The lungs are common target organs for metastatic tumors. Generally, metastatic lung tumors present as single or multiple pure solid nodules on chest CT scans. GGO on chest CT are often a finding of inflammatory disease, pulmonary edema, AAH, or AIS in the lungs. In our case, a lung metastatic tumor from gastric cancer presented as a GGO-dominant pulmonary nodule on a thin-section chest CT. Although it is unusual for metastatic lung tumors to present with GGO on CT, there have been several reports of lung metastases from melanoma, thyroid carcinoma, pancreatic cancer, cholangiocarcinoma, malignant phyllodes tumor, sarcoma, gastric cancer, breast cancer, and malignant schwannoma presenting with GGO [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. Metastatic lung tumors exhibit GGO on chest CT by two mechanisms. First, tumor cells grow to replace the alveolar epithelium, and the alveolar structure is preserved, similar to the AIS of the lung. It has been previously reported that pulmonary metastases of melanoma, thyroid carcinoma, pancreatic cancer, and cholangiocarcinoma exhibit this form of proliferation [1,2,3,4,5,6,7,8]. Second, tumor cells infiltrate and proliferate mainly within the alveolar septum, with only slight destruction of alveolar structures. Pulmonary metastasis from malignant phyllodes tumors and sarcomas has been reported to involve this form of proliferation [9, 10]. In addition, there have been a few reports of pulmonary metastases from gastric signet-ring cell carcinoma presenting with GGO on chest CT through this form of extension [11, 12]. Based on the pathological findings, our case was considered to present as GGO on chest CT by a second mechanism, similar to the previously reported pulmonary metastasis from gastric cancer. However, in previous reports, the pulmonary metastatic lesions were diffusely present, just like interstitial pneumonia, and there are no reports of a few isolated nodules like this case. In addition, previous reports have not mentioned changes in size over time.

In this case, the tumor volume-doubling time of the nodule in S6 was 259 days. In previous reports, the median tumor volume-doubling time of GGO in primary lung cancer was reported 400–1800 days [15,16,17,18], and the nodule in the S6 segment in our case grew faster than the typical GGO in primary lung cancer. Because of its unusually rapid growth and previous history of gastric signet-ring cell carcinoma, we might have considered lung metastasis of gastric cancer as a differential diagnosis.

To the best of our knowledge, this is the first report to demonstrate the growth rate of GGO in lung metastases from gastric cancer. Even if the nodule is GGO-dominant on the chest CT, a metastatic lung tumor should be considered as a differential diagnosis if it is growing rapidly and the patient has a history of cancer in other organs.