Patient 1
A 64-year-old man was diagnosed with gastric cancer after complaining of epigastric pain. During a detailed examination of his gastric cancer, a lung tumor was found. He was then referred to our hospital.
He had smoked 20 cigarettes per day from 20 to 64 years of age, and did not have a drinking history. The results of a hematological examination were within the normal ranges. The patient’s carcinoembryonic antigen (CEA) level was elevated to 72.4 ng/mL, whereas his neuron specific enolase, cytokeratin 19 fragment (CA19-9), and pro-gastrin-releasing peptide levels were within the respective normal ranges.
Endoscopically, the gastric lesion was a type 2 tumor of 30 mm in size. Contrast-enhanced computed tomography (CT) showed irregular thickening of the stomach wall, two swollen lymph nodes at the splenic hilum and along the splenic artery, and a solid tumor of 10 mm in size in the upper lobe of the right lung. 18F-fluorodeoxyglucose-positron emission tomography (FDG-PET) showed the accumulation of FDG in swollen mediastinal lymph nodes near the main bronchus, as well as the lung tumor, gastric tumor, and swollen lymph nodes in the abdominal cavity. Bronchoscopic lung biopsy and endoscopic ultrasound-guided fine needle aspiration biopsy of the swollen mediastinal lymph node were performed.
The histopathological examination of biopsy specimens from the gastric tumor and lung biopsy specimen revealed poorly differentiated adenocarcinoma and invasive adenocarcinoma, respectively (Fig. 1a, d). Mediastinal lymph node biopsy showed poorly differentiated adenocarcinoma (Fig. 1h). Immunohistochemical staining of the gastric tumor, lung tumor, and mediastinal lymph node were performed (Fig. 1b, c, e–g, i–l).
The gastric and lung tumors were diagnosed as discrete primary cancers, because of the different histopathological findings. The clinical stages of the cancers were cStage IIIB (T4a N2 M0) gastric cancer according to the Japanese Classification of Gastric Carcinoma, 3rd English edition [11], and stage IIIB (T1a N3 M0) lung cancer according to the Union International for Cancer Control-TNM, 8th edition.
After a multidisciplinary team conference, curative chemoradiotherapy was indicated for the lung cancer. Four courses of cisplatin plus S-1 and thoracic radiation (60 Gy in 30 daily fractions) were performed. After chemoradiotherapy, the lung tumor shrank and the gastric cancer did not show any obvious change. Hence, gastrectomy was performed. At gastrectomy, the patient participated in the multiple omics analysis for “Project HOPE”, and whole exome sequencing (WES) and gene expression profiling (GEP) were conducted [12, 13]. Macroscopically, the lesion was an ulcerated tumor of 55 mm in size with raised clear margins (Fig. 2a). The raised margin of the tumor was covered by non-cancerous mucosal tissue. Microscopically, the tumor consisted of solid type poorly differentiated adenocarcinoma (Fig. 2b). Chemotherapy showed no therapeutic effect. A histopathological examination revealed pStage IIIB (T4a N2 M0) gastric cancer. The patient underwent adjuvant chemotherapy with S-1; however, CT showed lymph node recurrence in the abdomen six months after gastrectomy. The patients died 11 months after gastrectomy.
The gene mutational signature of the gastric tumor showed signature 4 of COSMIC mutational signature (version 2; https://cancer.sanger.ac.uk/cosmic) (Fig. 3). Signature 4 is associated with smoking and has been found in lung cancer, head and neck cancer, and liver cancer [14]. However, there are no reports of primary gastric cancer showing signature 4. We were concerned about the possibility of a metastatic gastric tumor from lung cancer, because the patient had lung cancer and a smoking history. TTF-1 is well-known to be a transcription factor that is highly expressed in thyroid, brain and lung cancer. Thus, additional TTF-1 immunohistostaining of the gastric tumor biopsy and gastrectomy specimens was performed. Both the gastric tumor biopsy and gastrectomy specimens were TTF-1 (+), suggesting that the gastric tumor was metastasis from lung cancer (Additional file 1: Fig. S1a, b). However, TTF-1-positive primary gastric cancer could not be completely ruled out because there are reports describing TTF-1-positive gastric adenocarcinoma [15, 16].
To confirm the origin of the gastric tumor, we conducted a somatic mutation comparison of the two tumors after obtaining permission from the institutional review board. DNA extracted from formalin-fixed paraffin-embedded tissues of the lung biopsy and resected stomach were compared using a custom 409-gene panel sequencing including cancer related genes. Detailed experimental protocols have been previously described [13]. To exclude germline variations, blood-derived WES results were used as a control. Panel sequencing revealed 97 and 87 mutations in the lung and gastric tumors, respectively (Additional file 2: Table S1). Of these mutations, 5 and 7 mutations in the lung and gastric tumors, respectively, were driver mutations including likely-pathogenic mutations (tier 1 and 2 in our previous study) [13]. Of the 97 lung tumor mutations, 28 completely matched (including chromosomal location and pattern of the mutation) the mutations of the gastric tumor. Of these mutations, two were driver mutations. The gastric tumor was diagnosed as the metastasis from lung cancer based on the immunohistochemistry findings and the concordance of the somatic mutations.
Patient 2
A 47-year-old woman was diagnosed with cStage IIIB (T4a N2 M0) gastric cancer according to the Japanese Classification of Gastric Carcinoma, 3rd English edition [11].
We performed distal gastrectomy, D2 lymph node dissection, dissection of the lymph nodes around the hepatoduodenal ligament, and cholecystectomy. We also resected the bilateral ovaries, because enlargement of the ovaries was detected during the operation. A histopathological examination revealed pStage IV (T4a N2 M1) gastric cancer. Peritoneal lavage cytology was negative for cancer. The enlarged ovaries were diagnosed as metastasis from gastric cancer.
After gastrectomy, the patient underwent adjuvant chemotherapy with S-1 for one year. A colon tumor was detected by a detailed examination after a fecal occult blood test, which was performed for screening purposes 6 years after gastrectomy.
Endoscopically, the colon lesion was a 40 mm submucosal tumor-like elevated tumor. A histopathological examination of the biopsy specimen showed poorly differentiated adenocarcinoma and signet ring cell carcinoma. No other tumor or metastatic lesion was detected by CT or FDG-PET. Although the colon tumor was suspected to be metastasis from gastric cancer, the possibility of primary colorectal cancer could not be completely ruled out. If the tumor was colon cancer, the clinical stage was IIA (T3 N0 M0) according to the Union International for Cancer Control-TNM, 8th edition.
The patient underwent laparoscopic sigmoidectomy with D2 lymph node dissection. The patient participated in “Project HOPE” and WES and GEP were conducted [12, 13]. Macroscopically, the lesion was a 27-mm submucosal-like nodule. Microscopically, the tumor was signet ring cell carcinoma (Fig. 4a). No mucosal lesion was observed, and the tumor cells existed mainly in the submucosa. Immunohistochemical staining of the sigmoid colon tumor and previously resected gastric cancer were performed (Fig. 4b–g, i–n). After matches were made according to the histological type and the mucin phenotype, the colon tumor was thus suspected to have been metastasis of gastric cancer. However, the possibility of primary colorectal cancer could not be completely ruled out because the mucin phenotype of both the colon tumor and gastric cancer was a gastrointestinal type, not a gastric type. The WES results showed that the colon tumor had no specific mutational signature (data not shown).
To confirm that the origin of colon tumor, we conducted a somatic mutation comparison of the two tumors, as we did with Patient 1. DNA was extracted from formalin-fixed paraffin-embedded tissues resected in gastrectomy and sigmoidectomy. Panel sequencing revealed 11 and 11 mutations in the gastric and colon tumors, respectively (Additional file 2: Table S1). None of them were driver mutations. Of the 11 mutations in the gastric tumor, 4 completely matched those of the colon tumor.
Based on the results of immunohistochemical staining and comparison of the somatic mutations, the colon tumor was diagnosed as metastasis from gastric cancer.