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A man in his 50s presented with persistent left flank pain, imaging revealed a non-functioning hydronephrotic left kidney with multiple calculi and thinning of the renal parenchyma, prompting a radical nephrectomy. Histopathological examination unexpectedly identified two distinct neoplasms—a type 1 papillary renal cell carcinoma (PRCC) and primary mucinous adenocarcinoma arising from the pelvicalyceal system. Immunohistochemistry confirmed their separate epithelial origins. Neither tumour had metastasised. Chronic irritation from nephrolithiasis and hydronephrosis may have promoted the development of the mucinous adenocarcinoma, while sporadic genetic alterations likely underpinned the PRCC. This case highlights the need for thorough pathological assessment and, specifically, to consider multiple primary tumours in patients with chronic renal pathology. Early surgical intervention and careful histopathological evaluation are crucial for accurate diagnosis, informing disease management and potentially improving patient outcomes.