Multifocal Primary Neoplasms in Kidney Allografts: Evaluation of Two Cases.

Renal cell carcinoma (RCC) is the fifth most common malignancy in kidney transplant recipients, with increased risk arising due to immunosuppression. RCC occurrence in kidney allografts is much less common when compared with the native kidneys. Multifocal RCC in allograft kidneys is rarely described. In this report, we discuss two cases of multifocal renal neoplasms in allograft kidneys. Case 1 had three distinct neoplastic lesions of >5 mm, and case 2 had four. Using the World Health Organization 2016 classification of adult renal tumours, case 1 had one clear-cell (cc) RCC (grade 3) and two papillary adenomas; all confined to the kidney. Case 2 had a nodular lesion classified as ccRCC (grade 4) with focal rhabdoid differentiation and some infiltration of renal sinus fat; a cc tubulopapillary RCC; a multilocular cystic renal neoplasm of low malignant potential; and a mucinous tubular and spindle cell carcinoma; the last three all confined to the kidney. This is the first report of mucinous tubular and spindle cell carcinoma in a kidney allograft. When considering multifocal RCC with discordant histology, it is likely that these represent independent tumourigenic events.