LINEAR-ACCELERATOR PHOTON-BEAM QUALITY AT OFF-AXIS POINTS

Transmitted intensity through water was measured in a narrow-beam geom etry for different energy x-ray beams from commercial accelerators. In order to accurately obtain the attenuation coefficient of the inciden t beam using transmission data, a novel formula was developed based on consideration of beam hardening in phantom. The value of the attenuat ion coefficient obtained by fitting transmission data to this formula was found to be independent of the absorber thickness used in experime nts, whereas the attenuation coefficient obtained from the traditional formula, I(x)=I-0 exp(-mu x), changed by up to 7% with absorber thick ness for a given beam. The beam hardening coefficient obtained from ou r formula indicates that the attenuation coefficient in water changes by about 0.33% per cm near the surface for the high-energy photon beam s studied. Variations in beam quality with off-axis distance were subs equently investigated using the new formula. Results show that the att enuation coefficient at the water surface increased by about 15% for 1 5 and 18 MV beams, and by 11%-13% for 6 MV beams, when the off-axis di stance at 100 cm from the source was changed from 0 to 18 cm. Consider ation of the physics of bremsstrahlung production suggests that these variations should be mainly determined by the shape of the flattening filter, i.e., by the path length of rays traversing the filter in diff erent directions. This expectation was confirmed by observing that the attenuation coefficient at the phantom surface can be related to the ray path of the beam in the flattening filter using the new transmissi on formula. (C) 1997 American Association of Physicists in Medicine.