Therapy imaging: source sizes of radiotherapy beams.

We have developed a novel method, which employs large lead collimators and computed tomography reconstruction techniques, to measure the intensity distributions of x-ray sources of radiotherapy devices. Using this method, we have measured the intensity distributions of x-ray sources from 60Co, 6-, 18-, and 25-MV radiotherapy devices. The x-ray sources of the accelerators were all elliptical in shape, but varied in eccentricity, and the sizes of the accelerator sources varied from 0.7 to 3.3 mm full width at half-maximum. The 60Co source was circular in shape and 20 mm in diameter, however, the output from this source was not uniform across its face. The modulation transfer functions (MTF's) (at the image plane) calculated for the accelerator sources, assuming an image magnification of 1.2, had similar magnitudes at low spatial frequencies as the MTF's of the metal plate/film detectors commonly used for therapy imaging. However, the source MTF's declined much more rapidly at high spatial frequencies. Therefore, for geometries commonly found in radiotherapy, the loss in spatial resolution due to the x-ray source was at least equal to that caused by electron and photon scatter within the metal plate/film detectors.