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

OBJECTIVE. Abdominal extended field radiotherapy requires exact field shaping. Conventional treatment planning is difficult to adapt to individual anatomy, whereas three-dimensional planning is time-consuming. The authors introduce a method with digitally reconstructed radiographs of spiral CT data to facilitate radiotherapy planning. METHODS. Twenty-two patients underwent imaging with a standardized CT protocol, and digitally reconstructed radiographs were calculated in central beam projection using a maximum intensity projection algorithm (MIP-DRR), For comparison, the expected error from parallel projection was calculated depending on object thickness and field length. RESULTS. The contrast-enhanced protocol used in spiral CT produces a good rendition of all relevant structures. The resulting MIP images have a geometry identical to standard simulation films and to the linear accelerator, whereas standard MIPs with parallel projection show significant distortion compared 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 sufficient contrast enhancement in all relevant structures and provides all necessary anatomic information for individual beam shaping.