Most thymic carcinomas present initially with cough, fatigue, chest pain, fever, loss of appetite, and weight loss . Thymic carcinoma is a rare carcinoma of the thymus arising in the thymic epithelium. It has the similar malignant characteristics with other organ, which tend to have capsular invasion and metastases .Previous study reported one-third of thymic carcinoma patients had lymph node involvement or distant metastasis . The incidence of extrathoracic metastases for thymic carcinoma is approximately 3–6%, which makes diagnosis difficult . Most of the thymic carcinomas often metastasize to bones, lung, pleura, liver, or lymph nodes [3, 5]. There were no specific signs for the metastasis of thymic carcinoma. Previous studies reported lymph node metastasis was common and the anterior lymph nodes were first involved, with subsequent progression to the intrathoracic lymph nodes, and then the extrathoracic lymph nodes . Bone metastasis were characterized by osteolytic bone destruction combined with soft tissue mass formation . Pleural and lung metastasis was present as multiple small nodules. MRI of the brain with intravenous gadolinium showed heterogenetic enhancing nodule with central necrosis and large edema surrounding . To our knowledge, the metastasis of thymic carcinoma to the small intestine was extremely rare and only one study had reported in the literature .
There are limited data about the incidence of the small intestinal metastasis in patients with thymic carcinoma and the mechanism of distant metastasis from thymic carcinoma to small intestine remains unclear. Previous study suggested that extrathoracic metastases may occur because the tumor cells penetrated and spread from the great vessels in thymic carcinoma . In our case, we indicated that hematogenous metastasis is the primary pathway leading to the metastasis of thymic carcinoma to the small intestine. Differential diagnoses for small intestinal masses included metastatic lesion, GIST, lymphoma and adenocarcinoma. GIST is the most common mesenchymal tumor in the gastrointestinal tract . GIST typically presents as a submucosal tumor of the gastrointestinal wall, occasionally accompanied by mucosal ulcer and rupture of tumor . Patients may have hematemesis, melena, hematochezia, or signs and symptoms of anemia . Cavity and fistula formation may occur, which results in luminal enlargement and communication of the cavity or fistula with the small intestinal lumen . The outer margins of GIST are typically sharply defined and the inner margins may show smooth. Moreover, GIST seldom had regional lymph nodes metastasis . Our case showed the irregular outer and inner margins of the mass, and lymph node involvement in the adjacent mesentery and right cardiophrenic angle. Lymphoma is the second most common tumor of the small intestine neoplasm . Lymphoma usually presents as a homogeneous soft tissue mass without necrosis. The intestinal lymphoma often has thickened wall and presents as aneurysmal dilatation.. Lymphoma often shows mild enhancement and preservation of the fat plane . And the inner margins of lymphoma are smooth, unlike as the mass presenting with irregular inner margins in our case. In addition, lymphoma rarely present with liver metastasis. Intestinal adenocarcinoma typically shows luminal narrowing, which may result in intestinal obstruction.
As regard to the treatment for thymic carcinoma, surgery remains the mainstay of treatment, and radiation and chemotherapy also have been applied widely as adjuvant and palliative procedures . For resectable patients, total thymectomy and complete tumor excision is recommended . For patients with metastatic thymic carcinoma, platinum-based chemotherapy is recommended for first-line therapy . Therefore, the patient was started on gemcitabine plus cisplatin regimen.
In conclusion, the metastasis of thymic carcinoma to the small intestine is rare and easily misdiagnosed. When a mass communicated with the small intestinal lumen is seen on CT, a suspicion of metastatic small intestinal neoplasms should not be overlooked and we should make accurate differential diagnosis from the other small intestinal tumors. Abdominal and chest CT is helpful to make the preoperative diagnosis and accurate treatment.