Correction method for in-air output ratio for output variations occurring with changes in backscattered radiation.

PURPOSE: The in-air output ratio (S(c)) for a rectangular field is usually obtained using an equivalent square field formula. However, it is well-known that S(c) obtained using an equivalent square field formula differs slightly from the measured S(c). Though several correction methods have been suggested for the monitor-backscatter effect, the authors propose a more simple correction method for a rectangular field. METHODS: For rectangular fields and equivalent square fields, the authors assumed that the output variation was the product of six output variations for each backscattering area at the top of the collimator jaws, and the correction factor was the ratio of the output variation for a rectangular field to the output variation for an equivalent square field. The output variation was measured by using a telescope measurement. RESULTS: The differences between the measured and corrected S(c) ranged from -0.20% to 0.28% for symmetric rectangular fields by applying the correction factor to S(c) obtained using an equivalent square field formula. This correction method is also available for asymmetric rectangular fields. CONCLUSIONS: The authors propose a method to correct S(c) obtained using an equivalent square field formula, and a method to obtain the output variation for a field defined by collimator jaws.

New shielding materials for clinical electron beams.

Since lead has recently been recognized as a source of environmental pollution, we have investigated new electron shielding materials that do not contain lead. We compared the shielding thicknesses of a hard plate and a sheet composed of the new materials with that of lead for electron beams. The shielding thickness was evaluated as the thickness required for shielding primary electrons. The comparison revealed the shielding ability of the hard plate and sheet is approximately equivalent to 1.0 and 0.9 times that of lead, respectively. The thickness (in millimeters) required for shielding by the hard-plate, as well as the thickness of lead, is related to approximately half of the electron-beam energy (in MeV). The shielding ability of the sheet is also equivalent to that of Lipowitz alloy. Moreover these materials are environmentally friendly, and can be easily customized into arbitrary shapes. Therefore they can be used as lead substitutes for shielding against electron beams.