原發性肺癌是臨床上常見的癌症，目前研究對於肺癌的診斷、臨床及病理表現、治療等等日新月異。然而，原發性肺癌合併腸胃道轉移的個案則是相當罕見，因為其常以無症狀或非特異性症狀表現，在臨床上容易被忽略或誤診。原發性肺癌合併胃部轉移與原發性胃癌在組織學的表現十分相似，需要依靠免疫組織化學染色（包含腫瘤細胞角蛋白7 (CK7)、甲狀腺轉錄因子-1 (TTF-1)、腫瘤細胞角蛋白20 (CK20)）協助鑑別診斷。此篇病例報告呈現一位原發性肺癌合併胃部轉移的個案，臨床表現包含長久的消化不良以及體重減輕，在追蹤期間被診斷為化療造成的不適，最後經免疫組織化學染色確診為原發性肺癌合併胃部轉移。

Primary lung cancer is very common and fatal. However, cases of primary lung cancer with gastrointestinal metastases are relatively rare. We present a case of gastric metastasis from primary lung adenocarcinoma with clinical manifestations including dyspepsia and weight loss. The patient was initially thought to undergo chemotherapy-induced side effects but later diagnosed as gastric metastasis from primary lung cancer (GMLC) through special immunohistochemical staining of tissue for thyroid transcription factor-1 (TTF-1) and cytokeratin-7 (CK-7). Since the gastrointestinal (GI) tract remains a rare site of metastasis of primary lung cancer, GMLC is primarily described in case series. The incidence is considered underestimated, and the main reason is that it mostly presents with asymptomatic and nonspecific symptoms which sometimes may be regarded as the side effects of chemotherapy, hence misidentified as primary gastric cancer. Complications such as acute gastric bleeding and gastric perforation occur in patients with gastric metastasis and may result in death in severe cases. The presentation of gastric metastasis is difficult to differentiate from primary gastric cancer under endoscopy, leading to misdiagnosis and inadequate treatment. As the histological appearance of primary lung cancer with gastric metastasis is highly similar to that of primary gastric cancer, immunohistochemistry (IHC) staining is needed to assist in differential diagnosis.