Rhabdoid tumour: a malignancy of early childhood with variable primary site, histology and clinical behaviour

Aims: To correlate the immunostaining for INI1 protein and mutations in INI1 gene in possible rhabdoid tumours (RT) and atypical teratoid/rhabdoid tumours (AT/RT) seen at the Royal Children's Hospital in the last 10 years, and to study the clinicopathological features of those patients with negative nuclear staining. Methods: Twenty tumours showing suggestive histological and/or immunohistochemical features of RT and AT/RT were selected. Immunohistochemistry for INI1 and molecular investigations for INI1 mutations were performed. The clinical features, histology and immunohistochemistry in those patients with negative nuclear staining were studied. Results: In seven tumours the nuclei stained uniformly for INI1. In none of these was an INI1 mutation detected. In 13 tumours nuclei showed no staining. In only ten of these was material available for molecular studies. Mutations were detected in nine. In these 13 patients, the primary tumour was in the central nervous system (CNS) in seven, in the soft tissue in three, in the liver in two and in the kidney in one. The age of presentation varied from 19 days to 7 years. Only five tumours showed large areas of rhabdoid cells. Most showed extensive non-diagnostic areas. In two an alternative diagnosis, ependymoma or myoepithelial carcinoma of soft tissue, was initially suggested. All the CNS tumours were positive for EMA, GFAP, and SMA. There were no long term survivors, but an occasional patient showed excellent response to intensive chemotherapy. Conclusions: In this small series, there is a strong correlation between the loss of INI1 immunostaining and the presence of an INI1 mutation suggesting that the former is a reliable marker for RT and AT/RT in children. As relatively few tumours showed uniform populations of rhabdoid cells, and some showed features suggesting another diagnosis, INI1 staining should be checked in all high grade CNS tumours and malignant extraCNS tumours where the diagnosis is unclear. The prognosis of RT is poor but medium term remission can be achieved in some patients with aggressive treatment.