The Effectiveness of Postoperative Radiotherapy on Thymoma: A Retrospective Cohort Study

Summary: Thymomas and thymic carcinomas are the most common neoplasm in the anterior mediastinum, representing 1.5% of all malignancies. The 15-year survival rate is 12.5% in those with invasive thymoma and 47% in those with noninvasive thymomas1. Thymomas are categorized into Masaoka clinical stages I-IVa/b based on capsule invasion and metastasis; the current mainstay of treatment for early stage thymomas is surgical resection while for late stage thymomas both resection and postoperative radiotherapy (PORT) has been utilized within 3 months of surgery. Due to low prevalence of this disease and lack of randomized control trials, there is uncertainty about the efficacy of PORT and when to use it. The objective of this study is to determine the progression-free survival and local control benefits of treating thymoma patients with PORT compared to those treated without. Data was collected from patients treated at Northwestern Memorial Hospital from January 2015 through May 2024 using the Northwestern Electronic Data Warehouse and internal Northwestern Radiation Oncology database. CPT codes were used to identify diagnosis of thymoma or thymic carcinoma and for resections of mediastinal tumors. Duplicates between the two databases were removed. Chart reviews were conducted to identify variables of interest, including the following: sex, birth date, surgical resection date, diagnosis, paraneoplastic syndrome, carcinoma subtype, tumor size, Masaoka stage, resection status, neoadjuvant/adjuvant chemotherapy, PORT dosage and fractions, PORT treatment duration, local recurrence, distant metastasis, and survival. Outcomes of disease progression and local control benefits are time-to-event and were modeled with Kaplan Meier survival analysis and additional predictors were modeled with the Cox proportional hazards model. 35 patients were identified as having received resection for thymoma or thymic carcinoma without undergoing PORT, while 42 patients made up the intervention arm having received both resection and PORT. The 5-year survival rate in the control arm is 88.6% (n=31) while it is 92.9% (n=39) in the intervention arm. The distant metastasis rate in the control arm is 20% (n=7) versus 21.4% (n=9) in the intervention arm. Further data analysis will be conducted in the interim. Thymoma and thymic carcinomas are extremely rare, leading to limited availability of RCTs that elucidate the effects of PORT on treatment. This study demonstrates PORT may show benefit in improving 5-year survival rates after surgical resection. Future statistical analysis will elucidate concrete effects of PORT on the progression of this disease and help us better understand the predictive factors that result in better outcomes for those treated with PORT. Shivani Kumaresan (she/her/hers), n/a (Presenting Author) - Northwestern Feinberg School of Medicine; Yirong Liu (she/her/hers), MD, PhD (Co-Author) - Northwestern University; Tarita Thomas (she/her/hers), MD (Co-Author) - Northwestern University