Answer to Question #4235 Submitted to "Ask the Experts"

If a ⁶⁰Co teletherapy unit gets stuck in the "expose" position, do I wear a lead apron into the room and how much dose will I receive?

Lead aprons are generally designed for protection from diagnostic x rays, which have a much lower energy than ⁶⁰Co teletherapy. The typical lead apron thickness is 0.5 mm, which is approximately two half-value layers for diagnostic x ray (that is, will reduce exposure to below one-fourth of the incident radiation). The half-value layer in lead for ⁶⁰Co is 12 mm. The use of a standard lead apron will do very little to reduce exposure to ⁶⁰Co. In addition, if a source is stuck in the "expose" position, time is of the essence when removing the patient from the beam.  The additional time for a staff member to put on a lead apron would result in a significantly higher radiation dose to the patient who is still in the radiation beam.

Typical patient exposure in the radiation beam of a  ⁶⁰Co teletherapy unit is ~80-160 R/min and the maximum amount of scatter radiation at one meter from the patient is 0.6% of the incident radiation beam (~hundreds of mR/min).

The United States Nuclear Regulatory Commission maintains a Nuclear Events Database that includes incidents for all devices that utilize radioactive sources.

References

• Introduction to Health Physics, Second Edition, Cember H., p. 309.
• Personal conversation with John Freshcorn, ABR certified Therapy Medical Physicist
• Structural Shielding Design and Evaluation for Medical Use of X-Rays and Gamma Rays of Energies up to 10 MeV, National Council on Radiation Protection and Measurements Report No. 49, p. 59.

Ken "Duke" Lovins, CHP