SRS372 - Primary renal MALT lymphoma mimicking urothelial carcinoma: a rare case of asymptomatic presentation

Abstract Primary renal mucosa-associated lymphoid tissue (MALT) lymphoma is extremely rare and often mimics upper tract urothelial carcinoma (UTUC) on imaging. Most cases present with haematuria or flank pain. We report a unique case of incidentally discovered renal MALT lymphoma in an asymptomatic patient, managed surgically for presumed UTUC. A 67-year-old woman with a background of UC treated with infliximab underwent a CTAP to investigate a suspected stricture, incidentally revealed a left renal mass and ipsilateral adrenal lesion. She was asymptomatic with no urinary complaints. A CT urogram confirmed a filling defect in the renal pelvis and distal ureter, while cystoscopy showed no bladder tumour. Given radiological features suspicious for UTUC, a laparoscopic left radical nephroureterectomy was performed. Histopathology revealed a low-grade B-cell non-Hodgkin lymphoma, favouring extranodal marginal zone lymphoma (MALT) arising from the renal pelvis. Immunohistochemistry showed positivity for CD20, PAX5, and IgM, with negative staining for CD5, CD10, BCL6, and cyclin D1. Margins were negative. A haematopathology second opinion was requested to rule out follicular lymphoma due to overlapping features. Postoperative recovery was uncomplicated. Renal function remained stable and follow-up MRI showed no evidence of metastases. The case was discussed in the haematology MDT. The lymphoma was fully excised and there was no systemic involvement, so no further treatment was required. Renal MALT lymphoma is a rare diagnostic entity that may be radiologically indistinguishable from UTUC. This case highlights the need for awareness of atypical pathologies in renal masses and the importance of multidisciplinary input to guide appropriate management.