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it is well known that the most common sites of gastric carcinoma recurrence are the liver , lungs , lymph nodes , and peritoneum.1 although metastasis to the posterior mediastinal lymph nodes is occasionally encountered after gastrectomy , recurrence in the anterior mediastinum is extremely rare .
we report a very rare case of a single thymic metastasis from gastric cancer occurring 5 years after a distal gastrectomy with lymph node dissection .
1 ) , for which he had undergone a distal partial gastrectomy 5 years previously .
histological examination of the resected specimen had revealed moderately differentiated adenocarcinoma invading the muscularis propria with 5 regional lymph node metastases at stations 1 ( 3/7 ) , 3 ( 1/1 ) , and 7 ( 1/1 ) .
the disease was stage iib as defined by the japanese gastric cancer association.2 the patient received postoperative adjuvant chemotherapy with the oral fluoropyrimidine s-1 ( 100 mg / body ) .
four years and 8 months after the initial operation , a computed tomography scan from the chest to the pelvis demonstrated a tumor in the anterior mediastinum ( fig .
2 ) . with 18f - fluorodeoxyglucose positron emission tomography computed tomography , a focal hypermetabolic lesion with a maximum standardized uptake value of 3.6 was identified in this lesion ( fig .
the patient underwent an anterior mediastinal tumor resection by video - assisted thoracic surgery ( fig .
4 ) ; the preoperative diagnosis was thymoma , thymic carcinoma , or malignant lymphoma in the thymus .
histopathology of the tumor revealed adenocarcinoma cells that were similar to those of the gastric cancer resected 5 years previously .
furthermore , immunochemistry indicated that tumor cells were positive for cytokeratin 20 and cdx-2 ( c and d ) and negative for cytokeratin 7 .
thymic metastasis was considered likely based on the location of the tumor ( fig . 5 ) .
the patient is currently taking the oral fluoropyrimidine s-1 ( 100 mg / body ) as adjuvant chemotherapy to prevent recurrence .
in this report , we describe a solitary thymic metastasis from a previously resected gastric cancer , which is an extremely rare occurrence .
the thymus is an essential organ that controls cellular immune function ; therefore , it has been considered almost impossible for a tumor to metastasize to it .
however , detailed study of the thymic structure has revealed that it is not absolutely safe from tumor metastasis.3 as described by clark,4 the parenchyma of the thymus is associated with a blood - thymus barrier that prevents it from making direct contact with antigens or cancerous cells , thereby seemingly excluding the occurrence of cancer metastasis .
however , the septum of the thymus comprises interlobular connective tissue with blood vessels , lymph ducts , and nerves that theoretically enable contact with other parts of the body through the blood and lymphatic systems .
there are relatively few reports3,5,6,7 of thymic metastases , but gastric , breast , laryngeal , and lung carcinoma metastasis to the thymus has been reported at autopsy.8 in an autopsy series by middleton,8 7 cases of thymic metastasis occurred in 102 carcinoma cases , which is an incidence of approximately 7% .
to our knowledge , no antemortem cases of thymic metastasis from gastric cancer have been previously reported . in our case
this is reasonable because the thymus degenerates after adolescence and is replaced by fatty tissue .
therefore , our conclusion that the tumor originated in the thymus is based on the metastatic tumor 's location .
it is important to detect metastatic tumors as soon as possible because some patients can be successfully palliated with appropriate treatment and may even achieve prolonged survival .
the recognition that gastric cancer can metastasize to unusual sites , such as the thymus , can facilitate an accurate , prompt diagnosis and appropriate treatment .
the peritoneum is the most frequent site of recurrence for gastric cancer after gastrectomy , followed by the liver and lymph nodes . 
 in contrast , metastasis to the thymus is rare . 
 annual surveillance with computed tomography was performed on a 67-year - old man who previously underwent a distal gastrectomy and d2 lymph node dissection for gastric cancer at tottori university . 
 five years after the initial operation , an anterior mediastinal tumor was detected by computed tomography . 
 the patient underwent video - assisted thoracic surgery to remove the tumor . 
 histopathology revealed adenocarcinoma cells similar to those of the gastric cancer resected 5 years previously . 
 thymic metastasis was considered likely based on the location of the tumor . 
 the recognition that gastric cancer can metastasize to unusual anatomic locations , such as the thymus , can facilitate an accurate , prompt diagnosis and appropriate treatment .