A simple method of obtaining off-axis half-value layers for megavoltage photon beams

We describe a simplified technique for the acquisition of off-axis beam-quality data (i.e., half-value layers in water) for medical linear accelerators. A measurement protocol is presented in which an ordinary beam-scanning water phantom is used to accurately position an ionization chamber at appropriate measuring positions. Attenuation coefficients are calculated by performing a regression on the measured data. The technique uses the smallest fields for which lateral electron equilibrium can be established.