Clinicopathologic Characteristics of Intraglomerular Malignancy in Kidney Biopsies.

Introduction: Intraglomerular malignancy (IGM) is a rare finding, with current data limited to case reports and small, often postmortem, series. This study aimed to characterize the clinicopathologic features of kidney biopsy cases with IGM. Methods: Renal biopsy cases diagnosed with IGM from January 2000 to June 2023 at Cedars Sinai Medical Center were retrospectively reviewed. Demographic data, clinical characteristics, and pathologic data were collected, and cases were divided into hematologic (HEME) versus non-hematologic (non-HEME) malignancy. Results: We identified 9 patients with IGM. Five were hematolymphoid in origin, and 4 were from metastatic solid tumor (2 carcinomas from the lung, 1 neuroendocrine tumor of likely lung origin, and 1 from the head and neck). All patients presented with proteinuria and hematuria, and 89% had renal dysfunction. The median serum creatinine was 2.9 (IQR 1.7-5.7) mg/dL. All non-HEME patients had an established malignant diagnosis at the time of kidney biopsy, whereas all HEME cases were initially or concurrently diagnosed at the time of biopsy. Two of the non-HEME biopsies showed extracapillary hypercellularity due to malignant cells, a feature not seen in HEME cases. Of the 8 patients with follow-up available, 7 (88%) died within a median of 69 (IQR 4-161) days. Conclusion: IGM is a rare presentation of disseminated malignancy, often indicating advanced disease with poor prognosis. Nephropathologists should be aware of IGM as a histologic mimicker of endocapillary hypercellularity or cellular crescent formation. Similarly, the provision of complete clinical history is critical for accurate biopsy assessment and to avoid this diagnostic pitfall. Given its high mortality rate and the short interval between tissue diagnosis and death, the identification of IGM should prompt urgent medical attention.