Background

Thymomas are relatively rare mediastinal tumors arising from epithelial cells and accounting for approximately 0.2 to 1.5% of all malignancies[1]. Thymomas are typically indolent-growing tumors and have relatively good prognoses; however, their variable clinical patterns sometimes make it difficult to decide on optimum therapeutic strategy. We report on two patients who developed pulmonary metastasis with thymoma.

Case presentation

Case 1: the patient was a 74-year-old man without myasthenia gravis. An anterior mediastinal mass had been incidentally noticed on chest computed tomography (CT) approximately 10 years previously (patient age, 64 years). The patient refused surgery, and he was conservatively followed-up by annual CT scan. At the age of 74 years, a chest CT scan revealed a mediastinal mass, approximately 3 cm in diameter, and a 2 cm well-defined hilar nodule in the left upper lobe (Figure 1). The anterior mediastinal tumor was diagnosed as type A thymoma according to the World Health Organization (WHO) classification following percutaneous needle biopsy. Positron emission tomography (PET)-CT showed moderate 18 F-fluorodeoxyglucose (FDG) uptake in both lesions. We made a preoperative diagnosis of thymoma with intrapulmonary metastasis or thymoma combined with primary lung cancer. Therefore, thymo-thymectomy and left upper lobectomy were performed via a median sternotomy with transsternal extension. Both resected tumors were shown histologically to be type A thymoma (Figure 2), at stage IVb according to the Masaoka staging system[2]. The patient did not receive any postoperative adjuvant therapy because the resection had been complete. There has been no evidence of recurrence in four years.

Figure 1
figure 1

Computed tomography (CT) scan shows an anterior mediastinal mass with calcification, approximately 3 cm in diameter, and a 2 cm well-defined hilar nodule in the left upper lobe.

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Figure 2
figure 2

Macroscopic finding of the pulmonary tumors. The cut surface shows a completely encapsulated mass.

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Case 2: the patient was a 68-year-old man with myasthenia gravis. He presented with invasive thymoma infiltrating into the right lung, pericardium, and right brachiocephalic vein at the age of 61 years. We performed extended thymectomy with combined resection. The tumor was diagnosed as type B3 thymoma according to the WHO classification. Histologic examination confirmed that the tumor was directly invasive only into the wall of the right brachiocephalic vein. The patient was shown to have stage III disease according to the Masaoka staging system. Adjuvant radiation (40 Gy) was performed postoperatively. A pulmonary nodule occurred seven years following the initial operation (patient age, 68 years) (Figure 3). PET-CT revealed moderate FDG uptake in the lesion of the right lower lobe. We diagnosed his disease preoperatively as pulmonary metastasis of invasive thymoma or second primary lung cancer. Therefore, right lower lobectomy was performed via right thoracotomy. The resected tumor was histologically the same as the former. He has been followed-up after the second operation, and there has been no evidence of recurrence at two years.

Figure 3
figure 3

Another computed tomography (CT) scan shows a 2.5 cm well-defined nodule in the right lower lobe at seven years.

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Discussion

Several factors have been identified to influence the prognosis of thymomas after resection, such as clinical Masaoka stage, WHO histology, completeness of resection, and tumor size[3].

The first case in this report was cytologically benign, although the type A thymoma produced lung metastasis in the ten-year follow-up period. Generally, stage IVb thymoma is not an indication for surgery; however, surgery is recommended for a good prognosis if the tumor appears completely resectable[4]. Kondo and associates also demonstrated that adjuvant chemotherapy or radiotherapy did not improve the prognosis in patients with totally resected stage III and IV thymoma and thymic carcinoma[4]. We consider that surgery is recommended for completely resectable thymoma with a few lung lesions. We do not perform adjuvant chemotherapy or radiotherapy in patients with totally resected stage III and IV thymoma and thymic carcinoma. After resection, surveillance for recurrence should include annual chest CT scan for at least ten years.

The second case was cytologically malignant; hence the type B3 thymoma produced a lung metastasis even after the initial surgery combined with adjuvant radiotherapy. There is still no standard guideline for the management of recurrent pulmonary metastases. Recurrence rates after resection of thymomas range from 5 to 50% and are related to the initial clinical stage[5]. The pattern of recurrence of resected thymomas includes local recurrence, pleural implants, and distant metastases. Local and distant recurrences occur with similar frequencies irrespective of the stage of the initial thymoma[2]. Complete resection of the local recurrent lesions seemed to be more effective for long-term survival[2, 3, 6]; likewise, complete resection of the recurrent pulmonary metastases may also result in a good prognosis[7]. We attempt surgery for lung metastases after the initial total thymectomy if the lesions appear completely resectable; whereas, it is necessary for us to discuss the indication for recurrent lung metastases after initial lung metastasectomy. In our opinion, types B3 and C thymomas have to be considered for re-resection because of higher risk of relapse. Alternative therapies (chemotherapy and radiotherapy) should be considered in the unresectable and the medically inoperable cases.

Long-term follow-up for thymoma patients is essential because thymomas are potentially malignant tumors.

Conclusions

Here, we have reported two cases of metastasis from thymomas and reviewed the current literature. Surgery is the recommended option for a good prognosis for limited stage IVb patients if the thymomas with lung metastases appear completely resectable.

We emphasize the need for careful long-term follow-up of the patients who have undergone surgery to detect recurrences as early as possible. Complete resection of small numbers of recurrent pulmonary metastases may also result in a good prognosis.

Consents

Written informed consent was obtained from the patients for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.