2038 - Inducing Abscopal Response by Local Radiotherapy and Granulocyte-Macrophage Colony-Stimulating Factor as Monotherapy in Patients with Pretreated Advanced Thymic Epithelial Tumors: A Retrospective Anal

M. Fan^1,2, H. Li^2,3, J. Chen³, B. Wang^1,3, and J. Wen^1,3; ¹Department of Radiation Oncology, Fudan University Shanghai Cancer Center, Shanghai, China, ²Fudan University Shanghai Cancer Center, Shanghai, China, ³Department of Oncology, Shanghai Medical College, Fudan University, Shanghai, China

Purpose/Objective(s): The study aimed to investigate the induction of abscopal effect in patients with pretreated advanced thymic epithelial tumors through local radiotherapy in combination with granulocyte-macrophage colony-stimulating factor (GM-CSF) alone. Materials/

Methods: In this retrospective serial, thymic epithelial tumor patients with at least three distinct measurable sites of disease, treated with concurrent radiotherapy (35 Gy in ten fractions or 30 Gy in ten fractions, over 2 weeks) to one metastatic site and GM-CSF (125 µg/m² subcutaneously injected daily for 2 weeks, starting during the second week of radiotherapy), were recruited. The course was repeated, targeting a second metastatic site. No systemic treatment was applied. The primary endpoint of this study was the proportion of patients exhibiting abscopal effect and treatment response. The treatment response was evaluated according to ITMIG-Modified RECIST (version 1.1). Secondary endpoint included therapeutic safety and treatment-related toxicities. Adverse events were reported using CTCAE (version 5.0). Experimental endpoint includes changes of peripheral immune markers during and/or after treatment by blood routine test.
Results: Twenty patients from April 28, 2016, to January 11, 2021, were enrolled in this study, with 2 type B1 thymoma (10.0%), 4 type B2 thymoma (20.0%), 4 type B3 thymoma (20.0%), 9 thymic squamous cell carcinoma (45.0%) and 1 thymic atypical carcinoid (5.0%). Five out of twenty patients (25.0%) had obvious abscopal effect. And the median time for abscopal effect occurrence was 23 days. Six patients (30.0%) were evaluated as partial response at 1-month assessment after treatment. The median overall survival time was not reached, and the median progression free survival time was 12.0 months(95%CI:5.4-18.6) for all patients. Seven = Grade 3 adverse events (6 Grade 3, 1 Grade 4) occurred but were controllable. Lymphocytes may relate to abscopal effect based on blood routine results.
Conclusion: Combining local radiotherapy with GM-CSF alone successfully induced abscopal effect in patients with pretreated advanced thymic epithelial tumors, resulting in favorable long-term outcomes, with adverse events being manageable.