Technical note: leakage radiation in radiotherapy--what is an acceptable level in the electron mode?

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The International Electrotechnical Commission (IEC) sets standards for the safety of medical electrical equipment. IEC Publication 601–2–1 which applies to medical electron accelerators is under revision. One of the requirements in the current standard (IEC, 1981) relates to the amount of leakage radiation that is allowed through the beam limiting devices. Clause 6.1.3 of this document states that for equipment in which the area denned by the primary collimators, M, shielded by the adjustable beam limiting devices exceeds 500 cm2 (all modern equipment), the product of the average absorbed dose resulting from leakage radiation and the area, M, shall not exceed one tenth of the product of the absorbed dose on the central axis and the area of a 10×10 cm field. This requirement applies to accelerators operating in both the X-ray and electron modes and has since been incorporated in ICRP recommendations (ICRP, 1985). The requirement is shown graphically by the solid curve in Figure 1. For modern equipment, the area shielded by the beam limiting devices, M, covers a large area of the patient. It is worth noting that outside this area the IEC (1981) leakage requirement in the patient plane is much stricter. An average of only 0.1% of the central axis absorbed dose is allowed in areas outside M, which may be occupied by the patient. Discussions are currently taking place to consider relaxing the requirement on leakage radiation through beam limiting devices for electron therapy.