Compensating filters for high energy x rays

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Abstract
In order to produce individualized compensating filters for use with high energy X-rays, Pb was studied as a filter material using 10-MV X-rays. Lead filters could be constructed for 10-MV X-rays by extending concepts developed for 60Co .gamma. rays. A quantity called the effective attentuation coefficient (.mu.eff) was determined for a range of field sizes, initial depths in water, and lead thicknesses and for 100-cm SSD [skin-source distance] and 150-cm SSD treatment techniques. Slight variations of .mu.eff 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 [root mean square] error of 2 mm. Tests were then performed on filters whose shapes were calculated using the effective attentuation coefficient. Compensating filters restored the flatness of 10-MV X-ray beams to better than 5%, which was better than comparable filters using 60Co .gamma. rays. The technique is presented as generally reliable for constructing compensating filters for use with high energy linear accelerators. [The concept of using a compensating filter to effectively flatten the irregularly shaped surface of a radiotherapy patient is elegant.].

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