Accuracy of the dose delivery in prostate cancer patients-using an electronic portal imaging device (EPID)

Background: To correct patient positioning errors (setup errors) during prostate cancer treatment using EPID and fiducial gold markers, to improve the accuracy of the dose delivery in these patients. Materials and Methods: Fifteen patients with localized prostate carcinoma after implantation of fiducial gold markers in their prostate gland underwent the five-field IMRT planning technique. The plan was prepared in accordance with ICRU 50 guidance (PTV to receive 95-107% dose). The software program reconstructed the three-dimensional position of the markers from the different Beams Eye Views (BEV). The discrepancies of the seeds’ positions (prostate surrogate) between plan and daily images were calculated three dimensionally. Then, necessary corrections were applied to match the prostate fiducial markers in the portal image with the BEV image in the planned one by moving the couch in the X, Y and Z directions. Results: Data from 15 patients and 469 fractions of radiotherapy were analyzed in this study. Two sets of data were available from EPID software before and after 3D set-up corrections. The mean of the population displacement in Left /Right (L/R), Anterior/Posterior (A/P) and Crania/Caudal (C/C) directions were 0.5, -1.0 and 2.4mm before, and -0.1, -0.5 and 0.9mm after corrections, respectively. The systematic and random errors for the measured populations in the three mentioned directions were 2.4, 2.7 and 2mm and 6.4, 5.9 and 6.1mm before corrections, and 1.1, 2.4 and 1.4mm and 3.8, 3.9 and 3.6mm after corrections, correspondingly. Conclusion: This study provides further evidence that using gold markers in the prostate improves dose delivery to the prostate. Also, it has been demonstrated that the EPID can be a powerful tool in the reduction of treatment setup errors and the quality assurance and verification of complex treatments.