Image-guided prostate radiotherapy: analysis of inter- fractional setup errors

Introduction: The accuracy of the patient setup and tumor location are major factors that determine the efficacy of radiotherapy treatment planning. In the prostate radiotherapy, reproducibility of the prostate location for each treatment fraction is problematic owing to varying filling of the rectum and bladder. According to this, the aim of this study was to measure the inter- fractional setup errors using gold markers implanted into the prostate and electronic portal imaging device (EPID). The results were used to determine optimal CTV-PTV (clinical target volume-planning target volume) margin. Materials and Methods: Image-guided three-dimensional conformal radiotherapy (3DCRT) was performed by daily online verification of implanted fiducial prostate markers using a megavoltage EPID. In this retrospective study, a total of 359 image-guided treatment fractions from 15 prostate cancer patients were analyzed. PTV was defined as an isotropic margin of 5 mm around of CTV. Daily electronic portal images (EPIs) compared to reference digitally reconstructed radiographs (DRRs), the setup errors were determined and online corrections applied. Mean displacements, population systematic and random errors were calculated. CTV-PTV margin was determined using van Herk’s formula. Results: The mean shifts in the vertical, longitudinal and lateral directions were 0.31 mm, 0.22 mm and – 0.41 mm, respectively. The population systematic and random errors in the three mentioned translational directions were 0.70 mm, 0.80 mm and 1.00 mm and 2.11 mm, 2.24 mm and 2.20 mm, correspondingly. CTV-PTV margin of 3.23 mm, 3.60 mm and 4.04 mm in the vertical, longitudinal and lateral directions were calculated. Conclusion: The calculated PTV margin was less than 5 mm, therefore PTV margin applied in this study provides a sufficient PTV coverage. Daily EPIs combined with gold markers implanted into the prostate provide an effective tool for verifying of the prostate position immediately prior to treatment delivery.