From Inoperable to Remission: Complete Response in Advanced Thymoma <scp>B3</scp> With Immunotherapy—A Case Report

Thymoma is a rare thymic epithelial tumour, often detected incidentally on imaging. We report a 30-year-old female presenting with cough and chest discomfort, whose evaluation revealed a large anterior mediastinal mass compressing the heart and superior vena cava. Histopathology confirmed type B3 thymoma. The patient underwent thymectomy with residual disease, followed by adjuvant radiotherapy and prolonged pembrolizumab immunotherapy. Serial imaging demonstrated marked tumour regression, with residual metabolically inactive tissue on PET scan, consistent with non-viable remnants. This favourable outcome is notable given the incomplete (R2) resection, where prognosis is typically poor. The case illustrates the effectiveness of a multimodal approach incorporating immune checkpoint blockade in advanced thymoma, an area with limited clinical evidence. Reports on pembrolizumab use after incomplete thymectomy remain limited in the global literature, and this case highlights its potential role in extending disease control and improving outcomes in aggressive thymoma.