Digitally reconstructed radiographs from abdominal CT scans as a new tool for radiotherapy planning

OBJECTIVE. Abdominal extended field radiotherapy requires exact field shapi ng. Conventional treatment planning is difficult to adapt to individual ana tomy, whereas three-dimensional planning is time-consuming. The authors int roduce a method with digitally reconstructed radiographs of spiral CT data to facilitate radiotherapy planning. METHODS. Twenty-two patients underwent imaging with a standardized CT proto col, and digitally reconstructed radiographs were calculated in central bea m projection using a maximum intensity projection algorithm (MIP-DRR), For comparison, the expected error from parallel projection was calculated depe nding on object thickness and field length. RESULTS. The contrast-enhanced protocol used in spiral CT produces a good r endition of all relevant structures. The resulting MIP images have a geomet ry identical to standard simulation films and to the linear accelerator, wh ereas standard MIPs with parallel projection show significant distortion co mpared to the treatment process. CONCLUSIONS. Because of the integration of the geometry of the radiotherapy treatment, the described central beam projection method might be used as a new tool for abdominal radiotherapy planning. The CT protocol offers suffi cient contrast enhancement in all relevant structures and provides all nece ssary anatomic information for individual beam shaping.