MEGAVOLTAGE IMAGING WITH LOW Z-TARGETS - IMPLEMENTATION AND CHARACTERIZATION OF AN INVESTIGATIONAL SYSTEM

The poor quality of stereotactic radiotherapy portal images is a limit ing factor in precise image registration. To alleviate this problem, a low atomic number (Z) target was implemented on our Siemens MXE linea r accelerator. This investigational system was used to assess the perf ormance of various target materials by filming an aluminum contrast ob ject. Beryllium, carbon and conventional target materials were studied . The bremsstrahlung spectra of these materials were simulated using M onte Carlo techniques. These spectra were used to calculate the depend ence of narrow beam contrast on phantom thickness for verification of the data measured from film. A Monte Carlo simulation of the beryllium spectrum in a wide beam geometry was used to evaluate the effect of p hantom-to-film distance on contrast. Although the same degree of contr ast improvement with distance was not realized in practice, the improv ement in image quality rivaled that achieved using a scatter reduction grid. A comparison of conventional localization images of the head an d neck of an anthropomorphic phantom with images produced with a beryl lium or carbon target and a mammography film and screen system support s earlier suggestions that the technique is clinically useful. (C) 199 8 American Association of Physicists in Medicine. [S0094-2405(98)01010 -4].