Pembrolizumab-associated eosinophilic polyserositis with pericardial involvement during adjuvant therapy for clear cell renal cell carcinoma: a case report and narrative review

Background : Pembrolizumab is a standard adjuvant option for patients with high-risk clear cell renal cell carcinoma after nephrectomy. Serositis is an uncommon immune-related adverse event under programmed cell death 1/programmed death ligand 1 blockade. Pericardial involvement is the best-characterized serosal compartment, and cohort data suggest a several-fold increased risk of pericardial events compared with non-immune checkpoint inhibitor-treated controls. However, eosinophil percentages in serosal fluid are rarely reported, leaving the incidence and clinical significance of eosinophilic serositis poorly characterized. Case presentation: A 55-year-old man with clear cell renal cell carcinoma (metastatic disease with no evidence of disease after nephrectomy and bone metastasectomy) received adjuvant pembrolizumab (200 mg every 3 weeks) per the KEYNOTE-564 regimen. After six cycles he developed anasarca with large bilateral pleural effusions, mild ascites, and a small pericardial effusion. Pleural fluid was exudative and eosinophil-rich (20%), consistent with eosinophilic pleural effusion, with negative cytology and microbiology. Adenosine deaminase was mildly elevated (42.8 U/L) but tuberculosis evaluation was negative. Transthoracic echocardiography showed preserved biventricular function and no tamponade. Pembrolizumab discontinuation, thoracentesis, and systemic corticosteroids (methylprednisolone 1 mg/kg with taper) resulted in rapid clinical and laboratory improvement. Conclusions : This case supports pembrolizumab-associated eosinophilic polyserositis with pericardial involvement as a rare phenotype within the cardio-oncology spectrum. In a targeted review of published immune checkpoint inhibitor-associated serositis/pleuritis reports, effusion eosinophil percentage was explicitly reported in only a small minority of cases, a gap that may obscure a mechanistically and therapeutically distinct inflammatory subgroup. We provide a diagnostic and management framework and highlight emerging options, including interleukin-5 axis blockade, for eosinophil-driven disease.