Failure Patterns Relative to Radiation Treatment Fields for Stage II–IV Thymoma

Abstract

The optimal radiation therapy (RT) field design for thymomas remains unclear. Here we report the failure patterns in stage II-IV thymoma after RT at two tertiary referral centers, classified according to the new International Thymic Malignancy Interest Group definitions. We reviewed 156 stage II-IV patients with thymoma treated with definitive (n=24) or adjuvant (n=132) RT. All RT was delivered without elective nodal irradiation (median dose 5040 cGy). Intrathoracic failures were classified as (1) in-field failures (within 100% isodose line [IDL]), (2) marginal recurrences (<100% and ≥50% IDL), and (3) out-of-field failures (outside the 50% IDL). The median follow-up was 61 months. Surgical margins were positive in 39%. The median tumor size was 9 cm. The 5-year cumulative incidence of all intrathoracic failures was 24% (n=34). Failures occurred within the RT field (n=5), marginally (n=1), out-of-field (n=22), and synchronously in- and out-of-field (n=6). The 5-year cumulative incidence of in-field failures was 7%. These were associated with Masaoka stage and tumor size. Macroscopically positive margins were associated with more local failures. Intrathoracic failures occurred most commonly in the pleural space (n=29) and lymph nodes (n=9). Patients with more advanced stage, and those treated with intensity-modulated radiation therapy had more intrathoracic failures. RT dose and chemotherapy did not impact failure patterns. Although there were few in-field failures in patients who received RT for stage II-IV thymomas, a high rate of out-of-field intrathoracic failures still occurred. Further study is necessary to identify treatment approaches that prevent pleural recurrences.