Sarcomatous Wilms' tumour with clear cells and hyalinization. A study of 38 tumours in children from the SIOP nephroblastoma file.

The International Society of Paediatric Oncology (SIOP) nephroblastoma trial and studies 1, 2 and 5 include 33 sarcomatous Wilms' tumours with a clear cell pattern and five with a hyalinizing pattern among 909 renal tumours. Bone metastases developed in 22 (67%) and one patient (20%) respectively compared with 14 patients (1.6%) in the rest of the series. Both subgroups had clear cells and hyalinization. A constant and characteristic feature of the clear cell tumours was an alveolar vascular pattern which aids in the recognition of this tumour even in fibrous and sclerotic forms. Sarcomatous Wilms' tumour with a hyalinizing pattern had homogeneous areas of sclerosis with clustering of tumour cells but lacked the vascular pattern. Two tumours showed atypia similar to that in malignant rhabdoid tumour of the kidney but had no cytoplasmic inclusions. Five year recurrence-free survival for the entire series was 68%, for sarcomatous clear cell tumours, 42% and for sarcomatous tumours with hyalinizing pattern, 20%.