Thorotrast was the trade name of a radioactive thorium dioxide suspension radiographic contrast media widely used between the 1930s and 1940s. It was popular for its use as an intravascular contrast agent, particularly for cerebral angiography. An estimated 2-10 million people are thought to have received it 8. Umbrathor was another thorium dioxide-based contrast medium which was principally used for assessment of the GI tract.
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Physics
Thorium-232, the most abundant thorium isotope, is highly radiopaque and produces excellent subject contrast due to its high atomic number (Z=90) relative to biological tissues, resulting in differential photoelectric absorption 7. It is an alpha emitter, decaying to radium-228, with a physical half-life of 1.405 x 1010 years and estimated biological half life of 400 years 8. Its distribution within the body varies according to administration route but when administered intravascularly, it is predominantly retained within the reticuloendothelial system (liver, spleen, bone marrow, and lymph nodes) 8. Its use is associated with an increased risk of malignancy but ironically, its initial popularity was in part due to its perceived safety 7,8.
Radiographic features
Plain radiograph of the abdomen may demonstrate fine, irregular metallic densities distributed throughout the liver, spleen, and peripancreatic lymph nodes, representing thorium deposition 4. There may be evidence of autosplenectomy 5.
History
The predecessor, Umbrathor, was a suspension of thorium dioxide which had a tendency to precipitate in solution and therefore was unsuitable for intravascular injection. In the late 1920s, Moabit Heyden, in collaboration with Berlin University, began developing a more stable suspension of thorium dioxide which was eventually marketed as Thoratrast in 1931 6. The Nobel Prize laureate and Portuguese neurologist Antonio Egas Moniz, also known for pioneering the prefrontal leucotomy (lobotomy), played an important role in popularizing its use in cerebral angiography 6.
Complications
Hepatic angiosarcoma is the classic Thorotrast-related malignancy. Other associated cancers include:
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