Good Response of Advanced Thymic Carcinoma with Low PD-L1 Expression to Chemotherapy plus Pembrolizumab as First-Line Therapy and to Pembrolizumab as Maintenance Therapy: A Case Report.

Thymic carcinoma is a rare malignant tumor with a poor prognosis. No standard treatment is currently available. The present case was a 64-year-old male smoker with no symptoms referred to our hospital because of abnormal chest radiological findings. The CT study showed a tumor between the anterior mediastinum and the right lung upper lobe, multiple nodular shadows along the right pleura, and pleural effusion. A CT-guided needle biopsy revealed squamous cell carcinoma. However, the differential diagnosis between thymic carcinoma and primary lung cancer was difficult. Treatment with carboplatin, nanoparticle albumin-bound paclitaxel, and pembrolizumab was initiated. The CT scan showed tumor shrinkage and good clinical response after four treatment cycles. Therapy was switched to maintenance therapy with pembrolizumab alone. Imaging studies showed further tumor shrinkage after twelve cycles of maintenance therapy with pembrolizumab. Sixteen cycles of maintenance therapy were continued without performance status deterioration. An abnormal radiological finding was detected after a twelve-month exacerbation-free period. The diagnosis was thymic carcinoma. Treatment with lenvatinib was initiated, and tumor-size reduction was observed. This is the first report of a case showing a successful maintenance therapy with pembrolizumab after effective first-line therapy with a combination of carboplatin-based chemotherapy plus pembrolizumab in advanced thymic carcinoma.