Compensating filters for high energy x rays

In order to produce individualized compensating filters for use with high energy x rays, a study was undertaken to study lead as a filter material using 10-MV x rays. As was expected, it was found that lead filters could be constructed for 10-MV x rays by extending concepts developed for cobalt-60 gamma rays. A quantity called the "effective attenuation coefficient" (mueff) was determined for a range of field sizes, initial depths in water, and lead thicknesses and for 100-cm SSD and 150-cm SSD treatment techniques. Slight variations of mueff with field area and filter thickness were discernable. However, a single value of this parameter could reproduce the original depth of missing tissue data with an rms error of 2 mm. Tests were then performed on filters whose shapes were calculated using the effective attenuation coefficient. Compensating filters restored the flatness of 10-MV x-ray beams to better than 5%, which was better than comparable filters using cobalt-60 gamma rays. The technique is presented as generally reliable for constructing compensating filters for use with high energy linear accelerators.