positron emission tomography showed abnormal uptake of 18F-fluorodeoxyglucose in these lesions. Considering this evidence, the cystic lesions were diagnosed as multiple lung metastases. Various differential diagnoses should be considered when diffuse cystic lesions are found in the lungs after the administration of immune checkpoint inhibitors.

Large cell neuroendocrine carcinoma (LCNEC) of the lung with epidermal growth factor receptor (EGFR) mutation is rare, and few cases have been treated with EGFR tyrosine kinase inhibitors (TKIs). We report the treatment of combined LCNEC with adenocarcinoma harboring an EGFR mutation with EGFR-TKIs and bevacizumab. Our patient was a 70-year-old asymptomatic woman who underwent surgical resection of the lung for combined LCNEC with adenocarcinoma harboring an activating EGFR mutation 11 months previously. Magnetic resonance imaging (MRI) and positron emission tomography revealed metastatic lesions in the brain and lung. The patient was diagnosed with recurrence of combined LCNEC with adenocarcinoma. The brain lesion was irradiated, followed by administration of afatinib. Eight months after irradiation, brain MRI revealed ringed enhancement and perilesional edema after radiotherapy without new metastatic lesions. We switched treatment to erlotinib and bevacizumab, resulting in maintenance of stable disease for 10 months. Overall, the disease was controlled for 18 months with EGFR-TKIs and bevacizumab. Combination treatment with EGFR-TKIs and bevacizumab could be a treatment option for LCNEC of the lung harboring EGFR mutations, especially with brain metastasis.