Case Report of MPO+ ANCA Vasculitis with Pauci-immune GN Associated with Invasive Ductal Carcinoma of the Breast

Background: Malignancy-associated vasculitis usually presents in the form of polyarteritis nodosa or leukocytoclastic vasculitis. However, ANCA vasculitis associated with malignancy is rare. Here, we present a case of MPO+ ANCA vasculitis with pauci-immune GN associated with invasive ductal carcinoma of the breast. Case Presentation: A 66-year-old female with a history of rheumatoid arthritis, Hashimoto's thyroiditis, and psoriasis presented with multiple joint pain, body aches, petechial rash, paresthesia and numbness, and deranged renal function a month after diagnosis of localized left breast invasive ductal carcinoma. Renal biopsy showed crescentic pauci-immune glomerulonephritis, and serology was positive for Perinuclear Antineutrophil Cytoplasmic Antibody (P-ANCA) and myeloperoxidase (MPO). The disease course was complicated by diverticulitis with peritonitis and intraperitoneal abscess collection, which required laparoscopic peritoneal lavage and additional interventional radiology-guided drainage of the abscess. We treated the patient successfully with steroids, rituximab, and mastectomy for left breast malignant lesions, resulting in the resolution of symptoms, normalization of inflammatory markers, and ANCA seroconversion. Conclusion: Treating ANCA-associated Vasculitis (AAV) in surgical emergencies like bowel perforation can be challenging. Individualized treatment strategy tailored to patients' acute needs is crucial. In this case, we considered malignancy-associated vasculitis and pursued treatment that fit the patient's clinical situation in a multidisciplinary approach.