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High-energy x-ray beam calibrations using scaling.

When calibrating high-energy x-ray beams, it is convenient to measure in a plastic phantom and convert the result to absorbed dose in water at a specified depth, field size, and distance from the source. By combining the scaling theorem with the observation that TAR is independent of the distance to the source, it is demonstrated that the measurement in plastic can be made at the same source-detector distance but scaling depth and field size. Experiments to support this were made in acrylic and water with 6- and 25-MV x rays and at different distances from the source. The agreement between the calibrations in acrylic and water was comparable to that using the current national protocol, which recommends the scaling of depth only while using the same field size and source-detector distance in the two media and correcting for the difference in scatter dose. It was confirmed that the scaling factor may be the ratio between the electron densities of acrylic and water, or the ratio between the linear attenuation coefficients. A method to determine this factor directly is also described.