抄録
The tissue-maximum ratio for extremely small X-ray beams was examined using a 4 MV linear accelerator applied to square fields and its usefulness in the output factor was investigated. Furthermore, measurements in a water phantom and the effects of a low density phantom (average electron density 0.30 relative to water) on the measured values were assessed. A cylindrical watertight ionization chamber was used and the sensitive volume with air cavity was 0.006cm3. Marked discrepancies were found between the measured values in water and low density phantoms for the depth of the maximum dose and the output factor of an extremely small field. The measurement in the low density phantom has no advantage in practical use; therefore, we used the small ionization chamber to obtain data for an extremely small field.
