Comparison of x-ray characteristics beam from a medium energy linear accelerator and a high energy linear accelerator

Abstract

The radiotherapy community is by no means in agreement as to what is the optimum energy for accelerators used in radiation therapy. Photon radiation in the energy range 4-6 MV is typically classified as standard megavoltage while radiation in the range 20-25 MV is considered to be supervoltage. Recently, however, therapy accelerators have been developed which produce photon energies between lo-18 MV and are substantially less expensive than the 25 MV units. This presentation will compare the beam characteristics of an 18 MV x-ray beam from a new generation Varian Associates Clinac 20 linear accelerator to those of a 25 MV x-ray beam produced by a Varian Associates Clinac 35 linear accelerator. Percentage depth dose, tissue-phantom ratios, output factors, and field s mmetry/flatness measurements were made for both accelerators using 0.1 3 cm ion chambers in a water phantom. Isodose distributions were computed from central axis depth dose data in conjunction with dose profiles measured at selected depths and compared with measured isodoses obtained with an automatic isodose plotter for both machines. Variation of output with field size and distances will be compared. Attenuation data for shielding, wedges, and compensating materials for both machines will be provided. Surface doses and the position of the dose maximum as a function of field size and target-surface distance will be given. The data indicates that through improved target and flattening filter design the 18 MV x-ray beam has almost identical central axis depth dose characteristics as the higher energy 25 MV x-ray beam for field sizes larger than 10 x 10 cm 2. The table below compares central axis depth dose values for various field sizes and selected depths for the two x-ray beams.