A case of seronegative microscopic polyangiitis following the diagnosis of renal pelvic carcinoma.

The incidence rate of malignancy in patients with antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is higher than that in the general population. Malignancy has been indicated to be a risk factor or inducer of AAV. Herein, we report the case of a healthy 84-year-old man with seronegative microscopic polyangiitis (MPA) after the diagnosis of renal pelvic carcinoma. Four weeks before admission, his estimated glomerular filtration rate (eGFR) was 85 ml/min/1.73 m², and no hematuria or proteinuria was detected. Renal biopsy on admission revealed invasive urothelial carcinoma of the right renal pelvis. On day 15, his eGFR decreased to 30 ml/min/1.73 m² without any incitement. The renal specimen extracted via right robot-assisted nephroureterectomy indicated the presence of ANCA-associated glomerulonephritis. On day 37, urinary protein/urinary creatinine level of 6.48 g/gCre, serum albumin level of 2.1 mg/dL, and eGFR of 20 ml/min/1.73 m² indicated the presence of nephrotic syndrome. His blood sputum was analyzed via chest computed tomography, which revealed alveolar hemorrhage. Although his myeloperoxidase-ANCA was negative, he was diagnosed with MPA based on the 2022 American College of Rheumatology/European League Against Rheumatism classification criteria. This is the first case report of  MPA or AAV complicated with renal pelvic carcinoma. The clinical indicators demonstrated that renal pelvic carcinoma preceded the onset of MPA. The spatial proximity of both diseases indicated that renal pelvic carcinoma had some influence on MPA development via the mechanism of inflammatory cytokines or neutrophil extracellular traps. Our report may be useful in elucidating the mechanism of MPA development.