ctv to ptv margins based on cbct method for prostate cancer of patients treated with vmat technique

Introduction: patient repositioning in treatment radiotherapy is one of the main factors that increase error of target irradiation. however additional margin is necessary to consider the uncertainties created along and around x, y and z-axis.material and methods: set-up and random errors were calculated in translational and rotational axis for a sample of 20 prostatic patients; using daily igrt-cbct method. the aim of this study was to determine the additional margin that should be added from clinical target volume (ctv) to prevent toxicity and increase the irradiation precision in radiotherapy. the van hark formula (ptv margin =2.5σ +0.7σ) was used for all patients to perform ptv margin for prostatic localization.results: the research performed for a sample of 20 consecutive patients. with respect to systematic error along the lateral axis, longitudinal and anterior-posterior was 2.32, 2.42 and 3.54 respectively. the random error was 1.82, 2.19 and 1.76° along lateral axis, longitudinal and anterior-posterior respectively. the rotational systematic error was 1.49, 2.04 and 2.14° around lateral, longitudinal and anterior-posterior axis respectively. the random error was 1.78, 1.75 and 1.63° around lateral, longitudinal and anterior-posterior axis respectively. the calculated safety margin to cover clinical target volume (ctv) taking the prostate variability into account measured 7.55, 8.08 and 10.79 mm for lateral, longitudinal and anterior posterior respectively and 7mm would be enough in the posterior side. rotational set-up errors for almost 95% of patients were between -2° and 2°.conclusion: the calculated safety margin in all direction was smaller than 1 cm except in anterior side that was 1 cm or more.