Severe mesangiolysis with prominent microaneurysm formation following total body irradiation conditioning for bone marrow transplantation.

Patients who undergo hematopoietic stem cell transplantation (HSCT) face a high risk of acute kidney injury (AKI), which significantly increases the likelihood of developing chronic kidney disease (CKD). Proteinuria is also frequently observed after HSCT and is associated with an increased risk of CKD progression. Thrombotic microangiopathy (TMA), stemming from endothelial injury caused by factors such as total body irradiation (TBI), calcineurin inhibitors, tyrosine kinase inhibitors, and graft-versus-host disease, is a major contributor to both AKI and CKD. We report a case of a woman in her early thirties who developed progressive kidney dysfunction and subnephrotic-range proteinuria three months after HSCT. Kidney biopsy findings included marked mesangiolysis, endothelial cell detachment, subendothelial expansion, and diffuse segmental capillary microaneurysms, which are highly suggestive of TMA due to TBI administered as part of the myeloablative conditioning regimen prior to HSCT. She underwent conservative management for chronic kidney disease, but her kidney function progressively declined. In patients presenting with a progressive decline in kidney function following HSCT, TMA should be included in the differential diagnosis. A kidney biopsy is valuable for determining the underlying cause of HSCT-associated TMA, as treatment strategies vary depending on the etiology. Prominent mesangiolysis with capillary microaneurysms, as observed in this case and the previously reported irradiated patients, may serve as a distinctive histological marker of radiation nephropathy.