Neoplastic Complications Under mTOR Inhibitors in Kidney Transplant Recipients: 2 Case Reports.

Malignancy remains a major cause of late morbidity and mortality in kidney transplant recipients, largely due to chronic immunosuppression and impaired tumor immune surveillance. Mammalian target of rapamycin inhibitors have antiproliferative and antiangiogenic properties and are frequently used in recipients considered to be at increased oncologic risk. However, their protective effect against de novo malignancy is not absolute. Here, we report 2 cases of severe malignancies that developed in kidney transplant recipients after conversion from calcineurin inhibitors to sirolimus following polyomavirus-associated nephropathy. The first patient, a 55-year-old man, developed prostate adenocarcinoma 6 years after transplant and 4 years after conversion to sirolimus. The diagnosis was established during evaluation for severe anemia and graft dysfunction. The second patient, a 35-year-old woman, developed primary central nervous system posttransplant lymphoproliferative disorder 4 years after transplant and 2 years after conversion to sirolimus. Histopathologic examination confirmed an aggressive lymphoma without detectable Epstein-Barr virus infection. These cases illustrate that mammalian target of rapamycin inhibitor-based immunosuppression does not eliminate the risk of solid or hematologic malignancy. Cumulative immunosuppressive exposure, viral complications, and delayed conversion may contribute to persistent oncogenic risk. Careful long-term oncologic surveillance and individualized immunosuppressive management remain essential in kidney transplant recipients.