Previous reports of sites of mediastinal metastasis of a malignant tumor have noted the mediastinal lymph nodes [1], thymus [3], and pericardium [4, 5]. Thymic metastasis can be determined based on surgical or pathological findings. Metastasis to the mediastinal lymph nodes is diagnosed based on pathological findings showing the presence of a primary tumor [6], while that to the pericardium is diagnosed based on surgical findings, cytology findings of pericardial effusion, and/or pathological findings of a biopsy specimen taken from the pericardium [4, 5]. Additionally, metastatic tumors to the mediastinum related to various types of carcinomas have been reported to appear as an abnormal enlarged mass in chest CT images, with increased uptake noted in PET findings [7, 8]. In the present case, while the clinical history of BC and imaging findings for diagnosis suggested that the mediastinal tumor might be metastasis from BC, the malignant thymic epithelial tumor was considered to indicate a differential diagnosis for the isolated mediastinal tumor; thus, surgical resection was performed. As noted above, we were unable to precisely define the metastatic site based on pathological findings. However, intraoperative gross findings suggested that the tumor was located in pericardial fat tissue; thus, a diagnosis of mediastinal metastasis of BC was determined. The metastasis pathway in the present case is not clear. Common metastatic sites include the liver and lungs, as well as others in cases with a small cell carcinoma related to BC [2], indicating a hematogenous spread. Supradiaphragmatic lymph node metastasis from ovarian cancer has been reported [9]; thus, BC might share the same lymphatic route. Postoperative recurrence and findings of metastasis in the mediastinum, liver, and pericardium in the present case indicate that both lymphatic and hematogenous spreading are possible.
A small cell carcinoma of the bladder is very rare, comprising less than 1% of all malignant bladder tumors, and most often shows metastasis to regional lymph nodes, bone, liver, lung, or brain tissue, with other sites also seen [2]. Because of its rarity, the initial diagnosis of the primary tumor in the present case was urothelial carcinoma. Only a few cases of BC metastasis other than small cell carcinoma to mediastinal lymph nodes have been reported [10,11,12], and, to the best of our knowledge, there is no previous report of isolated mediastinal metastasis from a small cell carcinoma of the bladder. A small cell carcinoma related to BC is known for an aggressive clinical course, with early vascular and muscle invasion, and high propensity for metastasis; thus, the prognosis is poor, with an overall 5-year survival rate ranging from 8 to 25% [2]. In the present case, multiple metastasis lesions were noted at 4 months after chest surgery, even though the mediastinal tumor had been isolated based on preoperative PET imaging findings. Unfortunately, it was not possible to predict the poor prognosis of this patient because the small cell carcinoma was diagnosed after resection of mediastinal metastasis. With a primary aim of diagnosis as well as local disease control, surgical resection should be generally selected for isolated mediastinal metastasis from an extrathoracic carcinoma [12], as seen in the present case. Actually, the chemotherapeutic regimen for this patient was determined based on the pathological diagnosis of primary bladder cancer, which the mediastinal tumor provided an opportunity to re-evaluate. For surgical treatment of the condition, a less invasive thoracoscopic approach would have been preferable. However, an open thoracotomy was performed so as to avoid intraoperative injury to the large tumor, likely resulting in intrathoracic dissemination of cancer cells.
In conclusion, we performed surgery for mediastinal metastasis of bladder cancer, which was diagnosed as a small cell carcinoma developed from a metastatic tumor. Surgical resection for an isolated mediastinal tumor should be considered because its histological diagnosis can be useful for deciding appropriate therapeutic options.