Sarcoid-like reaction to avelumab during therapy for metastatic Merkel cell carcinoma

A man in his 60s with metastatic Merkel cell carcinoma was treated with avelumab. After 8 months, he achieved complete resolution of cutaneous lesions. At 13.5 months, he developed erythematous chest papules with biopsy-proven non-necrotising granulomas. Imaging revealed bilateral hilar and mediastinal lymphadenopathy, and serum calcium was elevated at 3.02 mmol/L. This is consistent with a sarcoid-like reaction to avelumab. High-dose prednisolone resolved sarcoid lesions and normalised calcium. However, avelumab rechallenge triggered recurrent hypercalcaemia and renal deterioration, raising concern for immunotherapy-related sarcoid nephritis. Avelumab was permanently discontinued after 2 years, and the sarcoid-like reaction remained inactive thereafter.