Planning target volume margins for prostate radiotherapy using daily electronic portal imaging and implanted fiducial markers

Background: Fiducial markers and daily electronic portal imaging (EPI) can reduce the risk of geographic miss in prostate cancer radiotherapy. The purpose of this study was to estimate CTV to PTV margin requirements, without and with the use of this image guidance strategy. Methods: 46 patients underwent placement of 3 radio-opaque fiducial markers prior to prostate RT. Daily pretreatment EPIs were taken, and isocenter placement errors were corrected if they were >= 3 mm along the left-right or superior-inferior axes, and/or >= 2 mm along the anterior-posterior axis. During-treatment EPIs were then obtained to estimate intra-fraction motion. Results: Without image guidance, margins of 0.57 cm, 0.79 cm and 0.77 cm, along the left-right, superior-inferior and anterior-posterior axes respectively, are required to give 95% probability of complete CTV coverage each day. With the above image guidance strategy, these margins can be reduced to 0.36 cm, 0.37 cm and 0.37 cm respectively. Correction of all isocenter placement errors, regardless of size, would permit minimal additional reduction in margins. Conclusions: Image guidance, using implanted fiducial markers and daily EPI, permits the use of narrower PTV margins without compromising coverage of the target, in the radiotherapy of prostate cancer.