gms | German Medical Science

The accuracy of a patient’s position in the treatment room is fundamental for an effective radiotherapy. Reproducible positioning in every fraction can be achieved by different fixation systems and can be controlled by making images of the patient. Especially if the target volume is able to move relative to the bony structure (respiratory movement or movement due to organ filling), image guidance in the treatment room is essential. A possibility for verification of a patient’s position is the use of the treatment beam (MeV) for making radiographs on films or electronic portal imaging devices (EPID). With a low energy X-ray source (keV) and an additional flat panel detector mounted to the gantry of a linear accelerator a new alternative for making portal images (PI) was developed. When the gantry is rotated around the patient, the X-ray source can be used as cone-beam CT (CBCT) for a volume scan (Elekta Synergy & XVI-System, Elekta, Crawley, U.K.). The aim of this study was to compare the dose delivered to the patient by MeV PI (iView CT, Elekta), keV PI and CBCT (X-ray volume imaging). 5 MU were delivered for ap and lateral PI with the treatment beam. For keV PI and CBCT the manufacturer’s presets for pelvis images and CBCT were used. The dose measurements were done with 5 patients treated for prostate cancer with intensity modulated radiotherapy (IMRT). For ap and lateral PI one ionization-chamber was placed on the patients’ skin close to the irradiation source and another chamber inside the rectum. The dose measurement for CBCT was done only inside the rectum. The av. dose for ap MV PI was 57.78±1.17 mGy at the surface and 33.90±1.18 mGy in the rectum, for lateral MV PI was 69.42±1.41 mGy at the surface and 31.69±1.75 mGy in the rectum. The dose measurements for the kV PI resulted in approx. 1-2% of the dose for the MV PI: for ap PI the dose was 0.75±0.13 mGy at the surface and 0.19±0.08 mGy in the rectum, for lateral PI it was 1.12±0.24 mGy at the surface and 0.13±0.04 mGy in the rectum. For a 360° volume scan with the CBCT the dose in the rectum was 17.23±2.76 mGy. The results are summarised in Table 1 [Tab. 1]. Conclusion: for very fast check of the patient’s position it is adequate to aquire ap and lateral images with a keV source: the image quality is better and the additional dose is much less than with MeV portal imaging. The CBCT is well suited for detailed position verification and the dose inside the patient is still much less than with conventional MeV PI.