Patterns Of Failure After Adjuvant Radiation Therapy Based On Tumor Bed With Margin For Stage Ii-Iv Thymic Carcinoma

This study was conducted to evaluate the suitability of radiation target volume based on “tumor bed with margin” in patients with thymic carcinoma who were treated by surgery and postoperative radiation therapy (PORT). The records of 54 patients with Masaoka-Koga stage II-IV thymic carcinoma, who received surgical resection at one institution, from Jan. 2000 to Dec. 2014, were retrospectively reviewed. The number of patients was 22 in stage II, 23 in stage III, and nine in stage IV. The median PORT dose was 54 Gy in 27 fractions and the target volume was confined to the primary tumor bed only, while did not include the regional lymphatics nor pleuro-pericardial spaces electively. The clinical outcomes including patterns of failure and prognostic factors were analyzed. After median follow-up of 61 months, recurrence was observed in 24 (44.4%) patients; pure local failure within the PORT volume was founded in only one (1.9%) patient who had gross residual lesion after surgery, isolated regional recurrences in adjacent mediastinum or lymph nodes in 4 (7.4%) patients, regional lymphatic recurrences accompanied by pleural or lung recurrences in 4 (7.4%) patients, pleuro-pericardial seedings in 6 (11.1%) patients, and distant metastases were founded in 9 (16.7%) patients. The locations of distant metastases were mostly lung in 7 patients, chest wall in one patient, and multiple bone metastases out of thorax in only one patient. Overall survival rate at 5 and 10 years was 84.1% and 62.8%, respectively. Recurrence free survival rate at 5 and 10 years was 50% and 45%, respectively. The younger age (<60 vs. ≥60 years; HR 0.148; 95% CI, 0.043-0.506; p=0.002) and squamous cell carcinoma pathology (Squamous vs. Non-squamous; HR 0.264; 95% CI, 0.079-0.884; p=0.031) were favorable prognostic factors for overall survival, and the higher Masaoka stage (III vs. I; HR 5.017; 95% CI, 1.567-16.067; p=0.007, II vs. I; HR 2.399; 95% CI, 0.893-6.444; p=0.083) and larger tumor diameter (≥ 9cm vs. < 9cm; HR 2.726; 95% CI, 0.981-7.580; p=0.055) were predictable factors for poorer recurrence free survival on multivariate analyses. Radiation toxicity was mild in most patients and no severe toxicity was registered. PORT confined to the primary tumor bed only is suggested to be optimal in patients with Masaoka stage II-IV thymic carcinoma considering acceptable isolated loco-regional recurrence and radiation-related morbidity. Development of effective treatment strategy to encounter frequent pleuro-pericardial seeding and distant metastases to elevate the survival outcome may be warranted.