Role of radiation therapy in locally advanced thymoma

Thymoma

Thymomas and thymic carcinomas are thymic epithelial tumors that constitute approximately 15% of all mediastinal masses. From 28 to 66% of thymomas cause chest symptoms as the initial manifestation; the rest are discovered on routine chest roentgenograms or during investigations prompted by the presence of a paraneoplastic syndrome. Forty percent of patients with thymoma have one or more paraneoplastic syndromes, including myasthenia gravis, pure red cell aplasia, and hypogammaglobulinemia. Extrathymic malignant lesions develop in up to 20% of patients. Traditional histologic classifications have not accurately predicted tumor behavior; a recently developed classification based on cellular differentiation toward thymic medullary or cortical epithelium may correlate better with prognosis. Nevertheless, the prognosis is best predicted by stage of the tumor determined intraoperatively and is poorer in patients with incomplete resection than in those with complete resection of the thymoma. In addition to surgical intervention, irradiation and chemotherapy have important roles in the management of thymomas, particularly in advanced stages. In this article, the clinical manifestations, diagnosis, pathologic features, staging, and treatment of thymomas are reviewed, and the prognosis of affected patients is discussed.

Current therapy for thymoma

Thymomas are the most frequently encountered neoplasm of the anterior mediastinum. Thymic neoplasms can be divided into 3 broad categories based on histologic appearance and behavior. Thymomas are tumors with benign-appearing cellular features. If there is no gross or microscopic invasion beyond the capsule, the tumor is considered benign. Any tumor with gross or microscopic invasion through the capsule is considered malignant. In a third category, thymic tumors which have histologically malignant epithelial features, are considered thymic carcinomas and generally have an adverse prognosis. Microscopically, thymomas consist of a variable component of lymphocytic, epithelial, and fusiform (spindle) cell elements. The tumors may be classified according to the predominant cell type or are considered mixed if no cell type predominates. Prognosis is most strongly linked to clinical stage. The role of adjuvant radiotherapy or chemotherapy in the treatment of such tumors is controversial, but malignant thymomas are generally responsive to radiotherapy. Associated myasthenia gravis does not adversely affect the prognosis of thymoma, and medical attention to the myasthenia may contribute to earlier diagnosis of the thymoma. Cisplatin-based chemotherapy may be used to treat advanced disease with a modest response rate.

Aggressive treatment of intrathoracic recurrences of thymoma

Between 1974 and 1988, 21 patients with intrathoracic recurrences of thymoma received radiotherapy with radical intent; surgery was always attempted when considered feasible: 11 patients were partially (6 cases) or totally (5 cases) resected before irradiation, while in the other 10 radiotherapy was the only treatment. In 7 cases the recurrence was confined to the anterior mediastinum, 9 had pleural nodules without mediastinal lesions and 5 had both mediastinal and pleural lesions. Mediastinal recurrences were treated by opposed parallel mediastinal fields with 2/3 of the dose delivered through the anterior port: doses ranged between 38 and 44 Gy; a boost of 10-16 Gy was given in patients not radically resected. Pleural nodules were treated with a variety of techniques according to the extent of the lesions. The 7-year survival of the whole group was 70%; 5 patients died: 4 with intrathoracic progression and one with distant metastases. The survival was 74% in the 11 patients having received surgery, either radical or subtotal, and 65% in the 10 patients treated with radiotherapy alone: the difference is not significant. Patients with Karnofsky index greater than 70 had a significantly better survival (100%, versus 28%, p = 0.0015). This is a selected series of patients presenting recurrences still amenable to a radical treatment either by surgery and radiotherapy or by radiotherapy alone: the results confirm that an aggressive approach is warranted in patients in good general conditions with recurrences confined to the mediastinum and/or 1 hemithorax.

Thymoma: treatment and prognosis

Thirty-six patients with pathologically confirmed thymoma were treated at M.D. Anderson Cancer Center from 1962 to 1987. The tumors were staged based on invasion and intrathoracic dissemination. Twenty-one patients had total resection, five had subtotal resection, and 10 had biopsy alone. Twenty-two patients had definitive megavoltage radiation therapy with a median dose of 50 Gy. The 5-year, disease-free survival by stage was 74% for Stage I (n = 11), 71% for Stage II (n = 8), 50% for Stage III (n = 10), and 29% for Stage IVA (n = 7) (p less than 0.03). The 5-year, disease-free survival by extent of surgery was 74% for total resection, 60% for subtotal resection and 20% for biopsy only (p = 0.001). There were 15 patients with recurrences: two in Stage I, two in Stage II, five in Stage III, and six in Stage IVA. The median months to relapse, for those who failed treatment, were 46, 36, 2, and 13 for Stages I, II, III, and IVA respectively. Of the patients with recurrences four had a total resection, two subtotal resection, and nine biopsy only. Only one patient had distant metastases as the first site of relapse without intrathoracic relapse. For the eight patients who relapsed following radiation therapy, four were in the radiotherapy field. All four of the in-field failures were in patients who had a partial response. There were insufficient numbers of patients to determine a dose response to radiotherapy. For patients with invasive, incompletely resected disease, a multimodality approach may be necessary for long term, disease-free survival.