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Memorandum from the Institute of Physical Sciences in Medicine

At present, the dosimetry of low- and mediu-energy X-radiation for radiotherapy purposes in the UK is based on the method recommended in International Commission on Radiation Units and Measurements (ICRU) Report 23 (1973), and incorporated into the current Hospital Physicists' Association (HPA) code of practice (1983). The essential points are summarized here using the same symbols and units as in the code of practice.

For an X-ray beam between 150 kV and 300 kV (0.5 mm Cu and 4 mm Cu half-value layer (HVL)), ICRU recommended that output measurements should be made using an ionization chamber with its centre 5 cm deep in a water phantom. Absorbed dose is given by:or by:where D = absorbed dose to water in grays at the position of the centre of the chamber, with the chamber replaced by water, R = instrument reading, corrected to the same standard ambient conditions as the calibration factor, N_k = calibration factor to convert the instrument reading to air kerma in grays for standard ambient conditions (at present 20°C, 1013 mbar, and 50% RH for calibrations at NPL), N_x = calibration factor to convert the instrument reading to exposure in röntgens for standard ambient conditions, F = a conversion factor, in rad/röntgen, which depends on the HVL of the X-ray beam being used.

Key Words: Dosimetry for radiotherapy • Back-scatter • Low-energy X-radiation

Received for publication March 1, 1991.