Rapidly Recurrent Retroperitoneal Cystic Lesion With Mass Effect in a Kidney Transplant Recipient: A Diagnostic and Management Challenge

Complex cystic lesions arising from the kidney and adrenal gland are often found incidentally on cross-sectional imaging, as the majority are benign and asymptomatic. Large cystic lesions pose a diagnostic challenge, especially when initially simple and later demonstrating internal heterogeneity. This is consistent with radiological complex lesions, particularly in medically complex transplant patients, as there is no evidence-based consensus regarding when medical or surgical intervention is beneficial over regular monitoring. These challenges are amplified in solid-organ transplant recipients maintained on chronic immunosuppression, where altered immune surveillance may influence lesion behavior and clinical presentation. This report describes the case of a chronically immunosuppressed kidney-transplant recipient who was incidentally found to have an unusually progressive retroperitoneal lesion causing mass effect on the left native kidney, pancreas, stomach, and spleen. The lesion demonstrated progressive growth and increasing complexity on serial imaging and rapidly recurred following image-guided aspiration. Despite minimal early symptoms, the patient ultimately required an exploratory laparotomy with en bloc resection of the mass with left adrenalectomy and nephrectomy to fully excise a 20 cm cystic lesion. Pathologic evaluation was inconclusive despite extensive histologic and immunohistochemical analysis and review by multiple attending pathologists; no evidence of malignancy was identified. This case was characterized by rapid enlargement, recurrence after aspiration, and significant mass effect despite minimal symptoms. Such atypical behavior may be influenced by the patient’s immunosuppressive regimen, which is known to impair immune surveillance, inflammatory signaling, and reparative responses. This case highlights the need for careful surveillance and a low threshold for surgical intervention in transplant recipients with atypical or rapidly progressive cystic lesions.