Abstract The clinical and pathological features of a left renal tumour and a subsequent retroperitoneal tumour in a 2-year-old boy are presented. The nephrectomy speci- men showed typical features of a triphasic Wilms’ tu- mour with focal heterologous elements and intralobar nephrogenic rests. The tumour was assessed as clinical stage III and post-operative chemotherapy and radiother- apy were administered. A retroperitoneal mass, detected following completion of postoperative therapy, was mainly made up of skeletal muscle and mature adipose tissue. Nests of epithelium resembling ameloblastic nests and a unique structure reminiscent of a developing tooth were present. Key words Differentiation · Odontogenic · Nephroblastoma · Organogenesis · Teratoma · Wilms’ tumour Introduction Wilms’ tumour (WT) is the most common renal neo- plasm of childhood, arising from the metanephric blaste- ma, which is totipotent and can differentiate into a vari- ety of tissues. Heterologous elements are common in WT and might be mistaken for evidence of organogene- sis, but unequivocal organoid differentiation (for exam- ple, bronchus, skin, and tooth) has not been described. In fact, the presence of unequivocal organogenesis in a re- nal tumour is considered to be pathognomonic for a renal teratoma [2]. The retroperitoneum is a common extragonadal site for teratomas, but renal teratomas are rare. We report a patient with a stage III triphasic WT, which was fol- lowed after 6 months of chemotherapy and radiotherapy by a retroperitoneal tumour showing a unique micro- scopic structure reminiscent of tooth development. Case report A 2-year-old black male infant presented with a 10×9 cm, firm, non-tender, left upper quadrant abdominal mass. He was anaemic and malnourished, weighing 7.6 kg. A CT scan of the abdomen confirmed a partially cystic but well-encapsulated mass arising in the left kidney. The right kidney, ureter and bladder were normal, but the left kidney failed to excrete the contrast medium. Following pre-operative chemotherapy, with actinomycin D and vincristine, surgery was performed; a large left-sided renal tu- mour was encountered, which had clearly penetrated the capsule at the upper pole. Tumour encased the renal pedicle and extended in- to the ureter. The left renal vein was thrombosed, but the inferior vena cava was clear of tumour when pre-operatively assessed with ultrasound. Total nephrectomy was performed, and a para-aortic lymph node was biopsied. The lesion was operatively staged as III according to the NWTS staging criteria. The child received post- operative radiotherapy and chemotherapy according to the SIOP protocol. Six months after primary surgery, at completion of chemother- apy and radiotherapy, a CT scan of the abdomen showed evidence of tumour recurrence at the hilum of the right kidney. No intrare- nal tumour was detected on imaging studies. At surgery a large soft tumour mass was found in the left renal bed, with extension across the midline, anterior to the aorta and in front of the right re- nal hilum and adrenal gland. The bulk of the tumour was close to the right renal hilum, but the right kidney was clearly uninvolved. Debulking of the mass was achieved. A CT scan done 3 months after debulking showed no residual tumour. The patient was dis- charged and did not return for follow-up. Genetic analysis was not performed. Results The nephrectomy specimen measured 14×12×6 cm and weighed 500 g. Cut sections revealed a predominantly D. Govender Department of Pathology, University of Natal Medical School and King Edward VIII Hospital, Durban, South Africa G.P. Hadley Department of Paediatric Surgery, University of Natal Medical School and King Edward VIII Hospital, Durban, South Africa Mailing address: D. Govender ( ✉ ) (e-mail: govendh@med.und.ac.za, Tel.: +27-31 2604493, Fax: +27-31 252711), Department of Pathology, University of Natal Medical School, Private Bag 7, Congella 4013, South Africa Virchows Arch (1999) 435:67–70 © Springer-Verlag 1999 CASE REPORT D. Govender · G.P. Hadley Recurrent Wilms’ tumour or retroperitoneal teratoma? A challenging case Received: 22 January 1999 / Accepted: 25 March 1999