Atypical thymic carcinoid manifesting with nephrotic-range proteinuria in a 7-year-old boy

Nephrotic-range proteinuria as a paraneoplastic syndrome (PNS) is an exceptional presentation, especially in children. It is usually associated with hematologic malignancies. Solid tumors are very rare causes of proteinuria. We present the case of a 7-year-old boy with an extremely rare atypical thymic carcinoid accompanied by nephrotic-range proteinuria as PNS. The kidney biopsy was consistent with minimal change disease (MCD). Tests for a neuroendocrine tumor were performed due to symptoms of hypercortisolemia and an elevated concentration of chromogranin A in the serum. The chest computed tomography revealed a tumor in the anterior mediastinum, which was diagnosed as an atypical thymic carcinoid. A complete resolution of the nephrotic-range proteinuria was observed within 1 week after the first thoracoscopic surgery, with almost complete reduction of the tumor mass. This extremely rare case shows that MCD can occur as a PNS even in children. Nephrotic-range proteinuria can be a symptom of malignant solid tumor. This case highlights the possibility of secondary causes of MCD in children.