Answer to Question #390 Submitted to "Ask the Experts"

Q

There is a case in California where a 250MV accelerator therapy unit is apparently making the Cerrobend blocks radioactive. Have you heard of this problem? Could you tell me what elements are involved and the activation process making them radioactive? What radiation is the Cerrobend emiting? Are there any references? Thank you.

A

Cerrobend is a metal alloy used as a lead substitute material for making patient blocks for radiation oncology treatment. Cerrobend melts at a lower temperature than lead and is generally less toxic than lead. It does contain some metals which can constitute a health hazard over long periods of use, particularly cadmium. These heavy metals can become radioactive when placed in a high-energy photon beam due to the production of photoneutrons. These neutrons are produced when the photon energy exceeds 6 to 8 MV. Most radiation oncology treatment machines produce x rays between 6 and 25 MV. Photon accelerators operating above 15 MV produce a measurable number of neutrons and some detectable activation. There are one or two articles in the literature which deal with the doses to radiation oncology technologists handling these blocks and working in these types of accelerators. Radiation oncology equipment does not operate at 250 MV since photons of this energy would pass through the patient without depositing sufficient energy for treatment. There is a specialized facility at Loma Linda, California, which is using proton therapy (a very different modality) and you may wish to call them directly if this is the facility in your question.

Jean St. Germain, CHP