Natural History and Treatment of Malignant Thymoma

Cowen and colleagues provide a very good review of thymomas. Although thymomas are the most common of the anterior mediastinal tumors, these tumors as a group are relatively uncommon. The authors focus their discussion on the pathology, staging, and management of thymomas.

As the authors point out, surgery is indicated in all situations for complete excision if possible and, if not, for accurate diagnosis and debulking. The authors discuss radiation therapy at great length and conclude that all stage IB and higher thymomas should be treated with postoperative radiation therapy. They also indicate that thymoma is a chemotherapy-sensitive tumor. Of their cohort of 149 patients with nonmetastatic thymomas, approximately half received postoperative chemotherapy consisting of cyclophosphosphamide (Cytoxan, Neosar), doxorubicin, and cisplatin (Platinol).

Diagnostic Difficulties

We have experienced problems with the accurate diagnosis of anterior mediastinal tumors. The four tumors that can be found in the anterior mediastinum are thymomas, lymphomas, teratomas, and substernal thyroid tumors. It can be difficult to distinguish lymphomas from thymomas especially when thymomas are of a lymphatic nature. Often, a fine-needle biopsy does not provide enough of a tumor sample to enable the pathologist to make this distinction. Although the true-cut needle biopsy technique may provide an adequate sample, at times it may be necessary to do a limited anterior thoracotomy.

One does not need to do extensive surgery to excise a lymphoma. In contrast, thymoma excision requires good surgical exposure and complete removal of the thymus. At times, major venous reconstruction is necessary. One frequent site of invasion is the crossing left innominate vein and its junction with the superior vena cava. By various vascular techniques, it is possible to remove portions of these veins and patch them. If necessary, one can resect and reconstruct the superior vena cava by placing a diverting cardiac catheter from the cephalad veins through the right auricular appendage. This permits superior vena cava replacement by either a prosthesis or a spirally wound autologous vein to create a satisfactory autologous graft.

Indications for Radiation, Chemotherapy

We agree with the authors that if the capsule has been invaded or penetrated, postoperative radiation therapy is indicated. In addition, if excision of the tumor is suspected or known to be incomplete, we recommend the use of chemotherapy to supplement radiation therapy.

The survival rate for completely excised noninvasive thymomas should approach 100%. Completely excised capsular-invaded or minimally invasive tumors treated with postoperative radiation therapy should have a 5-year survival rate of approximately 80%. In patients with more extensive tumors that have not been totally excised, surgery, radiotherapy, and chemotherapy produce an anticipated 5 year survival rate in the range of 25% to 50%. Thymomas that have largely metastasized more distantly in the chest usually are serious problems, and even when treated with surgery, radiotherapy and chemotherapy, have a long-term survival rate under 25%.

References:

Batata MA, Martini N, Huvos AG, et al: Thymomas: Clinicopathologic features, therapy, and prognosis. Cancer 34(2):389-396, 1974.

Beattie EJ, Raskin NM: Thymoma, in Cameron JL (ed): Current Surgical Theory 3, pp 511-516. Hamilton, Ontario, Canada, Decker BC, 1989.

Goldberg M, Burke RL: Tumors of the mediastinum, in Harvey JC, Beattie EJ (eds): Cancer Surgery, pp 258-272. Philadelphia,WB Saunders, 1996.