Thorotrastosis. Light micrograph of a section through a spleen with thorotrastosis. This is a malignant (cancerous) change in tissue caused by a deposit of thorotrast, a diagnostic material injected into patients from 1929 to around 1960. It was used as a radiocontrast agent (a substance used to image tissues that would otherwise not show up on X-rays, such as blood vessels). Thorotrast contained thorium oxide, and is harmful because the thorium 232 isotope emits damaging alpha particle radiation. As well as spleen cancer, it causes nervous system tumours, leukaemia and blood disorders.