[A Case of Early Breast Cancer Which Transient CEA Elevation after Treatment with Immune Checkpoint Inhibitors].

The efficacy of perioperative treatment with pembrolizumab for early-stage triple-negative breast cancer(TNBC)at high risk of recurrence was demonstrated in the KEYNOTE-522 trial, and it's indication was expanded in Japan as of September 2022. We report a case in which transient elevation of CEA was observed following treatment with the pembrolizumab. The patient was a 41-year-old woman who presented to a local clinic with a mass in the right breast. She was diagnosed with right breast cancer and referred to our department for further treatment. The clinical stage was T2N0M0, and the tumor was ER-negative, PgR-negative, and HER2-negative, confirming a diagnosis of TNBC. We initiated perioperative treatment in accordance with the KEYNOTE-522 regimen. She underwent a total mastectomy of the right breast with sentinel lymph node biopsy, achieving a pathological complete response. Postoperatively, she was scheduled to receive 9 courses of pembrolizumab. During postoperative therapy, an elevation in CEA was observed. Although recurrence was suspected, further evaluation revealed no evidence of recurrence. Instead, thymic enlargement was noted. As follow-up continued, CEA levels began to decline and the thymic enlargement showed a tendency to regress. These findings suggest a potential association between immune checkpoint inhibitor and thymic enlargement, as well as a possible link to the observed CEA elevation.